Chapter 10: The Human Eye and the Colourful World
The Human Eye and Its Functions
The human eye functions like a camera, using a convex lens to focus light on the retina. Light first passes through the cornea, where most refraction occurs, and then through the eye lens, which fine-tunes the focus. The iris controls the amount of light entering through the pupil. The retina contains light-sensitive cells that convert light into electrical signals sent to the brain. The eye’s ability to adjust its lens shape for viewing near and far objects is called accommodation. The least distance for comfortable near vision is about 25 cm, while the far point is infinity. Cataracts can cloud the lens, leading to vision loss, but surgery can restore sight.
Defects of Vision and Their Correction
The three main refractive defects are:
- Myopia (nearsightedness): Distant objects appear blurry because light focuses before the retina. Corrected using a concave lens.
- Hypermetropia (farsightedness): Nearby objects appear blurry as light focuses behind the retina. Corrected with a convex lens.
- Presbyopia: Age-related loss of accommodation, affecting near vision. It can be corrected with bifocal lenses, combining both concave and convex lenses. Modern alternatives include contact lenses and surgeries.
Optical Phenomena in Nature
Light refracting through a prism bends and splits into a spectrum (VIBGYOR) due to dispersion, with red bending least and violet most. This explains phenomena like rainbows, formed by refraction, dispersion, and internal reflection in raindrops. Atmospheric refraction causes twinkling of stars and the apparent early sunrise and delayed sunset. Scattering of light, especially by tiny atmospheric particles, makes the sky appear blue and the Sun red at sunrise and sunset. Blue light is scattered more than red due to its shorter wavelength.