How the Eye Forms an Image

0:00How does your eye make a picture of the world?

0:05Some people think the eye sends out rays to see. It does not.

0:10Instead, light bounces off objects, like this tree, and travels into the eye.

0:17First, the cornea, the clear curved front of the eye, bends the light.

0:23The iris is the coloured ring. The hole in its middle, the pupil, lets light in.

0:30The iris makes the pupil smaller in bright light and bigger in dim light.

0:35The lens bends the light more. It changes shape to focus on near or far things.

0:42The light focuses on the retina, a layer of light-sensitive cells at the back of the eye.

0:48The rays cross over, so the image on the retina is upside down.

0:53The retina has two kinds of light-sensitive cells: rods and cones.

1:00Rods work in dim light, but they cannot tell colours apart.

1:04Cones sense colour. Three kinds respond most to red, green and blue light.

1:11The rods and cones turn light into electrical signals, and the optic nerve carries them to the brain.

1:20The brain makes sense of the signals and turns the picture the right way up.

1:25Where the optic nerve leaves the eye, there are no rods or cones. This is the blind spot.

1:32We do not usually notice it, because the brain fills in the gap.

1:37So, the cornea and lens focus the light, the retina senses it, and the brain sees the picture.