Light and seeing

Light

When a light globe is turned on it gives out light.

There are many other things that produce light. Candles, fluorescent tubes, the Sun, television screens, computer screens and glow-worms are just some of them.

Light travels in all directions from the thing producing the light.

A very narrow beam of light is referred to as a light ray. Diagrams of light rays indicate the direction in which the light is travelling.

If light is travelling from a light globe, then we can draw light rays spreading out from the globe.

Usually light rays travel away from the source of the light in straight lines until they hit something.
Light globe showing rays of light radiating

Sometimes you can see light from the Sun passing through clouds and the sunbeams appear as straight lines.

After light has been reflected off an object, such as a tree or a book, it still travels in straight lines, but in a new direction.

If the light enters our eyes, we see the object (ie our eyes can detect light). For example, if light from a light globe enters our eyes, we can see the globe.

If light from the Sun strikes an object such as a tree or a house, the light is usually reflected off that object. If light from a light globe hits an object such as a book or a clock, it is usually reflected off that object.

We can see things such as books and trees because of the light that is reflected off them. If light was not reflected off them into our eyes, we could not see them.

Transparent, opaque and translucent

If light strikes a sheet of clear glass, most of it will pass through the glass. Glass is said to be transparent. Similarly if light strikes the surface of some pure water, it will pass through the water. Water is also transparent.

However, if light strikes the surface of a block of wood or sheet of steel, it will not pass through those substances. Things that light will not pass through are said to be opaque.

Sometimes light will pass through a substance, but the light is scattered. The light is said to be scattered by the substance. Things that scatter light as it passes through them are said to be translucent.

Frosted glass and the pearl glass used on light globes are examples of translucent materials. Many plastic ice cream containers are translucent.

The eye

Our eyes have a small window at the front through which the light can enter. It is called the pupil. The pupil appears black because it is dark inside our eye.

At the back of our eye is a sensitive sheet of nerves called the retina. These nerves can detect light when it strikes the retina.

Our eye forms an image on the retina. This image is like a small picture of what you are looking at. If you are looking at a house, then an image of the house is focused by the lens of the eye onto the retina. The nerves in the retina send messages about the image to the brain, and your brain enables you to interpret what you are looking at.

Our eyes can detect images only because of the light that enters into them. If you close your eyes, the light cannot enter them, and you can detect nothing but darkness.
The eye

© Commonwealth of Australia, 2003