Chapter 10 Human Eye Colourful World

The human eye is a complex optical instrument that works similarly to a camera. Light enters through the transparent cornea, which does most of the refraction. It then passes through the pupil, a hole in the iris that controls the amount of light entering. The iris adjusts the pupil size — it becomes smaller in bright light and larger in dim light. The crystalline lens, held by ciliary muscles, further focuses light onto the retina, the light-sensitive layer at the back of the eye. The retina contains rod cells (sensitive to dim light and give black and white vision) and cone cells (sensitive to colour and require bright light). The optic nerve carries signals from the retina to the brain, where the image is interpreted. The image formed on the retina is real, inverted, and diminished, but the brain interprets it as upright.

The power of accommodation is the ability of the eye's lens to change its focal length in order to focus on objects at different distances. This is achieved by the ciliary muscles adjusting the shape of the crystalline lens. When looking at a nearby object, the ciliary muscles contract, making the lens thicker and increasing its converging power so it can focus close objects on the retina. When looking at a distant object, the ciliary muscles relax, making the lens thinner and reducing its converging power to focus far objects. The nearest point at which the eye can see clearly is called the near point (about 25 cm for a normal adult eye), and the farthest point is called the far point (infinity for a normal eye). The range between the near point and far point is the range of vision.

The colour of the sky and sunsets is explained by the Tyndall effect — the scattering of light by particles in the atmosphere. Sunlight contains all wavelengths, but when it enters the atmosphere, molecules of nitrogen, oxygen, and dust scatter light. Shorter wavelengths (blue and violet) are scattered much more strongly than longer wavelengths (red and orange) because scattering is inversely proportional to the fourth power of wavelength. During the day, when the sun is overhead, the scattered blue light reaches our eyes from all directions, making the sky appear blue. At sunrise or sunset, sunlight travels a much longer path through the atmosphere. Most blue light gets scattered away before reaching us, leaving predominantly red and orange light, which is why the sun and sky appear reddish-orange at those times.

Dispersion of light is the phenomenon in which white light splits into its seven constituent colours (VIBGYOR — Violet, Indigo, Blue, Green, Yellow, Orange, Red) when passed through a prism. This happens because different colours of light have different wavelengths and travel at slightly different speeds in glass, causing them to refract by different amounts. Violet light bends the most (highest deviation) and red light bends the least (lowest deviation) because violet has the shortest wavelength and red has the longest. The band of colours produced is called the spectrum. A glass prism achieves this by refracting light twice — once when entering and once when exiting. A second prism can recombine the spectrum back into white light, proving that white light is a mixture of all colours. Isaac Newton first performed these experiments.

Myopia (short-sightedness) is a defect where a person can see nearby objects clearly but distant objects appear blurred. It happens because the image forms in front of the retina, either due to an elongated eyeball or an overly converging lens. It is corrected using a concave (diverging) lens. Hypermetropia (long-sightedness) is where distant objects are seen clearly but nearby objects are blurred because the image forms behind the retina, due to a shortened eyeball or weak lens. It is corrected with a convex (converging) lens. Presbyopia is an age-related condition where the ciliary muscles weaken and the lens loses flexibility, making it difficult to focus on nearby objects. It occurs usually after age 40 and is corrected with bifocal lenses. Astigmatism, another common defect caused by uneven curvature of the cornea, is corrected using cylindrical lenses.