Detecting light: cameras and the human eye

In a nutshell

Light incident on a source can be absorbed and transferred into energy. Ray diagrams can be used to show how pinhole cameras and convex lenses focus light. Photo-sensitive materials absorb light and convert the energy into a chemical change or electrical impulse.

Focusing light

Pinhole cameras

A pinhole camera is a box with a tiny hole in one end and a translucent (semi-transparent) screen at the opposite end. Light enters the box through the pinhole. It's then focused onto the screen.

The light is focused so that it looks like it meets at a single point. If the light wasn't focused at the pinhole, the image would appear blurred on the screen.

Science; Waves; KS3 Year 7; Detecting light: cameras and the human eye

1.	Object
2.	Translucent Screen
3.	Pinhole

The image produced is upside-down, laterally inverted and smaller than the object.

Example

Pinhole cameras are used for photography as well as to observe eclipses.

Convex lenses

Using a convex lens is another way of focusing incoming light to a single point. This point is known as the lens' focal point. It is made from a transparent material which refracts light as it passes in and out of the lens.

Science; Waves; KS3 Year 7; Detecting light: cameras and the human eye

1.	Incoming Rays
2.	Central line of lens
3.	Focal Length
4.	Focal Point

Example

If you wear glasses because you cannot see nearby objects (far-sightedness) then you'll be using a convex lens! They are also used for magnifying glasses and cameras.

Eyes

The eye is very important for focusing light in order to see. It works similarly to the convex lens, and focuses the light onto the focal point on the retina. The reason someone might need glasses is because their eye doesn't focus the light on the retina.

Science; Waves; KS3 Year 7; Detecting light: cameras and the human eye

1.	Object
2.	Pupil
3.	Iris
4.	Lens
5.	Retina
6.	Optic Nerve

Detecting light

Light waves carry energy which can be transferred to objects they come into contact with. This means that if a light ray hits a surface, it may transfer its energy to the surface. This process is called absorption.

Photo-sensitive materials detect light by absorbing light and converting the energy into a chemical change or an electrical impulse.

Example

Cameras focus light from an object onto photo-sensitive material by using a convex lens. The photo-sensitive material works by absorbing the light which transfers energy to the material. This causes a chemical change or, in modern day cameras, an electrical impulse.

Eyes also have photo-sensitive material in the retina. The retina is partly made up of cells that produce electrical impulses when they absorb light. These impulses are sent to your brain via the optic nerve.