The Human Eye and the Colourful World
This chapter covers how the human eye works, common defects of vision and their correction, and natural phenomena caused by light such as dispersion, scattering, and the colours of the sky and sunset.
The power of accommodation is the ability of the eye lens to adjust its focal length, becoming more curved to focus on nearby objects and flatter to focus on distant objects, achieved by the ciliary muscles.
Myopia (near-sightedness): distant objects appear blurred, corrected with a concave lens. Hypermetropia (far-sightedness): nearby objects appear blurred, corrected with a convex lens. Presbyopia: age-related loss of accommodation, often corrected with bifocal lenses.
Q. Why does the sky appear blue in the daytime?
Sunlight is scattered by air molecules and fine particles in the atmosphere. Blue light has a shorter wavelength and is scattered much more than red light (Rayleigh scattering), so when we look at the sky away from the sun, we mostly see scattered blue light.
Dispersion of light is the splitting of white light into its seven component colours (VIBGYOR) when it passes through a prism, because each colour bends by a different amount depending on its wavelength.
Q. Why does the sun appear reddish at sunrise and sunset?
At sunrise and sunset, sunlight travels a much longer distance through the atmosphere to reach us. Most of the blue light is scattered away along this longer path, leaving mainly red and orange light, which has a longer wavelength and is scattered the least, to reach our eyes directly.
A person who cannot see distant objects clearly is suffering from:
Correct answer: (B) Myopia
The scattering of light by colloidal particles to make their path visible is called the _______ effect.
Tyndall
The least distance of distinct vision for a normal human eye is about 25 cm.
True — for a normal eye, 25 cm is the minimum distance at which objects can be seen most comfortably and clearly without strain.