Light_Concept_4

Subject Facts

How the eye works is more a biological than a physical topic; but since the eye's functioning depends on most of the principles, outlined in concepts 1-3, it is worth discussing briefly here (see Figure 13).

A tough surface, the cornea, covers the front of the eye, Light enters the eye through a window the pupil (which is the black central part). The coloured ring around the pupil is the iris, which can become bigger or smaller to adjust the amount of light. allowed into the eye. In darkness or dim light the iris becomes thinner, allowing the pupil to open wider. In bright light the iris becomes wider, almost covering the pupil. Behind the pupil is a biconvex, transparent liquid-filled sack: the optic lens. Tiny muscles stretch or squash the kens, making it thicker or thinner so that images of different objects can be focused onto the back of the eye. The lens becomes thicker when focusing on near objects.

The inside back surface of the eye, the retina, is covered with light-sensitive cells. These cells respond to red, blue or green fight (which is why we only need those three to make white). and transmit signals to the brain via the optic nerve. It is the brain that actually 'sees'; it makes sense of the images, and interprets the relative intensities of red, blue and green fight as the various colours that we see.

Visual overload

When the light is too bright the iris expands to restrict the amount of tight getting into the eye. This can be inconvenient if you are passing directly from bright illumination to a dimly lit place for example, if you go from a sunlit garden into a house, you will be 'in the dark' for a few seconds until your irises open up to let more light in. In reverse, when you go from shaded place to a bright place, you are temporarily dazzled until your irises reduce the amount of light entering your pupils (in order to protect the retina from overexposure).

Your eyes can also adjust to an excess of one colour.The brain works on the principle that there is a range of colours in your field of vision, and that these change constantly as your eyes 'wander about. if one colour is much brighter than the others, or stays in a particular area of your vision for too long, your brain begins to say 'That cant be right better turn it down.' If you stare at a green cross on a white background for thirty seconds, then blink away and quickly look at a plain, white background (blinking a couple more times if necessary), you will see an after-image of a magenta (pink) cross, This is because your brain has 'turned down' the green receptors to avoid overload while looking at the green cross: when you look at a white background, your brain tunes out' a cross-shaped patch of green from the white — and so you see a magenta cross (made up of blue and red),

For this reason, the furniture in operating theatres tends to be draped in blue or green cloth. After staring into a blood-filled cavity, you will tend to go 'red-blind' and be unable to make things out. Looking away at something green or blue will bring your eyes back into colour balance.

Why you need to know these facts

Eyes are our means of detecting light and an understanding of how they work will increase children's ability to appreciate the science of life.

Vocabulary

Afterimage — negative image 'seen' as a result of parts of the retina closing down after overexposure to light or particular colours of light.

Cornea the tough outer covering of the eye.

Iris opens and closes to let more or less light into the eye.

Optic lens the clear opening that allows light into the eye. Optic nerve transmits the information received by the retina to the brain.

Pupil the black centre of the eye that allows light through to the lens.

Retina the internal surface of the eye that senses light.

Common Misconceptions

As children, we soon become aware that we use our eyes for seeing things, long before we have any scientific understanding of vision or the eye. How the eye works and how we use light to see things are frequently misunderstood. The early Greek philosophers had two conflicting theories about how we see things. The followers of Pythagoras believed that the eye contains light and the light comes out when we look at things. The alternative (and ultimately correct) view was held by the followers of Democritus: that light travels from a source to an object and then into the eye,

You see things because light comes out of your eyes, This classic misconception can be overcome by asking the child to go into a darkened room and find something. I can't, it's too dark in here? Light is clearly not coming from the child's eyes!

When light is in your eyes, you can look around and see things with it.

Repeat the 'darkened room' test, and point a small torch at the child's face. The child will see the torch but not the object. Ask where the torch needs to be pointed 'At the thing I want to find'. By this point the child should recognise that the light goes from the source to the object to his or her eyes.

Questions

Why do see spots when I blink after looking into a torch light?

Your eyes try to protect the light-sensitive cells of the retina from damage by blocking out very bright lights. First the black bit (the pupil) in the centre of your eye gets smaller to let less light in. Then the cells in the back part of your eye (the retina) start to close down, because they are getting overloaded. When you look away, the 'shut down' patches of your retina cause temporary dark spots on your vision, (NB Warn children not to stare at very bright lights, because of the damage this will cause to the retina. They should be especially careful when emerging from a dark room into bright light)

Why do people sometimes need glasses? When you look at things dose to you and then further away, the lenses in your eyes have to refocus by changing shape slightly. If you focus on your hand in front of your face, everything use becomes burred; then if you focus on something further away, your hand becomes burred. If you cant make the lenses In your eyes change shape enough the lenses in glasses can help you. Glasses have to be specially made for each person's eyes. (NB Warn children not to try on each other's glasses: this will cause blurred vision, as well as headaches.)

Teaching ideas

Opposite pictures

This activity can be linked to art work. Ask the children to use bright paint or collage materials to produce pictures that are only seen in the correct colours when you stare at them for a while and then look at a sheet of white paper to see the afterimage.

To find the 'opposites' of the colours they need, the children can make a colour wheel (red. blue and green with the secondary colours of light painted between them) to use as a visual reminder (see Figure 14). This could be made using a drawing or painting software program.

Alternative'y the children can stare at a 'true' colour version and then see Ets *opposite after-image on a white background