Key Stage 3 science · Physics – Waves

KS3 mirrors, lenses, the eye and colour: a guide for parents

Key Stage 3 pupils use the ray model, drawing light as straight lines, to explain how mirrors, pinhole cameras and lenses form images and how the human eye works. They also learn that light carries energy that can cause chemical and electrical effects, and that white light is a mixture of colours that a prism can split and that objects absorb and reflect in different ways. The treatment is qualitative, with no equations to learn.

Mirrors, lenses, the eye and colour, explained

The ray model treats light as straight lines with arrows that show the direction it travels. It is a simplification, but it is enough to explain a great deal. A pinhole camera is the simplest example. Light from each point of a bright object travels in straight lines through a tiny hole and lands at one spot on a screen behind it. Rays from the top of the object land at the bottom of the screen, and vice versa, so the image is upside down. A smaller hole gives a sharper image, but a dimmer one.

In a plane (flat) mirror, the image appears to be behind the glass, the same distance behind as the object is in front, and the same size as the object. It is called a virtual image because the light does not really come from there. Our brains simply assume that light travels in straight lines, so they trace the reflected rays back to a point behind the mirror. Pupils draw this by reflecting rays with equal angles and then extending the reflected rays backwards with dotted lines to find where the image seems to be.

Light also changes direction when it passes between transparent materials, such as air and glass or water. This is called refraction. It happens because light travels at different speeds in different materials. Entering glass from air at an angle, light slows down and bends towards the normal, and it bends away again on the way out. A convex lens is thicker in the middle than at the edges, so it bends parallel rays inwards to meet at a point called the focus. This is how a magnifying glass, a camera lens or a pair of long-sight glasses works. The treatment at this stage is qualitative.

The human eye uses the same ideas. Light is refracted by the cornea and the lens, and the iris controls how much gets in by changing the size of the pupil. The lens fine-tunes the focus by changing shape, and an upside-down image forms on the retina at the back of the eye. The retina contains light-sensitive cells, some for dim light and some for colour. They absorb light, which triggers chemical and electrical changes, and the signals travel along the optic nerve to the brain. A camera works similarly: a lens focuses an image on a sensor or film, which absorbs light energy and produces an electrical or chemical effect. Schools often add other examples of light causing change, such as solar cells producing electricity or sunlight fading coloured fabrics.

White light is a mixture of all the colours. A glass prism refracts different colours by different amounts, so it spreads white light into a spectrum, from red, which bends least, through orange, yellow, green and blue to violet, which bends most. Each colour corresponds to a different frequency, with red the lowest and violet the highest. An object looks coloured because of what its surface reflects and absorbs. A red object diffusely reflects mainly red light and absorbs the rest, while white surfaces reflect nearly all colours and black ones absorb nearly all. Under a light that lacks the colour it reflects, an object looks dark.

Words your child will hear

Ray: A straight line with an arrow used to show the path of light in a diagram.
Virtual image: An image that seems to be in a place where the light does not actually go, such as behind a mirror.
Refraction: The change in direction of light as it passes from one transparent material to another, caused by a change in speed.
Convex lens: A lens that is thicker in the middle and brings parallel rays together to a focus.
Retina: The layer at the back of the eye containing light-sensitive cells.
Spectrum: The band of colours, from red to violet, that white light is split into by a prism.
Frequency (of light): The number of waves per second. Each colour has its own, with red the lowest and violet the highest.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about the use of the ray model to explain imaging in mirrors, the pinhole camera, the refraction of light and action of a convex lens in focusing (qualitative); the human eye
Pupils should be taught about light transferring energy from source to absorber leading to chemical and electrical effects; photo-sensitive material in the retina and in cameras
Pupils should be taught about colours and the different frequencies of light, white light and prisms (qualitative only); differential colour effects in absorption and diffuse reflection

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

A worked example

A red apple looks red in white light. Explain why, and say how it would look in pure green light.

Start with the light source. White light is a mixture of all the colours of the spectrum, so it contains red, green, blue and every colour in between.
Describe what the apple’s surface does. Its skin reflects mostly red light, in all directions because the surface is rough on a tiny scale, and absorbs most of the other colours.
Link to seeing. The reflected red light enters the eye and is focused on the retina, where the light-sensitive cells respond. The brain interprets the signal as red.
Now change the light. Pure green light contains no red. The apple absorbs most of the green light that falls on it, and there is no red for it to reflect.
Work out the result. Very little light comes from the apple to the eye, so it looks very dark, close to black. A green leaf in the same green light would still look green.

The apple looks red in white light because it reflects red and absorbs other colours. In green light it looks very dark, because it absorbs green and has no red light to reflect.

Common mistakes and how to help

Thinking the image in a mirror is really behind the glass

The image looks as solid as the object and seems to sit in a space behind the mirror, so children believe the light goes there.

What helps: Try to catch the image on a sheet of paper held behind the mirror. Nothing appears, because it is a virtual image. A lens or a pinhole can form a real image on a screen, which makes a good contrast.

Believing a prism adds colour to the white light

The colours seem to come out of the glass, so the glass looks like the source.

What helps: The colours are already in white light, and the prism only separates them. A second prism, placed the other way round, can recombine the spectrum back to white.

Thinking light bends at the pinhole to turn the image upside down

An inverted image seems strange, and bending is the idea pupils have just learned.

What helps: In a pinhole camera the rays travel in straight lines and simply cross at the hole. Draw a ray from the top of an object through the hole: it must land at the bottom. The brain turns the eye’s upside-down image the right way up.

Saying a red object is red because it “absorbs red”

It is easy to mix up the colour that is taken away and the colour that is left.

What helps: The colour we see is the colour that is reflected. A red surface absorbs the other colours. Shine coloured torchlight on it in a dark room to test it.

How to help at home

Build a pinhole camera. An adult makes a small hole in one end of a cardboard tube or box with a pin or sharp pencil, and tapes greaseproof paper over the other end. In a dim room, point the hole at a bright window or lamp and look at the paper. The upside-down picture is the image. Never point it at the Sun.
The half-height mirror. Stand in front of a large mirror and use sticky notes to mark where the top of your head and your feet appear. You only need a mirror about half your height to see your whole body, and the distance does not change that. Ask your child to explain it with a ray diagram.
The vanishing coin. Put a coin at the bottom of an opaque mug and step back until the rim just hides it. Keep still while someone pours water in gently. The coin appears, because the light now bends at the water surface to reach your eyes. A straw in a glass of water looks bent for the same reason.
Make a spectrum. On a sunny day, stand a clear plastic tumbler of water on a windowsill so that sunlight passes through it on to a sheet of white paper. You may see a small rainbow. Do not look directly at the Sun.
Colour under coloured light. In a dark room, cover a torch with red, then green, cellophane and shine it on coloured toys or paper. Ask your child to predict how each will look before you try, and explain the results using reflection and absorption.

Leads on to

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