Waves, light and sound, Year 8: ray diagrams and the eye

An arrow

HintWithout it you cannot tell the lamp end from the eye end.

The whyLight travels from a source, or from an object that reflects it, to the eye. The arrowhead shows that direction.

The normal

HintAngles of incidence and reflection are both measured from it.

The whyIt is a construction line, usually drawn dashed. Measuring both angles from it shows that the angle of reflection equals the angle of incidence.

Behind the mirror, as far back as the object is in front

HintTrace the reflected rays backwards with dashed lines.

The whyThe reflected rays enter the eye as if they had come from a point on the far side of the glass. That is where the brain places the image.

No light really travels there

HintThey are a drawing aid for finding the image.

The whyThe dashed lines extend the reflected rays backwards to find the image. Real rays are solid lines; these imagined ones are dashed.

A ruler

HintLight travels in straight lines.

The whyA wobbly freehand ray gives the wrong angles and puts the image in the wrong place. Accuracy is the whole point of the diagram.

The retina

HintIt lines the inside of the eyeball and sends signals along the optic nerve.

The whyIts cells respond to light and send signals to the brain. The image that forms on it is upside down; the brain interprets it the right way up.

The cornea and the lens

HintOne is the clear front window; the other sits just behind the iris.

The whyThe curved cornea does most of the bending. The lens then makes the fine adjustment, changing shape for near and distant objects.

The pupil

HintIt looks black, and widens in the dark.

The whyThe iris makes it larger in dim light to let more light in, and smaller in bright light to protect the back of the eye.

In front of or behind the retina, not on it

HintA sharp image needs the rays to meet exactly on the light-sensitive layer.

The whyIf the rays meet too early or too late, each point of the object lands as a smudge. Glasses bend the light a little so that it focuses in the right place.

The sensor (or film)

HintIt is where the picture is recorded.

The whyIn both, a lens focuses light onto a light-sensitive surface. The camera's sensor records the picture; the eye's surface sends it to the brain.