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

An arrow

提示Without it you cannot tell the lamp end from the eye end.

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

The normal

提示Angles of incidence and reflection are both measured from it.

为什么It 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

提示Trace the reflected rays backwards with dashed lines.

为什么The 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

提示They are a drawing aid for finding the image.

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

A ruler

提示Light travels in straight lines.

为什么A 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

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

为什么Its 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

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

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

The pupil

提示It looks black, and widens in the dark.

为什么The 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

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

为什么If 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)

提示It is where the picture is recorded.

为什么In 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.