KS3 Physics: 2. Sound & Light
What is Light?
Light is all around us. It allows us to see. It carries warmth from the Sun. It provides the energy without which life on Earth would not be possible. (We will find out more about this last point when we study ecology later on this year. Ecology is the part of biology which deals with how living things interact with each other and their environment.) Light is everywhere, but what is it? This is a question that puzzled humans for centuries. The ancient Greeks thought that a fire shone out of human being’s eyes, making sight possible. We now know that we see things when light enters our eyes, and that light must bounce off objects on the way. But that still doesn’t tell us what light is.
Sir Isaac Newton, one of the most famous scientists who ever lived, believed light behaved as if it was made up of tiny particles, called corpuscles. His theory explained the results of his experiments, which showed how light split up and changed direction when shone through a glass prism (this is called refraction. We will learn more about these experiments later in this unit). Because Newton had made such important discoveries about matter, motion, and the laws that governed them, many people believed his corpuscular theory, i.e. that light behaves as if it is made of tiny particles, must be true.
Some, however, disagreed. A Dutch scientist, Christiaan Huygens, working around the same time as Newton, suggested that light actually behaved as if it was made up of tiny waves, like ripples in a pond. The wave theory of light explained everything that Newton’s theory did, as well as why light diffracts, i.e. spreads out when it goes through a gap (we shall learn more about this later too). The problem with Huygens’ theory, as we shall see, was that it also suggested the whole universe must be filled with an invisible substance called the aether, otherwise the ‘light waves’ would have nothing to travel through. Many scientists could not believe such a substance existed, so the argument remained unsettled. Was light made up of particles or waves? This question was not satisfactorily answered until more than 200 years later, by Albert Einstein and the theories of quantum physics. That is where our story is heading, but first we must look in more detail at the nature of sound and light.
What is a Wave?
To have any hope of understanding sound and light, we must first think in more detail about waves. Try to imagine a wave. You are probably thinking of the kind of waves you see in water: on the beach, in a boat, or in the bathtub. These examples are helpful illustrations of two important features of waves. Firstly, they are created by a disturbance. This word describes some sort of movement that transfers energy. Waves on the beach are caused by disturbances far out to sea: particles in the wind transfer kinetic energy to particles in the sea, causing the sea to move up and down. (This is why you are unlikely to see waves on a calm, still day.) Likewise, if you drop something in a bathtub, the object will transfer kinetic energy to particles in the water and they will move up and down, creating ripples. These are single, simple disturbances, but waves can also be created by regular movements. Imagine moving your hand up and down in water in a regular, repeating pattern. This would create regular, repeated waves. A regular, repeating up and down motion is called an oscillation; it could also be left and right, or backwards and forwards. Oscillations can be thought of as continuous disturbances, i.e. disturbances that transfer energy without stopping.
The second important feature of waves is that they require a material to travel through. We call this material a medium. In the examples we have looked at so far, the medium is water. An initial disturbance (e.g. dropping an object in the bath) causes energy to be transferred by particles moving up and down (or oscillating) in the water. Without a medium, it is impossible for energy to be transferred. Imagine you are an astronaut out in space. Space is a vacuum: there are no particles. You can move your hand up and down in a regular, repeating pattern, but energy would not be transferred because there is nothing to be disturbed. As we shall discover later, this is why, in space, no one can hear you scream.
Before we move on to look at examples of waves in more detail, we must address an important point. So far we have stated that:
- Waves are caused by disturbances, i.e. movements that transfer energy.
- Waves require a medium, i.e. a substance through which energy can be transferred by oscillations.
The waves we are talking about here are properly described as mechanical waves. These are waves that transfer energy mechanically, which is one of four ways energy can be transferred. (We learned about the four ways energy can be transferred in the Energy and Thermal Physics unit.) In the time of Newton and Huygens, mechanical waves were the only ones humans knew about. When Huygens suggested that light behaves as if it is made up of tiny waves, he was therefore referring to mechanical waves.
Much later, James Clerk Maxwell (who we also came across in the last unit) suggested that light behaves as if it is made up of a new type of wave — electromagnetic waves. These waves transfer energy by radiation, and do not require a medium to travel through. This insight was crucial to understanding the question: is light a particle or a wave?, but we will not explore it in detail until much later. For now, we will assume that both sound and light are mechanical waves, which are caused by disturbances and require a medium to travel through. It is important to understand how scientists like Newton, Huygens and Maxwell thought in order to understand their theories.
Sound Waves
How do we hear things? Why is it that when I move my mouth in a certain way, you can hear me talk? A clue comes if you hold your hand in front of your mouth and make an ‘ahhhhh’ sound. You should feel your breath on the palm of your hand. Somehow, when you make that sound, air begins to move.
Inside your throat, there is a part of your body called the larynx, or voice box. Make that ‘ahhhhh’ sound again, but this time put your fingers just above your adam’s apple. You should be able to feel some very slight vibrations. When you make the sound, your larynx moves back and forth: it oscillates (the words ‘oscillate’ and ‘vibrate’ are often used interchangeably). As we saw earlier, this creates a disturbance in the air inside your windpipe. Energy is transferred from your oscillating larynx to particles in the air. These particles collide with each other, causing the movement in air that you felt when you held your hand in front of your mouth. Like a ripple in a pond, the energy is transferred away from the initial disturbance. A sound wave has been formed: caused by the disturbance made by your larynx and travelling through the medium of air.
All sound waves are made and heard in this way: an initial disturbance causes air particles to move, and as the air particles bump into each other the sound wave spreads out (or propagates). It is worth noticing one important difference between a sound wave and a water wave. Imagine a water wave, e.g. a ripple caused by dropping an object in the bath. We noted earlier that the ripple spreads outwards as the water particles move up and down. This kind of wave is called a transverse wave. The particles move up and down but energy is transferred horizontally, across the surface of the water. See the diagram below for an illustration.
Sound waves work differently. Think of a speaker cone moving back and forth, pumping out sound. As the speaker cone oscillates, it collides with the air particles, causing a disturbance. They go on to collide with other air particles, and energy is transferred away from the speaker through the air. In this case, both the speaker cone and the air particles move in the same direction: back and forth. This kind of wave is called a longitudinal wave. The particles move back and forth in the same direction as energy is transferred. See the diagram below for an illustration.
Properties of Sound Waves
We have seen that sound waves are both mechanical and longitudinal. They are caused by disturbances that transfer energy through a medium. The particles oscillate back and forth in the same direction as energy is transferred. We can use this picture to explain differences between sounds. Why are some sounds louder than others? Why are some more high-pitched? As you might expect, louder sounds transfer more energy. They are caused by larger disturbances to the medium. If you clap very gently, you disturb the air particles around your hands only a little, meaning not much energy is transferred. This is a quiet sound. If you clap hard, the air particles around your hand are disturbed a lot, and lots of energy is transferred. This is a loud sound. We call the size of the disturbance to a medium the amplitude of the wave. Large amplitude sound waves are loud; small amplitude sound waves are quiet.
The loudness of a sound depends on the size of the amplitude, whereas the pitch of a sound depends on the frequency of the oscillations. Frequency is a very important word. We came across it in the Energy & Thermal Physics unit. There we saw that more frequent collisions between the particles and the walls of the container caused a greater outwards force, and therefore a higher pressure. By more frequent, we meant more collisions every second. This helps us understand what we mean by the frequency of oscillations: the higher the frequency of a sound wave, the greater the number of oscillations every second. You can see this when you look at a speaker. Most large speakers have two cones, one for high pitched sounds (the tweeter), and one for low pitched sounds (the woofer). The tweeter moves back and forth so fast you can’t really see it move: it oscillates many, many times every second. For this reason, it produces high frequency sounds. These also have a high pitch. The woofer moves back and forth much more slowly: you can see it vibrating as it gives out the lower pitched, bass notes. These are known as low frequency sounds.
We can represent the oscillations of particles in a medium using a wave trace. This shows how the displacement of a particle varies over time. If an object is displaced, it means it is moved from its original position. A disturbance causes particles in a medium to be displaced; a wave trace shows by how much and how frequently this happens. The height of the wave trace, measured from the zero line (each particle’s original position), tells us the wave’s amplitude. The amplitude can be thought of as the maximum displacement of each particle. As we saw above, sound waves with a large amplitude are loud. This means loud sounds are represented by tall wave traces, whereas quiet sounds are represented by short wave traces.
The top of each repeated part of the wave trace is called a peak, the bottom of each repeated part is called a trough. By measuring the distance between two peaks, two troughs, or indeed any two repeated parts of a wave, we can measure the time period. This is the time it takes for a particle to complete one oscillation, i.e. to move everywhere it’s going to go and back to where it started. The wider the gaps between the peaks and troughs, the longer the time period. This means oscillations are less frequent. Conversely, the narrower the gaps between peaks and troughs, the shorter the time period. This means oscillations are more frequent. We know from above that the frequency of oscillations determines the pitch of the sound. This means low pitched sounds are represented by wide wave traces whereas high pitched sounds are represented by ‘squashed up’ wave traces.
Sound Wave Effects
In this unit, we will look at three main wave effects: reflection, refraction and diffraction. Reflection happens when waves bounce off a surface. Refraction happens when waves move from one medium to another, slow down, and change direction. Diffraction happens when waves spread out after reaching an object or moving through a gap. For sound waves we will focus on two of these effects: reflection and diffraction.
The reflection of sound waves is called an echo. Imagine you are standing near a giant wall or a cliff (a large face of exposed rock, often found near beaches). You shout. After a moment, you are likely to hear a faint echo of that shout. How did this happen? As we saw earlier, when you shouted, your larynx produced a disturbance in the air which caused collisions between air particles that transferred energy away from your mouth. These disturbances to the air travelled all the way to the large wall. When the particles next to the wall were disturbed, however, they had no more air particles to collide with. Instead, they bounced into the wall and back again. The wall did not move. Then the chain of events began all over again. The particles that had bounced off the wall bounced back into the air particles next to them, and the disturbance rippled back towards you. Quite a lot of energy had been lost to the surroundings by the time the sound wave reached your ears, meaning it had a much lower amplitude. This is why it sounded quiet and faint.
We can use echoes to calculate the speed at which sound travels. If we measure how far away we are from the wall, and time how long it takes for us to hear the echo after we shout, we can work out how many metres the sound wave travelled every second. This is its speed. (We must remember that the sound wave has travelled to the wall and back: therefore if we are standing 100m away from the wall, the sound wave has travelled 200m by the time the echo reaches us.) This method can also be used in reverse. If we know how fast sound waves travel in a medium, we can time how long they take to return from an object to work out how far away from us it is. This is how radar systems work on ships and submarines, and how we can generate images of babies using ultrasound scanners.
Besides reflecting off surfaces to form echoes, sound waves also spread out when they go through a gap or around an obstacle. This is called diffraction, and is something all waves do. Huygens showed us the easiest way to think about diffraction. When a particle in a medium is disturbed, the energy spreads out in a circle from that point. As energy is transferred outwards like this, any particles a certain distance away from that point (e.g. 1 metre) are likely to be disturbed at the same time. We say that a group of particles all disturbed at the same time is called a wave front. If a wave front is blocked somehow, either by reaching a gap or an obstacle, the particles that are disturbed next to the gap or obstacle form new disturbances that spread out in a circle. Thus, straight wave fronts can become circular wave fronts when they reach the gap or obstacle. This is why we can hear someone speaking, even if they are around the corner and we can’t see them. The reason we can hear them but not see them is because, for diffraction to occur, the distance between the wave fronts, which we call the wavelength, must be similar to the size of the gap / obstacle. Sound waves have quite a long wavelength, whereas light waves (as we shall see) have a very, very short wavelength, so don’t spread out much if the gap is quite large.
Reflection & Refraction of Light
Before we begin to look at how light behaves, we must learn how we can represent light on a diagram. When light travels in a straight line, we call it a ray. We can draw rays of light using a pencil and a ruler. Each ray is represented with a straight line, with an arrow showing the direction in which it travels. Light rays are emitted by luminous objects, i.e. objects that give out their own light. The Sun is a luminous object, as is a light bulb, a mobile phone screen, or a candle. If we see an object, it is because light rays travel into our eyes. We can show this process by drawing light rays being emitted (given out) by luminous objects, reflecting from non-luminous objects, and entering our eyes. To do this correctly, however, we must make sure we follow certain rules.
When light reflects off an object or surface, the direction it travels in afterwards is not random. We can investigate the direction reflected light travels using a ray box, a mirror, and a protractor. If we shine light at the mirror from different angles to the normal, a straight line at a right angle to the mirror, then measure the angle between the normal and the reflected ray, our results should confirm the following relationship: the angle at which light hits a surface is the same as the angle at which it bounces off. We call the angle at which light hits a surface the angle of incidence. This is measured between the incoming ray and the normal. We call the angle at which light bounces off the angle of reflection. This is measured between the reflected ray and the normal. When we draw rays of light reflecting off surfaces, the law of reflection must always be obeyed: the angle of incidence is equal to the angle of reflection.
We can use a similar method to investigate refraction. As we stated earlier, refraction happens when light moves from one medium to another and changes direction. If we shine light from a ray box towards a rectangular glass block at different angles, we can see it change direction. We can again make measurements using the normal, a straight line at a right angle to the glass block. If we measure the angle between the normal and the ray of light going into the glass block (the angle of incidence again), then take the glass block away and measure the angle between the normal and the ray of light inside the glass block (the angle of refraction), we see that the angle of refraction is always smaller than the angle of incidence. This is often stated more simply: when light goes from air into glass, it bends towards the normal. The opposite effect occurs when light exits the glass and goes back into air: it bends away from the normal.
Glass can be shaped in a way that refracts all incoming light to the same point. An object that does this is called a converging lens. The shape of a converging lens — fatter in the middle than at the outer edges — is convex. Any rays of light entering a convex lens parallel to the normal will be refracted to the focal point. The distance between the focal point and the lens is called the focal length. The shorter the focal length, the more powerful the lens. This is because light must be refracted more if the focal point is closer to the lens. We measure the power of a lens in dioptres.
We can draw ray diagrams to show how convex or converging lenses produce an image. Depending on how far away the object is from the lens, these images have different properties:
- Real images are those in which light rays actually meet on the other side of the lens to the object. Real images can be captured on a screen. This is how cameras and projectors work.
- Virtual images are those in which the light rays don’t actually meet. They only appear to meet if you look through the lens at the object on the other side. Virtual images therefore appear on the same side of the lens as objects. This is how magnifying glasses work.
- Inverted images are upside down compared to the object. Real images are often inverted.
- Upright images are the same way up as the object. Virtual images are often upright.
- Magnified images are larger than the original object. We can calculate the magnification of a lens by dividing the height of the image by the height of the object. If the magnification of a lens is more than +1, the image will be magnified.
- Diminished images are smaller than the original object. If the magnification of a lens is between 0 and 1, the object will be diminished.
The opposite of a converging lens is a diverging lens. This is fatter at the outer edges than in the middle, so is also called a concave lens. Instead of focusing incoming parallel rays of light at one point on the other side of the lens, a concave lens causes incoming light rays to spread out. Concave lenses have an imaginary focal point on the same side of the lens as the incoming rays. The distance between this point and the lens is again called the focal length, and ray diagrams can be drawn in a similar way to with convex lenses.
Our eyes use refraction and lenses in order to see things clearly. The human eye is a little bit like a camera: it has a lens at the front, shielded by a protective layer called the cornea. At the back of the eye is a screen called the retina. For an object to be observed clearly, light must be focused on the retina. This means the light rays must meet on the retina. The lens and cornea refract incoming light by different amounts, depending on the angle at which light enters the eye. If we are looking at an object a long way away, the rays of light entering the eye are almost parallel. This means the cornea and lens do not need to refract them much to focus these rays on the retina. If we are looking at a nearby object, on the other hand, the rays of light need to be refracted a lot if they are to be focused on the screen at the back of the eye.
Some people do not have perfect vision, because their lens and cornea cannot always refract light from incoming objects by the right amount to focus the rays on the retina. The eyes of short sighted people refract light too much; their lens and cornea are too powerful. This means that distant objects appear blurry to them. The eyes of long sighted people do not refract light enough; their lens and cornea are not powerful enough. This means that nearby objects appear blurry to them. (Young people are much more likely to be short sighted, whereas older people often become long sighted). Short sightedness can be corrected with a diverging lens, as this spreads out the rays of light before they reach the eye. In effect, this means the lens has more refraction to do in order to focus the light, which is good for a lens that is too powerful. Long sightedness can be corrected with a converging lens, which brings light rays together before they reach the eye. In effect, this means the lens doesn’t have as much refraction to do in order to focus the light, which is good for a lens that isn’t quite powerful enough. If you were to look at the glasses of a short sighted person compared to those of a long sighted person, you would see the short sighted person would have concave lenses in their glasses, whereas a long sighted person would have convex lenses.
White Light & Colour
Refraction can be used to explain one of nature’s most beautiful phenomena: the rainbow. We can see refraction happen if we shine white light from a ray box at a glass prism (a triangle extended into three dimensions). The seven colours of the rainbow are produced (red, orange, yellow, green, blue, indigo, violet). We call these colours the visible light spectrum. In Newton’s time, scientists thought the colours of the spectrum were observed because white light reacted with impurities in the glass. Ingeniously, Newton showed this was not the case: by directing the colours produced by a prism towards a second prism, he demonstrated that the colours came back together again to produce a ray of white light. This changed our understanding of light forever. White light wasn’t a colour in itself: it was formed by the combination of the colours of the spectrum. Rainbows happen in the sky because white light from the Sun is refracted through raindrops, just like through a prism. This is why you only see them when it’s both sunny and rainy at the same time.
Once scientists understood that white light was made up of different colours, it became clear why certain objects appear to us in different colours. Take a tomato. Why does it appear red? The answer to this question lies in the idea of reflection, transmission, absorption and emission:
- When light is reflected by an object, it means it bounces off its surface (as we have seen).
- When light is transmitted by an object, it means it passes straight through.
- When light is absorbed by an object, it means it is taken in.
- When light is emitted by an object, it means it is given out.
So why does a tomato appear red? White light is emitted by a luminous object. It travels to the tomato. When it arrives, all the colours of the spectrum except red are absorbed by particles in the surface of the tomato. Only the red light is reflected and reaches our eyes, hence our eyes sense the tomato as if it only emits red light. It thus appears red.
We can use this principle to make colour filters. These are thin, coloured strips of a transparent (i.e. see through) material. Filters can make objects appear to be different colours: the effects we can put on photographs using apps like instagram are based on this idea. Filters work by absorbing some colours of light while transmitting others. Take a green filter: it absorbs every colour of the spectrum except green, which is transmitted. If you look at a white object (e.g. a fridge) through a green filter, only the green light passes through, so it appears green. What colour would you expect a tomato to look if you looked at it through a green filter? Tomato only reflects red light, but we know this is absorbed by the green filter. This means no light would pass through the filter. The tomato would therefore appear black. It is worth remembering these important points:
- Objects appear white when all the colours of the spectrum reach our eye.
- Objects appear black when none of the colours of the spectrum reaches our eye.
Diffraction of Light
We have seen a number of effects of light so far: reflection, refraction, colour and filters. Newton believed he could explain all of these in terms of his corpuscular theory, i.e. that light behaves as if it is made up of tiny particles. One effect the particle theory of light could not explain, however, was diffraction. We saw diffraction earlier. The reason we can hear people around a corner is that sound waves spread out when they pass through a gap or around an obstacle. This was explained by Huygens’ theory that each disturbance of particles within a medium causes energy to be transferred outwards in a circle from that point. As we have seen, Huygens believed that light was also a wave, and so would behave in this way. Many people did not believe him though, because his explanation relied upon the existence of a medium for light waves to travel through, which he called the aether. Newton had shown that light diffracted — he created a pattern known as Newton’s Rings — but he thought it could be explained by the behaviour of particles, not waves.
In the late 1700s, Thomas Young carried out a famous series of experiments showing what happened when light passed through two very narrow gaps (called slits). If white light was made up of particles, you would expect two bright, white lines to be formed on the screen behind. Instead, Young observed a repeating pattern of alternating bright and dark lines (called fringes — see below). He realised that this was very similar to the kind of pattern you would see if water was disturbed at two points next to each other. As the disturbances spread out from the two points, at some points the disturbances would add together to create a large disturbance in the water (this is called constructive interference). At other points, the disturbance from one point would cause water particles to move upwards, whereas the disturbance from the other point would cause the water particles to move downwards. The disturbances would therefore cancel each other out, and the water would appear to be still (this is called destructive interference). Young argued that the results of his experiments provided conclusive evidence that light did indeed behave as if it was made up of tiny waves. Instead of water particles moving up and down, he claimed that energy transferred by light caused oscillations in the aether.
Not only did the wave theory of light perfectly explain the results of Young’s own experiment; he also used it to offer an improved explanation of Newton’s results. If light behaved as if it was a wave travelling through a medium, then it would refract upon entering a prism (therefore splitting into the colours of the spectrum), and diffract after passing through a gap (therefore forming Newton’s Rings). Many applauded Young’s achievements, but many still could not believe that the whole universe was filled with an invisible substance called the aether. Incredibly detailed and precise experiments were set up to try and find the mysterious aether, but to no avail. It took two of the greatest physicists who ever lived — Maxwell and Einstein — to show what light really was.
Electromagnetic Radiation
Maxwell was a pioneer in the field of electromagnetism. He worked out the laws that show how electricity and magnetism are linked: we will learn about these effects in Year 8. From his calculations, he stumbled upon an incredible fact. If you knew the electric and magnetic properties of a material, they would tell you exactly how fast light travels in that material. Maxwell’s work led scientists to understand that light is in fact a new kind of wave: an electromagnetic wave. Later on, the work of Einstein and the field of quantum physics showed us that electromagnetic waves have nothing to do with the oscillations of particles. The initial disturbance that causes energy to be transferred actually takes place inside atoms (more on this below). Energy is not transferred mechanically by particles bumping into each other, but by radiation, which means it simply spreads out in all directions from a point. For this reason, electromagnetic radiation can travel through a vacuum (where there are no particles), and it does not need an aether to exist.
The light we see — visible light — is one of seven types of electromagnetic radiation classified in the electromagnetic spectrum. Each type of light is produced by energy changes that take place inside atoms: the higher the energy change, the higher the energy of the radiation produced. The highest energy radiation is gamma radiation, which can be used to treat cancers but can also cause cancer if not used safely. Next are X rays, which are used for medical imaging and in airport security. Next is ultraviolet radiation, which causes our skin to darken when it is absorbed and can be used to see ‘invisible ink’. Next is visible light, with violet the highest energy light and red the lowest energy. Next is infrared radiation, which heats things up when it is absorbed and is also used in remote controls. (This is the third main method of heat transfer. We studied the first two, conduction and convection, earlier this year). Next are microwaves, which are used to transmit mobile phone signals as well in ovens. Finally, the lowest energy radiation on the electromagnetic spectrum is that of radio waves, which are used to transmit radio signals as well as — confusingly — TV broadcasts.
High energy radiation like gamma and X rays is said to have a very high frequency. As we saw earlier, this means there are very many oscillations every second. For example, a gamma ray causes 1019 oscillations every second, which is ten billion billions. That is a lot of movement, and consequently a lot of energy. This means, (as we shall find out in Year 9), that gamma radiation and X rays also have a very short wavelength. We also came across this term earlier: it can be thought of as the distance between the wave fronts as a disturbance spreads out from a point. For example, the wavelength of X rays is similar to the size of atoms, around 0.1 nanometres, or 10–10 metres. This is equivalent to the size of the gap if 1 millimetre (the smallest gap on your ruler) was divided into 10 million equal pieces. The wavelength of visible light is between 400 and 700 nanometres, while radio waves can have wavelengths of 1 metre or more. These different wavelengths and frequencies all depend on the energy changes that take place inside atoms, but once electromagnetic radiation has been emitted by an atom, the waves always travel at the same speed. This speed is 300,000 kilometers per second. As Einstein would show in the early 20th century, nothing in the universe can travel faster.
In 1905, more than 200 years after Newton’s investigations into light, Einstein also solved the problem of whether light is a particle or a wave. He had been studying a phenomenon called the photoelectric effect, in which tiny particles called electrons could be emitted from a surface if ultraviolet light was shone at it (we will learn more about electrons in Year 8 and Year 9). At this time, many scientists had accepted Huygens’ theory that light was a continuous wave. If this was true, then the photoelectric effect should not happen if the light was very dim, because the dim light would not transfer enough energy to release the electrons. Likewise, the brighter the light, the more kinetic energy they expected the electrons to have once emitted. When scientists checked these deductions, however, they found that for light above a certain frequency, the photoelectric effect happened regardless of how bright the light was, but below that frequency, it did not happen at all. Similarly, they found that the brightness of the light did not affect the kinetic energy of the emitted electrons; again, it was the frequency that mattered.
From these observations, Einstein came up with the guess that is still considered true to this day: namely, that light behaves as if it is both a particle and a wave. He suggested light travelled in photons, tiny packets of energy that oscillate like waves. These photons are emitted when electrons inside atoms jump vertically upwards then fall back down. This jump is at a right angle to the direction in which the wave travels, meaning light is a transverse wave, and it is electromagnetic because it is caused by electrons. If you think all this is confusing, you’re not alone. Einstein’s idea, that light behaves as if it is both a particle and a wave, is called wave-particle duality. It is one of the most surprising and controversial ideas in physics, and forms the basis of quantum physics, which we will study in more detail in Year 11. Until then, it is fine to think of light as a wave: as we have seen in this unit, Huygens’ theory explains most behaviour of waves perfectly.
