Key Stage 3 science · Physics – Waves

KS3 light waves: speed, absorption and reflection explained

In Key Stage 3, light is treated as a wave that carries energy, and pupils compare it carefully with the water and sound waves they have already met. They learn that light can cross empty space at an enormous speed, and that when it meets a material it can be absorbed, scattered or reflected, which explains everything from why we can see a wall from any angle to why a wet road glares.

Light waves and reflection, explained

Light, water waves and sound waves have a lot in common. All of them carry energy from one place to another, all can be reflected, and all can be described by their frequency, wavelength and amplitude. Pupils are asked to say what is similar and what is different. Light is a transverse wave, like ripples on water, whereas sound is longitudinal. The biggest difference is that water and sound waves need a material to travel through, called a medium, but light does not. Light is one of a family of waves, called electromagnetic waves, that can travel through empty space.

That is how sunlight reaches us across about 150 million kilometres of nearly empty space. In a vacuum, light travels at about 300,000,000 metres per second, or 300,000 kilometres per second. That is roughly 900,000 times faster than sound in air, and nothing travels faster. Light slows down in materials such as glass and water. Because it is so fast, everyday light seems instant, and lightning is seen noticeably before its thunder is heard. Pupils use the speed of light with the equation time = distance ÷ speed to compare cosmic distances.

When light meets a material, one or more of three things can happen. It can be transmitted, passing through, as it does in clear glass, water and air, which are transparent. It can be absorbed, which means its energy is taken in by the material, usually warming it. Or it can be reflected, bouncing off the surface. Most real materials do a mixture. Translucent materials such as frosted glass let light through but scatter it so you cannot see clearly, and opaque materials let none through. A dark, matt surface absorbs more of the light that falls on it, which is why a black car seat gets hot in the sun.

Reflection comes in two kinds. Specular reflection happens at a smooth, flat surface such as a mirror, still water or polished metal. Parallel rays of light all hit at the same angle and bounce off at the same angle, so they stay parallel and an image forms. At any reflection, the angle of incidence equals the angle of reflection, both measured from the normal, an imaginary line at right angles to the surface. Diffuse scattering happens at a rough surface such as paper, cloth or a wall. Each ray still obeys the same rule, but the surface is uneven at a tiny scale, so the rays leave in all directions and no clear image forms.

This matters in daily life. Almost everything we see, from a page to a face, is visible because of diffuse reflection, which sends light to our eyes from whichever direction we look. A wet road glares because water fills the roughness and makes the surface smoother, so more light reflects specularly towards a driver’s eyes. Pupils practise drawing ray diagrams with straight lines, arrows and a normal, and learn to explain surfaces using exactly these terms.

Words your child will hear

Vacuum: A space with no particles in it. Light can travel through a vacuum but sound cannot.
Absorption: When a material takes in the energy of light, usually making it warmer.
Specular reflection: Reflection from a smooth surface, where parallel rays stay parallel and form a clear image.
Diffuse scattering: Reflection from a rough surface, where rays are sent off in many directions.
Normal: A line drawn at right angles to a surface where a ray hits it. Angles are measured from it.
Transparent, translucent, opaque: Letting light pass clearly through, letting some through but scattered, and letting none through.
Speed of light: About 300,000,000 m/s in a vacuum, the fastest speed possible.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the similarities and differences between light waves and waves in matter
Pupils should be taught about light waves travelling through a vacuum; speed of light
Pupils should be taught about the transmission of light through materials: absorption, diffuse scattering and specular reflection at a surface

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

The Sun is about 150,000,000 km from Earth. Light travels at about 300,000 km/s. How long does sunlight take to reach us?

Choose the equation: time = distance ÷ speed. Check the units match. The distance is in kilometres and the speed is in kilometres per second, so the time will come out in seconds.

Put the numbers in and calculate.

time = 150,000,000 ÷ 300,000
     = 500 s

Convert to minutes and seconds. There are 60 seconds in a minute, and 500 = 8 × 60 + 20.
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500 s = 8 min 20 s
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Make sense of it. The Sun we see is the Sun as it was just over eight minutes ago. For comparison, even if sound could cross space at 340 m/s (0.34 km/s), the same journey would take about 14 years.

Sunlight takes 500 seconds, which is 8 minutes and 20 seconds, to reach Earth.

Common mistakes and how to help

Thinking light travels instantly

Switching on a lamp lights up the whole room at once, so light seems to arrive with no delay.

What helps: Look at distances. Moonlight takes about 1.3 seconds to reach us and sunlight about 8 minutes. Light is very fast, but not infinitely fast.

Believing only shiny things reflect light

Mirrors and polished metal are the obvious examples, and children link “reflection” with seeing an image.

What helps: Ask how we see a matt book or a wall at all. We see non-luminous objects only because they reflect light into our eyes. Rough surfaces reflect light too, but they scatter it in all directions.

Thinking rough surfaces do not follow the law of reflection

No clear image appears, so it seems the light is behaving in a different way.

What helps: Look at crumpled kitchen foil. Each tiny flat patch acts like a little mirror, and each ray still reflects with equal angles, but the patches face different ways, so the light scatters.

Thinking absorbed light just disappears

A dark surface simply looks dark, and there is nothing to show where the light went.

What helps: Energy is never lost. Absorbed light transfers its energy to the material, which warms up. Compare a black and a white piece of paper left on a sunny windowsill and feel the difference.

How to help at home

Foil, flat and crumpled. In a dim room, shine a torch on smooth kitchen foil and then on foil that has been crumpled and opened out again. Watch the spot of light on a wall in each case. Talk about specular and diffuse reflection.
Sort materials with a torch. Hold different objects, such as a glass, tissue paper, a card and clingfilm, between a torch and a wall. Sort them into transparent, translucent and opaque, and notice how the shadows differ.
Dark against light in the sun. Leave a sheet of black paper and a sheet of white paper side by side on a sunny windowsill for twenty minutes, then feel both. The black one should be warmer, because it absorbs more light energy. Never look straight at the Sun.
Spoon reflections. Look at your reflection in the back and the bowl of a shiny spoon, then in a matt wooden spoon. Why do you see an image in one and not the other? Look around the kitchen for other smooth and rough surfaces.
Lightning and thunder. Next time there is a storm, ask why the lightning arrives first. Ask your child to use the speed of light and the speed of sound to explain, and count the seconds to estimate how far away it was.

Leads on to

Key Stage 4: Properties of waves
Key Stage 4: The electromagnetic spectrum