How the eye focuses light and why the sky is blue
Light enters the eye through the cornea, passes through the pupil, and is focused by the eye lens onto the retina, where light-sensitive cells send signals through the optic nerve to the brain. The ciliary muscles change the thickness of the lens so that both near and distant objects can be seen clearly. This ability is called accommodation. A normal eye sees clearly from about 25 cm up to infinity.
In myopia, or short-sightedness, distant objects look blurred and the defect is corrected with a concave lens. In hypermetropia, or long-sightedness, near objects look blurred and a convex lens is used. A glass prism splits white light into a spectrum of seven colours, called dispersion, because each colour bends by a different amount. Rainbows form when sunlight is dispersed by raindrops. The sky looks blue because tiny particles in the air scatter blue light much more than red light.
Why it matters
This topic explains why people need spectacles, how eye check-ups measure lens power in dioptres, and why danger signals use red light, which scatters least and is visible from far away. It also explains the reddish Sun at sunrise and sunset and the twinkling of stars caused by atmospheric refraction. It builds on the earlier chapter on reflection and refraction.
Key terms
- Accommodation
- The ability of the eye lens to change its focal length to see objects at different distances.
- Myopia
- A vision defect in which distant objects appear blurred, corrected using a concave lens.
- Dispersion
- The splitting of white light into its component colours when it passes through a prism.
- Scattering
- The redirection of light in many directions by small particles in its path.