Structure and Working of the Human Eye
The human eye acts like a camera. Light enters through a thin membrane called the Cornea. The dark muscular diaphragm behind the cornea is the Iris, which controls the size of the Pupil, regulating the amount of light entering the eye. The crystalline Lens focuses the light to form a real, inverted image on the light-sensitive screen called the Retina. The retina contains cells (rods and cones) that convert light into electrical signals sent to the brain via the optic nerve.
The ability of the eye lens to adjust its focal length using the ciliary muscles to see both near and distant objects clearly is called the Power of Accommodation. The least distance of distinct vision for a normal young adult is 25 cm.
Defects of Vision and their Correction
When the eye loses its power of accommodation, vision defects occur:
- Myopia (Near-sightedness): A person can see nearby objects clearly but cannot see distant objects distinctly. The image is formed in front of the retina. It is corrected using a Concave Lens of suitable power.
- Hypermetropia (Far-sightedness): A person can see distant objects clearly but cannot see nearby objects distinctly. The image is formed behind the retina. It is corrected using a Convex Lens.
- Presbyopia: An age-related defect where the ciliary muscles weaken, making it difficult to read or see nearby objects. It may require bifocal lenses (upper portion concave, lower portion convex).
Dispersion of Light by a Glass Prism
When white light passes through a glass prism, it splits into its seven constituent colors (VIBGYOR). This phenomenon is called dispersion. It occurs because different colors of light bend through different angles relative to the incident ray. Violet bends the most, and red bends the least. A rainbow is a natural spectrum caused by dispersion and total internal reflection of sunlight by tiny water droplets in the atmosphere.
Atmospheric Refraction
The Earth's atmosphere consists of layers of varying densities. Starlight entering the Earth undergoes continuous refraction as it passes through layers of gradually changing refractive index. This causes the apparent position of a star to be slightly higher than its actual position and leads to the Twinkling of Stars. Planets do not twinkle because they are closer and act as extended sources of light. Atmospheric refraction is also responsible for advance sunrise and delayed sunset.
Scattering of Light
When light hits tiny particles (like dust or water molecules) in the atmosphere, it is redirected in all directions. According to Rayleigh scattering, shorter wavelengths (blue light) scatter much more than longer wavelengths (red light). This explains why the sky is blue during the day. During sunrise and sunset, light travels a longer distance through the atmosphere, causing most of the blue light to scatter away, leaving the longer red wavelengths to reach our eyes, which explains the reddish appearance of the sun.