science education teaching booklet

Name: ________________ Discussion The great Italian scientist Galileo (1564-1642) notice while watching a lamp swinging in the cathedral of Pisa, that the time taken for each swing was the same, whether the swing was large or small. Since clocks were not invented until after Galileo had made this discovery, how do you think did he measure the time taken for the lamp to complete a swing? Before you can start an experiment you need to get your teacher’s signature, Beware you may be asked some questions so you will have to read it before starting Stop What is it that determines the period of the swing? Is it the size of the push? Is it the mass of the person sitting on the swing? Is it the length of the swing? SWING HIGH, SWING LOW Experiment Materials • • • • • • length of light string (at least 80 cm long) set of slotted masses (or various sized pendulum bobs) retort stand with bosshead (or a highstructure from which to suspend the pendulum) pair of scissors one-metre ruler stopwatch Part A: Does the push make a difference? Write down your hypothesis about the size of the push. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ • Set up your pendulum so it can swing freely. Start with the largest possible length and the smallest weight. • Record the mass and the length of the pendulum in the results table. The length should be measured from the top of the pendulum to the bottom of the swinging mass, as shown in the diagram. • Pull the mass aside so that the angle of release is about 20°. Take note of the height from which the mass is released so, that this angle of release is used throughout the experiment. • Release the pendulum. Measure the time taken for ten complete swings of the pendulum. Repeat your measurement at least twice so that you can find the average time for ten swings. Use this average to calculate the time taken for one complete swing (the period). Record all the measurements in your table. • Repeat this procedure, this time giving the mass a small push. • Repeat the procedure once more, giving the mass a larger push. 2 Part B: Do mass or length make a difference? Write down your hypothesis about the size of the effect of mass and length. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ • Using the same angle of release and pendulum length as before, determine the period'of the pendulum for several different masses. Use three trials for each mass. Record all data in your table. • Halve the length of the string and repeat your measurements for the different masses. Record the mass and length of the pendulum as well as the average times and period. • Halve the length of the string again and repeat your measurements, ensuring that all of the necessary data are recorded in the table Results Part A Mass of pendulum = _____ g Size of push Length of pendulum =______cm Average (s) Period (s) Time taken for 1 0 complete swings (s) Trial 1 Trial 2 Trial 3 No push Small push Larger push Part B Mass (g) Length (cm) Time taken for 10 complete swings (s) Trial 1 Trial 2 Trial 3 Average (s) Period (s) 3 Draw a line graph to show how the length of the pendulum affects the period. You need to graph data for only one mass. Period (s) 2 1.5 1 0.5 0 0 10 20 30 40 50 60 70 80 Length (cm) Discussion 1. How does the size of the push affect the pendulum's period? ________________________________________________________________________ ________________________________________________________________________ 2. How does the mass affect the period of the pendulum? ________________________________________________________________________ ________________________________________________________________________ 3. How does the length of the pendulum affect its period? ________________________________________________________________________ ________________________________________________________________________ 4. The period of most standard clock pendulums is one second. Use your graph to predict the length of a standard clock pendulum. ______________________________________________________________________ 4 5. Why is it a good idea to measure the time for ten swings rather than just one? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 6. Which variables must be kept constant when determining the effect of mass on the period of a pendulum? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Making waves When a wave is made in a still lake by dropping a rock into it, the wave spreads out. However, the particles of water do not spread out — they just move up and down. A wave is able to transmit energy from one place to another without moving any matter over the same distance. Before you can go on you need your teacher’s signature Check point 1 Moving Energy Without Matter Investigation Materials • deep tray • small cork • eye dropper Method • Half-fill the tray with water and place a small cork on the water surface. Use the eye dropper to release drops of water near the cork. Observe the motion of the cork and the motion of the waves made by the drops. Questions 1. Describe the motion of the cork. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2. Is there evidence to suggest that any water moves in the same direction as the waves? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 5 Two types of waves Waves on water are called transverse waves. Transverse waves can also he made on a slinky. As shown in the diagram below, the moving particles in a transverse wave travel at right angles to the direction of energy transfer. (The word 'transverse' means 'across'.) The diagram also shows that in a compression wave, the moving particles move backwards and forwards in the same direction as the energy transfer. Compression waves are also known as longitudinal waves. The material through which the waves travel is called the medium (the plural is 'media'). In the experiment on the opposite page the media are water and the slinky respectively. Direction of the motion of particles Direction of the energy transfer compression rarefaction Two types of energy transfer in a slinky: a transverse wave (top) and a compression wave (bottom) Wave terminology The frequency of a vibration or wave is the number of complete vibrations or waves passing a point in one second. The number of compressions made in a second gives the frequency of a sound wave. Frequency: The number of waves produced per second. The note middle C, for example, creates 256 vibrations, or compressions, every second. Frequency is measured in hertz (Hz), a unit named after Heinrich Hertz, the German physicist who, in 1887, was the first to detect radio waves. Thus, middle C has a frequency of 256 hertz. The frequency of a sound determines its pitch. High-frequency vibrations produce high pitch, and low-frequency vibrations produce low pitch. In the case of waves on water, the wavelength is the distance between two crests, or two troughs, or the distance between any two corresponding points on neighbouring waves. In the case of a compression wave, the wavelength is the distance between the centre of two neighbouring compressions, or two neighbouring rarefactions. X-File The human ear is capable of detecting frequencies between about 20 hertz and about 20 OOO hertz. Dogs have a much greater range of hearing and can detect frequencies between 15 hertz and 50 000 hertz. Cate cats hear even higher frequencies — up to 6O OOO hertz. Wavelength: The distance from one crest or one trough to the next. The distance between compressions — the wavelength — of the sound of the note middle C is about 1.3 metres. The wave length of sound made during normal speech varies between about 5 centimetres and about 2.5 metres. As the frequency of a sound gets higher — that is, as more compressions are produced per second — the compressions become closer together. Thus, low-frequency sounds have long wavelengths and high-frequency sounds have short wavelengths. The amplitude of a wave is the maximum distance that each particle moves away from its 6 usual resting position. Higher amplitudes correspond with louder sounds. Amplitude: The height of any wave, be it sound, water or electromagnetic. Properties of Wave Refraction — a wave can change speed when it passes from one material (or medium) to another. If the wave reaches this second medium at an angle, it will change direction. Both glass and water slow down light and speed up sound. Reflection—at least some of a wave always reflects from the surface of a material, whether straight or curved. For instance, light will refract through glass, but part of it will reflect. This is seen as glare. Absorption—waves can travel through solids, liquids and gases, depending on the wave and the material. A wave might be able to travel right through some substances. Other materials might absorb some or all the energy of the wave. This energy may then change those materials in some way. This diagram shows absorption of some of the wave energy. Diffraction—waves can diffract (as they move through a slit or around an object) if the wavelength is large enough. Sound waves can have a wavelength as small as 0.01 m or as large as 2 m. Light wavelengths are about 10-6 (a millionth) of a metre or a mirco metre (µm) and diffract little. Sound waves Sound is a compression wave. All vibrations caused by sounds travel by causing air particles to compress like the lower wave pattern shown in the slinky spring figure. The diagram on the right shows how a vibrating ruler makes compression waves in air. As the ruler moves up, a compression is created as air particles above the ruler are pushed together. Air particles below the ruler are Compression spread out, creating a rarefaction. When the ruler moves down, a Rarefaction rarefaction is created above the ruler, while a compression is created below it. Each vibration of the Compression ruler creates new compressions and rarefactions to replace those that are moving through the air. 7 Making Sound Place your hand over your voice box at the front of your throat, and say 'ahh' in a deep voice. You can feel your throat vibrating. Now say 'ahh' in a higher voice. • Does your throat vibrate as it did before? What difference do you notice? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Place a ruler over the edge of a desk and flick it. Notice that it makes a sound when it vibrates. Change the length that is over the edge of the table. Flick the ruler again. Notice that the sound is different. • Describe the difference between the sounds. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ • Which ruler made the lowest sound — the long one or the short one? ________________________________________________________________________ • Which ruler vibrated faster — the long one or the short one? ________________________________________________________________________ Activities 1. What causes all sound waves? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2. What is the difference between a compression and a rarefaction? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 3. What is the unit of frequency and what does it measure? ________________________________________________________________________ 4. What quality of sound does the frequency determine? 8 ________________________________________________________________________ ________________________________________________________________________ 5. Is a 'Mexican' wave, as seen among the crowds at some sporting events, a transverse wave or a, compression wave? _______________________________________________________________________ 6. What is the wavelength and amplitude of the transverse wave shown in the diagram Wavelength_____________________ Amplitude_____________________ Sound needs a medium to travel Because sound is transmitted as a compression wave, it can travel only through a medium that contains particles that can be forced closer together or further apart. Sound cannot be transmitted in a vacuum because there are no particles to push closer together or spread out. As sound travels through a medium, some of its energy is absorbed by the particles in the medium and is not transmitted to neighbouring particles. Sound travels more efficiently through materials that are elastic; that is, materials in which the particles tend to come back to their original positions without losing much energy. Speeding Sound The speed of sound in a particular medium depends on how close the particles are to each other and how easy they are to push closer together. In liquids and solids the speed is much greater because the particles are more closely bound together. The table below shows the speed of sound in some common substances at 0°. The speed of sound in air is greater at higher temperatures. At sea level in dry air at 0°C it is about 330 metres per second. At a temperature of 25°C it is about 350 metres per second. The speed of sound in air is lower at higher altitudes. At an altitude of 10 kilometres above sea level, it is about 310 metres per second. Substance Speed of sound (metres per second) 260 330 1300 1400 1500 4000-5000 4500-5500 5000 5000 about 6000 carbon dioxide (at 0°C) dry air (at 0°C) hydrogen (at 0°C) water sea water wood glass steel aluminium Before you can go on you need your teacher’s signature Check point 2 granite 9 SOUND IN DIFFERENT MEDIA Does the medium make a difference? Write down your hypothesis about where different mediums will make a difference. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Materials • • • • ticking watch metre ruler teaspoon (or spatula) cotton thread (or light string) Stop Before you can start an experiment you need to get your teacher’s signature, Beware you may be asked some questions so you will have to read it before starting Method • Place a ticking watch against your ear and listen to the tick. Have your partner slowly move the watch away from your ear until you can no longer hear the ticking. Measure the distance from your ear to this point. • Place a metre ruler gently against the same ear and rest the watch on it against the ear. Have your partner slowly slide the watch along the ruler to a point where you can no longer hear the ticking. Measure the distance from your ear to this point. • Tie about 80 cm of cotton thread to a teaspoon. Swing the teaspoon slowly so that it gently strikes the side of a bench, wall or cupboard. Listen to the sound made. • Place the free end of the cotton thread carefully against your ear and again gently strike the teaspoon against the same surface. Listen to the sound made. Discussion 1. What effect did the ruler have on the distance over which you could hear the sound of the ticking watch? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2. What difference does the cotton thread make to the sound heard when the spoon strikes a surface? 10 ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 3. Is sound conducted better through air or through solids? ________________________________________________________________________ 4. What property of the solids do you think makes the difference? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Echoes An echo is hearing a sound for a second time because the sound is reflected. Because sound takes a second to travel about 300 m, shouting loudly some distance in front of a cliff or large building will produce an echo seconds later. The echoes produced by a clap of thunder can often be heard many seconds after the first sound is heard. A small amount of echo can enrich the sound of someone singing or speaking. While, if there are too many echoes, the sound becomes fuzzy and confusing. When engineers design large concert halls they need to think very carefully about echoes. The walls need to absorb most of the sound that hits them but reflect just enough to give a pleasant enrichment to the music. Before you can start an experiment you need to get your teacher’s signature, Beware you may be asked some questions so you will have to read it before starting Stop Bouncing Sound Aim To find out how sound is reflected. clock Materials • 2 cardboard tubes • ticking watch or a clock • smooth wall table METHOD • Place the table along the wall. • Place the cardboard tubes and the watch as shown in the diagram on the right. • Place your ear at the end of the tube furthest from the watch and move the end facing the wall until you hear the watch ticking loudest. • Once you have this point, measure the angle of both tubes to the normal. • Change the angle of the tube and repeat the test. Results 11 Discussion 1. Compare the results of both tests. What did you notice about the angles to the normal when you could hear the watch ticking loudest? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2. Explain how the sound travels from the watch to your ear. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Sound and Technology Sound can be used for many purposes other than for talking and making music. Many animals have a sense of hearing that is much more sensitive than that of humans. Dogs are often used to guard people's homes because they can hear the slightest sound of an intruder. Bats use sound echoing off things around them to find their way while they are flying. Dolphins can use intense bursts of sound to stun fish they are hunting. People have built machines that use sound in surprising ways. For example, it is common for fishing fleets to use sonar equipment to find schools of fish. This is similar to the way that bats use sound. The sonar unit sends out a loud sound from a speaker under the water. The sound travels through the water and is reflected back by whatever it hits, just as sound echoes off a building or a cliff when you shout. (Movies about submarines often have 'ping' noises in the background—this is the sound of the sonar.) Scientists also use sonar to find fossils or oil under the ground. Instead of a loudspeaker they use an explosion to cause a loud noise that travels through the earth. Activities 1. Why are sound waves unable to travel through a vacuum? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 12 2. What properties of sound does sonar rely on? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 3. What is ultrasound and how is it useful ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 4. In general, how does the speed f of sound in solids compare with that in liquids and gases? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 5. Why do you think such a large range of speeds is given for wood and glass? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 6. Suggest a reason why the speed of sound in most woods is generally lower than the speed of sound in steel and aluminium? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 7. What do the data suggest about the effect of the density of a gas on the speed of sound? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Before you can go on you need your teacher’s signature Check point 3 13 Find each of the following words. ENERGY ABSORPTION FREQUENCY WAVELENGTH PITCH ELECTROMAGNETIC N E S N O I T A R B I V T A Y M A O M S E O T N M M S N L D R S O C M C I F A F A P T V A D A H H R R D I R C N E C T A N I R H VIBRATION REFRACTION DIFFRACTION LONGITUDINAL RHYTHM SOUND S I A T T E O C E C H C I F E N T R T E T F T I T I L U D F F O C W E N E R G Y C W T U U R A G E M S U U A E A O D F M T A C A L E E M I C O B M L R C I C T E E D A M I T S S P O E T G T I H C I N I F I M O R R Q M N I O E SPECTRUM VACUUM TRANSVERSE SLINKY REFLECTION RADIO T U E O G O N R E H U E O O N B R M A P R N A P S Y E C L N W E O H C T I P E T S T N M Y A M L M E R A R A N I I H C I V O F D A L A C S I I O O M Y E N D F N G N T S A R P N N N L R I R I S N B N O N O I T C E L F E R I N E I R U Y T R A N S V E R S E C RAREFACTION MEDIUM COMPRESSION DIFFRACTION T O R N T O I G A Y K N I L S R I F E D C E T D G O T N N C F E C N E N G H E I H U A F R S O E F U U L S U M C T R L I O B T T Special nature of light Light from the Sun travels through a vacuum before it reaches the Earth. The energy is not transmitted through a medium of particles. Instead, light consists of electric and magnetic fields, which vibrate at right angles to the direction of travel. It is therefore classified as a transverse wave. Light travels at 300 000 000 m/s through space. In the slinky spring experiment, all the transverse waves produced were vibrating in the one plane—the direction you moved your hand from side to side to produce them. Light sources emit light that is vibrating in all directions but when light is reflected from a surface, it sometimes vibrates in one direction only as shown in the diagram. The light is then said to be polarised. Polaroid sunglasses have lenses that only transmit light vibrating in one direction. They act like the slit shown in the diagram. What happens when one Polaroid lens is placed above another and rotated? Why? 14 Activities 1. Which waves, sound, light, or both: a b c d e f g h 1 j reflect? ____________________ refract? ____________________ travel through a vacuum? ____________________ are transverse? ____________________ are longitudinal? ____________________ travel at 300000000 m/s? ____________________ have compressions and rarefactions? ____________________ have crests and troughs? ____________________ can be absorbed? ____________________ need a medium to transmit them? ____________________ 2. P and S are two types of waves produced by earthquakes. P waves vibrate along the direction of the wave. S waves vibrate at right angles to the wave. Which kind is transverse, and which is longitudinal? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Light and wave properties When rays or beams of light hit smooth shiny surfaces they bounce off them. This is called reflection. The law of reflection says that the reflected rays always leave the mirror at the same angle that they hit. If we want to measure these angles, by convention we measure them from a line at right angles to the mirror. This is easier when the mirror is not straight. This line at right angles to the mirror is called the normal. The two angles have special names— the angle of incidence is the angle at which the rays hit the mirror and the angle of reflection is the angle at which the rays leave the mirror. Mirrors come in many different shapes and sizes. Flat mirrors are called plane mirrors and are the easiest to study. When you look into a mirror you see an image. Sometimes we are fooled by what we see. Look at some writing in a mirror and it looks the wrong way round. Hold your pen as if you are writing—do you appear right-handed or lefthanded in the image? The image in a plane mirror is exactly the same size as the object in front of it and is located exactly the same distance behind the mirror as the object is in front of it. Arrange a few objects in front of a mirror and look carefully at the image to check this. Bouncing Light Aim To show that the angles of incidence and reflection are equal. 15 Materials • plane mirror • power supply • sheet of paper • Plasticine Before you can start an experiment you • ruler • protractor and need to get your teacher’s signature, Beware pencil you may be asked some questions so you • ray box—single slit/multiple slits will have to read it before starting Method • Connect the ray box to a power supply. Place a single slit in the ray box so that a single beam of light is produced. • By attaching small pieces of Plasticine at each end of the mirror, fix it so that it stands vertically on the sheet of paper as shown in the diagram. Draw a fine pencil line along the reflecting edge (back) of the mirror. • Move the ray box until the beam of light can be seen to move across the paper, hit the mirror and reflect. • Make small dots with a pencil along the centre of the beam before and after it strikes the mirror. • Remove the ray box and mirror. Join up the dots with a ruler. At the point the rays hit the mirror, use a protractor to draw the normal. Measure the angles of incidence and reflection and record them in a table like the one shown. • Repeat for as many different angles of incidence as you can, giving each ray a number to distinguish between them. • Replace the single slit with the multiple one. On a separate sheet of paper trace three rays hitting the mirror. Make a similar table to record your results. Results Ray number 1 2 3 4 5 Angle of incidence Angle of reflection Stop Discussion What did you find about the angles of incidence and reflection? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Before you can go on you need your teacher’s signature Check point 4 16 With your knowledge of mirror try to complete the following diagrams An electromagnetic family Infra-red radiation is just one part of the family of waves that make up Visible Light the Red electromagnetic spectrum. Let's introduce the whole family, starting with the least Orange 'energetic' members and finishing with those that are the most 'energetic'. Yellow Green 17 Blue Indigo Violet (roygbiv) Radio waves include the low energy waves that are used to communicate over long distances through radio and television. They also include radar and microwaves. Infra-red radiation, invisible to the human eye, is emitted by all objects and is sensed as heat. The amount of infra-red radiation emitted by an object increases as its temperature increases. Visible light occours when oscillating atoms in the hot filament of a light globe emit a continuous spectrum of light: that is white light, which contains all the colours of the spectrum. Some elements emit coloured light when electrons in their atoms drop to lower energy levels. Light from the Sun contains a mixture of all the colours of the visible spectrum, but the light that is most useful in modern technology is the light obtained from lasers. Light can be detected by photographic film or by photocells, which contain substances that emit electrons when light energy falls on them. Like infra-red radiation, ultraviolet radiation is invisible to the human eye. It is needed by humans to help the body produce vitamin D; however, too much exposure to ultraviolet radiation causes sunburn. X-rays have enough energy to pass through human flesh. They can be used to kill cancer cells, find weaknesses in metals and analyse the structure of complex chemicals. X-rays are produced when fast-moving electrons give up their energy quickly. In X-ray machines this happens when the electrons strike a target made of tungsten. Gamma rays have even more energy than X-rays and can cause serious damage to living cells. They can also he used to kill cancer cells and find weaknesses in metals. Gamma rays are produced when energy is lost from the nucleus of an atom. This can happen during the radioactive decay of nuclei or as a result of nuclear reactions. It's natural Some electromagnetic radiation is emitted by all objects, including the sun. The higher energy waves, like ultraviolet radiation and X-rays are emitted naturally by stars. Our own sun emits ultraviolet radiation and X-rays. Gamma rays are emitted by radioactive substances and larger stars. All types of electromagnetic radiation can be produced artificially. Lasers Laser stands for 'light amplification by stimulated emission of radiation'. Lasers produce light of only one wavelength and therefore always of a single colour. This produces a high intensity beam of light that doesn't spread out a great deal— a property called coherence—as it moves away from its source. For example, a laser beam from the Earth directed towards the Moon 400000 km away spreads out only enough to light an area 10m wide on the Moon's surface. High intensity lasers are used to make finer cuts than is possible with a surgeon's scalpel. They can be used in industry in welding. In the recording industry, they are used for cutting and then reading compact discs. CD-ROM discs can store the equivalent of about 300 000 pages of text. Activities 1. Are electromagnetic waves transverse waves or longitudinal waves? ________________________________________________________________________ 2. List three properties that all electromagnetic waves have in common. 18 ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 3. List three differences between? sound waves and electromagnetic waves. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 4. The sound of a starting pistol can be heard easily by an observer one kilometre away. From that distance, the sound of the pistol is heard some time after the smoke is seen. Calculate to the nearest second: (a) how long it takes sound reach the observer? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ (b) how long it takes for the light scattered from the smoke to reach the observer? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 5. The speed of sound in air at a" temperature of 25°C is about 350 m/s. (a) How long would it take fo sound waves to travel from Sydney to Melbourne, a straight distance of about 700 km when the air temperature is 25°C? ________________________________________________________________________ ________________________________________________________________________ (b) Why doesn't it take this long for the sound to reach you by telephone, television or radio? ________________________________________________________________________ The Eye The human eye is roughly spherical in shape with a diameter of about 2.5 cm. Each is set deeply within the skull to protect it from external damage. When we say someone has brown or blue eyes we are referring to the colour of the iris, a set of delicate muscles that controls the amount of light entering the eye. The 'black' pupil is just a hole. 19 You may notice that it is difficult to judge the depth of water in a pool because of a special property of light called refraction. Light rays bend (or refract) when they pass into a medium of different density. For this reason, water appears more shallow than it really is. A similar situation exists with our eyes. Light is focused in our eyes because of refraction. As light moves from the air into our eye, it enters material of different density which causes it to bend. Light enters the eye through a transparent curved window at the front, called the cornea. Light rays are refracted strongly at the surface of the cornea. As the rays continue to pass through the eye they are refracted a little more by a convex lens until they form an upside-down image on the light-sensitive back surface known as the retina. Here there are millions of tiny cells which respond to light energy by sending tiny electrical signals to the brain via the optic nerve. Activities 1. Where does most of the refraction take place in the eye? ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 2. Explain why it is an important adaptation that the pupil of your eye grows very large under conditions of low light. ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ 3. Explain why the dot disappears when you perform the blind-spot experiment (see Activity). ________________________________________________________________________ ________________________________________________________________________ ________________________________________________________________________ Find your blind spot: Hold this page at arm's length. Close your left eye and stare at the cross on the left. Move the page slowly towards your face and at some stage you will be aware that the dot has disappeared. The image of the dot is falling on your blind spot! 20 Find each of the following words. CORNEA SPECTRUM ELECTROMAGNETIC PITCH CONVEX MEDIUM C R N U I O X N E L N L N N A E T L O O R R P E T I T N A I A T L S I V M C N E V L A C U O I I D T T P R S T E V N E E F I A C DIFFRACTION LONGITUDINAL CONCAVE LENS WAVELENGTH TRANSVERSE M E C A U R R V E A O N A O N E R E A L S P A R L S C C S S T E E E R N I H N E E F C N O E C A F N F P C L S F C E R R O C C P L V F T P O V R T O E E C C X N E L I E F N E A R M E D I U M E C P D S R G R C O U C C E T M V TRANSPARENT REFLECTION VACUUM FREQUENCY PUPIL DIFFRACTION T C T E E I S T M I I P V W I L I C A R Q T E I A N N Y A A I F O M D N U U N O G C G T C V S E N F T I E D I N N R A P U E P T S I R I N I T A E N M T U L E I R T C N C N D N T S A L M E C N F U V S Y A E E I C W R C N T I REFRACTION IRIS ENERGY G I I T P L T O C T O A I G R R N O I T C A R F F I D F M T U A N T N E R A P S N A R T M H M O V R T O C A M A E N R O C N I L M G E I E C C N M N F F Y R E F Skill Tester 1 Describe the differences between each of the pairs of waveforms below in terms of wavelength, frequency, amplitude and period. a b a ______________________________________________________________________ ______________________________________________________________________ b ______________________________________________________________________ ______________________________________________________________________ 2 Use the labels on the following diagrams to describe the diagrams. a 21 ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ ____________________________________________________________________________ 6a State the approximate wavelength of each of the components of the electromagnetic spectrum. gamma rays ________________ infrared radiation________________ radio waves ________________ visible light _________________ X-rays ______________ microwaves _________________ ultraviolet radiation ________________ b Complete the following diagram of the electromagnetic spectrum by marking the approximate positions of the components listed in parts a. Rule a line to match each statement with the correct term. (Start the line from the dots.) The letter through which the line passes gives the answer to the question at the bottom of the page.  1 The distance between the peak of one wave and the peak of the next.  2 As the wavelength of radiation decreases, the frequency and • • infrared O R translucent 22 A T __________ increase.  3 The type of radiation also known as heat.  4 The rays produced as a result of nuclear reactions.  5 A term used to describe an object that produces light.  6 Many light rays travelling together.  7 A term used for a substance through which light passes but you can’t see clearly through it.  8 What convex mirrors cause rays of light to do.  9 When vibrating particles pass heat through a solid object. 10 Heat passing through space. 11 The difference in frequency of sounds can be heard by the different _________. • gamma • wavelength R • • • energy E N beam conduction • • • • I F A pitch luminous radiation diverge C Sound travels through air as longitudinal waves. The area where the air particles are close together is called a compression and where they are further apart is a called a 3 _____ 10 _____ 7 _____ 6 _____ 11 _____ 4 _____ 8 _____ 2 _____ 9 _____ 1 _____ 5 _____ 23 Glossary Amplitude: The height of any wave, be it sound, water or electromagnetic. Compressions: Areas of high pressure where springs or particles are close together. Cornea: the clear window on the front of the eye. Almost 90% of the bending takes place as light first enters the eye! Crest: The top of a wave. Diffraction: Waves spread out after passing through a gap. Electromagnetic waves: These are electric and magnetic waves and do not need particles to pass the energy on. Frequency: The number of waves produced per second. Hertz (Hz): The unit used to measure frequency. Interference: When one wave meets another. Iris: This coloured muscular tissue contracts and relaxes according to how much light is present Lens: a clear elastic piece of tissue - it puts the 'finishing touches' to the image by changing its shape slightly so that rays from all objects are brought to a clear image. This is called accommodation. When you look at something very close you can feel the lens being squashed! Longitudinal waves: The particles oscillate along the direction in which the wave is travelling. Mechanical waves: These waves disturb particles — the particles pass the energy on. Opaque: Material that light cannot travel through, e.g. wood. Optic nerve: passes all the information from the light-sensitive cells in the retina to the brain. Where the optic nerve joins the retina there is a 'blind spot'. Pupil: The hole in the centre. Is large when there is little light and small when there is plenty Rarefactions: Areas of low pressure where the springs are stretched out. Reflection: Waves bounce back when they hit a solid object. Refraction: Waves change speed and direction if they pass through another substance. Retina: coats the back, inside surface of the eye. It is covered with millions of cells of two types, rods and cones, so called because of their shape Translucent: Material that lets some light through but also scatters it. Transparent: Description used of material through which light can travel, e.g. glass. Transverse waves: The particles move at right angles to the wave. Trough: The bottom of a wave. Wavelength: The distance from one crest or one trough to the next Sound waves compression (longitudinal) waves travel through all solids, liquids and gases, but are unable to travel through a vacuum speed in air between about 330 m/s and 350 m/s, depending on temperature Electromagnetic waves transverse waves unable to travel through some substances but travel through a vacuum speed in air about 300000000 m/s 24