The Human Eye and the Colourful World
What is tested
This domain tests the structure and working of the eye, accommodation, defects of vision, dispersion through a prism, scattering of light and atmospheric refraction. Expect reasoning about why the sky is blue and why a rainbow forms.
Core rules
- Light enters through the cornea, passes through the pupil controlled by the iris and is focused by the lens onto the retina. The image on the retina is real and inverted; the brain interprets it.
- Accommodation is the eye's ability to change lens focal length to focus near and far objects.
- Myopia is corrected by a concave lens; hypermetropia by a convex lens; presbyopia by bifocal lenses.
- A prism disperses white light into seven colours because different colours refract by different amounts. Red deviates least and violet most.
- Scattering makes the sky blue and sunsets red. The Tyndall effect shows scattering by colloidal particles.
Worked example
A myopic person has a far point 50 cm from the eye. The correcting lens must form the image of infinity at 50 cm on the same side, so f = -50 cm = -0.5 m. Power P = 1/f = 1/(-0.5) = -2 D. A concave lens of power -2 D is needed.
Traps
The image on the retina is not erect. Concave lenses diverge light and correct myopia. Dispersion is not the same as scattering. In a prism, red is least deviated, not most.