The Human Eye and the Colourful World
What the exam tests
The Human Eye and the Colourful World tests the structure and working of the eye, accommodation, defects of vision and their correction, dispersion of light, scattering, and atmospheric refraction. Questions often ask why the sky is blue or how myopia is corrected.
Core rules
- Light enters through the cornea, passes through the pupil (controlled by the iris) and is focused by the lens onto the retina. Rods detect dim light; cones detect colour.
- Accommodation is the eye lens changing focal length to focus near and far objects. The near point is about 25 cm for a normal adult eye.
- Myopia (near-sightedness): distant objects blur; the image forms in front of the retina; corrected by a concave lens of suitable power.
- Hypermetropia (far-sightedness): near objects blur; the image forms behind the retina; corrected by a convex lens.
- Presbyopia is age-related loss of accommodation; cataract is clouding of the lens.
- White light splits into seven colours in a prism because different colours refract by different amounts; red bends least, violet most.
- Scattering is inversely related to wavelength: blue scatters more than red. This explains the blue sky and the reddening of the sun at sunrise and sunset.
Worked example
A myopic person cannot see clearly beyond 2 m. Find the power of the corrective lens. The lens must form an image of a distant object at the person's far point, 2 m in front of the eye. For a concave lens, f = −2 m. P = 1/f = 1/(−2) = −0.5 D. The negative sign shows a diverging lens.
Traps that cost marks
- Using a convex lens for myopia; it is concave.
- Saying stars twinkle because of dispersion; it is atmospheric refraction.
- Claiming red scatters most; red scatters least.
- Forgetting that a rainbow involves refraction, dispersion and total internal reflection.