The Human Eye and the Colourful World

Lesson

Part 1 — how the eye forms an image, how it adjusts its focus, and how spectacles correct common defects of vision.

In this lesson you will

Diagram

Parts of the human eye

The iris is coloured (here brown); the pupil is the hole in its centre.

The eyeball is nearly spherical, about 2.3 cm across. Light passes through the cornea, the pupil and the eye lens to the retina.

Concept

Table of eye parts and their roles: cornea refracts most of the light, iris controls pupil size, pupil regulates light, lens adjusts focus and forms a real inverted image, ciliary muscles change lens curvature, retina converts light into electrical signals
Part	Role
Cornea	Transparent front bulge; most of the refraction of light happens at its outer surface
Iris	Dark muscular diaphragm that controls the size of the pupil
Pupil	Opening that regulates the amount of light entering the eye
Eye lens	Fine adjustment of focal length; forms a real, inverted image on the retina
Ciliary muscles	Change the curvature (and so the focal length) of the eye lens
Retina	Light-sensitive layer; its cells send electrical signals through the optic nerve to the brain