Quick Revision

The iris is a dark muscular diaphragm that controls the size of the pupil. In bright light, iris contracts to make the pupil smaller (less light enters). In dim light, iris expands to make the pupil larger (more light enters). The iris gives the eye its characteristic colour.

Presbyopia is a vision defect that occurs in old age (typically after 40 years). The ciliary muscles weaken and the eye lens loses its flexibility, reducing the power of accommodation. The person cannot see nearby objects clearly (near point recedes beyond 25 cm). Correction: Using bifocal lenses — the upper portion is concave lens (for distant vision) and the lower portion is convex lens (for reading/near vision). Sometimes a person may need both concave and convex corrections if they have both myopia and hypermetropia.

The sky appears blue due to the scattering of sunlight by the atmosphere (Rayleigh scattering). 1. Sunlight (white light) consists of seven colours (VIBGYOR). 2. When sunlight enters Earth's atmosphere, it gets scattered by tiny molecules of gases (N₂, O₂) and particles. 3. According to Rayleigh's law, the amount of scattering is inversely proportional to the fourth power of wavelength: Scattering ∝ 1/λ⁴. 4. Blue light has a shorter wavelength compared to red light. 5. Therefore, blue light is scattered much more (about 10 times more than red). 6. This scattered blue light reaches our eyes from all directions, making the sky appear blue. At sunrise/sunset, light travels through more atmosphere, so blue is completely scattered away and only red/orange reaches us — hence the reddish sky.

Myopia (near-sightedness/short-sightedness): A person can see nearby objects clearly but cannot see distant objects clearly. Cause: (1) Excessive curvature of the eye lens (too converging). (2) Elongation of the eyeball. In both cases, the image of distant objects is formed in front of the retina instead of on it. Correction: Using a concave (diverging) lens of appropriate power. The concave lens diverges the incoming rays slightly before they enter the eye, so the image shifts back onto the retina. Far point of the myopic eye becomes the normal far point (infinity) after correction.

Hypermetropia (far-sightedness/long-sightedness): A person can see distant objects clearly but cannot see nearby objects clearly. Cause: (1) Focal length of the eye lens becomes too long (lens becomes too flat). (2) Eyeball becomes too short. The image of nearby objects is formed behind the retina instead of on it. Correction: Using a convex (converging) lens of appropriate power. The convex lens converges the incoming diverging rays from nearby objects, so the image is formed on the retina. Near point of the hypermetropic eye becomes 25 cm (normal near point) after correction.