The Human Eye and the Colourful World: Class 10 Science Notes & NCERT Solutions

Structure and Working of the Human Eye

The human eye is like a sophisticated camera. Light enters through a thin membrane called the cornea, which does most of the light refraction. Behind the cornea is the iris, a muscular diaphragm that controls the size of the pupil, the opening in the center. The iris gives the eye its colour and regulates the amount of light entering. The light then passes through the eye lens, a convex lens made of a transparent, flexible material. The lens's curvature, and thus its focal length, is adjusted by the ciliary muscles. This ability to change the focal length is called accommodation. The lens focuses an inverted, real image of the object onto the retina, a light-sensitive screen at the back of the eye. The retina contains millions of light-sensitive cells: rods (sensitive to light intensity) and cones (sensitive to colour). These cells convert light energy into electrical signals, which are then sent to the brain via the optic nerve. The brain interprets these signals and processes the information, allowing us to perceive objects upright and in their true colours and size.

Common Defects of Vision and their Correction

Myopia (Near-sightedness)

A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. This occurs because the image of a distant object is formed in front of the retina. It is caused by either excessive curvature of the eye lens or elongation of the eyeball. This defect is corrected by using a concave lens of appropriate power.

Hypermetropia (Far-sightedness)

A person with hypermetropia can see distant objects clearly but finds it difficult to see nearby objects. This is because the light rays from a nearby object are focused at a point behind the retina. It is caused by either the focal length of the eye lens being too long or the eyeball being too short. This defect is corrected by using a convex lens of appropriate power.

Presbyopia

This is the defect of vision due to which an elderly person cannot see nearby objects clearly. It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens with age. Sometimes, a person may suffer from both myopia and hypermetropia. Such people require bifocal lenses, which consist of both concave and convex lenses.

Worked Example: Correcting a Vision Defect

1. Problem Statement — The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
2. Step 1: Understand the Defect and Goal — The person has myopia. Their far point is 80 cm, meaning they can see clearly only up to this distance. To correct this, we need a lens that can take an object at infinity (a distant object) and form its image at the person's far point (80 cm).
3. Step 2: Identify Given Values for the Lens Formula — The lens formula is 1/v - 1/u = 1/f. - Object distance (u) = ∞ (since the person wants to see distant objects). - Image distance (v) = -80 cm (the far point of the eye). By convention, distances measured in the direction opposite to incident light are negative. - We need to find the focal length (f).
4. Step 3: Calculate the Focal Length (f) — Substitute the values into the lens formula: 1/(-80) - 1/(∞) = 1/f Since 1/∞ = 0, we have: -1/80 = 1/f So, f = -80 cm. The negative sign indicates that the required lens is a concave lens.
5. Step 4: Calculate the Power of the Lens (P) — Power (P) is the reciprocal of the focal length in meters. First, convert f to meters: f = -80 cm = -0.8 m. Now, calculate the power: P = 1/f = 1/(-0.8) = -10/8 = -1.25 D. The power of the required lens is -1.25 Dioptres.

Key Natural Phenomena Involving Light

• Dispersion of White Light by a Glass Prism: When white light passes through a prism, it splits into its seven constituent colors (Violet, Indigo, Blue, Green, Yellow, Orange, Red - VIBGYOR). This happens because different colors of light bend through different angles. Red light bends the least, and violet light bends the most.
• Atmospheric Refraction: This is the refraction of light by the Earth's atmosphere. It causes phenomena like the twinkling of stars (as starlight passes through layers of air with varying refractive indices) and advanced sunrise/delayed sunset (the Sun is visible about 2 minutes before actual sunrise and 2 minutes after actual sunset).
• Scattering of Light: This is the process in which light is redirected in many different directions upon hitting particles. The colour of the scattered light depends on the size of the scattering particles. The blue colour of the sky is due to the scattering of sunlight by the tiny molecules of air in the atmosphere (Rayleigh scattering).
• Tyndall Effect: The scattering of a beam of light by particles in a colloid or a very fine suspension. This is why a sunbeam is visible in a dusty room.

Exam Tip: Mastering Ray Diagrams for Vision Defects

Ray diagrams for myopia and hypermetropia (and their correction) are very important for board exams. Always use a pencil and a ruler. First, draw the diagram for the defective eye, showing where the image forms (in front of or behind the retina). Then, draw the diagram for the corrected eye. Clearly show the corrective lens (concave for myopia, convex for hypermetropia) and draw the rays to show how the lens helps form the image correctly on the retina. Label all parts: the eye lens, retina, the corrective lens, and the path of the light rays with arrows.

Practice Questions with Solutions

• [object Object]
• [object Object]
• [object Object]
• [object Object]

Frequently Asked Questions