The human eye works very much like a camera. It collects light, focuses it, forms an image and sends information to the brain. Understanding its parts makes it easier to understand how vision happens.

• ★Cornea: The transparent curved front part of the eye. It is the first surface that light hits. The cornea does most of the bending (refraction) of light that enters the eye.
• ★Iris: The coloured circular part behind the cornea. It controls the size of the pupil and regulates how much light enters the eye. In bright sunlight the pupil becomes small; in dim light it becomes large.
• ★Pupil: The small black opening at the centre of the iris. It is simply a hole through which light enters the eye.
• ★Eye Lens (Crystalline Lens): A transparent, flexible, curved structure behind the pupil. It helps to adjust the focus depending on the distance of the object. When you look at something close, the lens becomes thicker. For far objects, it becomes thinner.
• ★Retina: A delicate layer at the back of the eye. It has light-sensitive cells that detect light and colour. These cells convert light into electrical signals.
• ★Optic Nerve: This nerve carries the electrical signals from the retina to the brain. The brain interprets them and forms the image you see.
• ★Blind Spot: A small region on the retina where the optic nerve leaves the eye. No light-sensitive cells are present here, so no image is formed at this spot.
• ★Eyeball: Almost spherical in shape with a diameter of about 2.3 cm.

Important point: The image formed on the retina is inverted (upside-down), but the brain automatically turns it the right way up.

The eye has the ability to adjust its lens to see objects clearly at different distances. This ability is called accommodation.

• ★For distant objects: The ciliary muscles relax, the lens becomes thin, and its focal length increases. This helps us see distant objects clearly.
• ★For near objects: The ciliary muscles contract, the lens becomes thicker, and focal length decreases. This helps us see close objects clearly.

Near Point: For a normal eye, the closest distance at which we can see clearly is about 25 cm.
Far Point: For a normal eye, the farthest point is infinity.

Cataract: A condition in which the eye lens becomes cloudy due to ageing. Vision becomes foggy or blurred. It is treated by surgery.

Sometimes eyes cannot form clear images because of changes in lens shape or eyeball size. These are called refractive defects.

A. Myopia (Near-sightedness)

A person with myopia can see near objects clearly but distant objects appear blurred.

• ★The image of a distant object is formed in front of the retina.
• ★Causes: – Eye becomes too long – Lens becomes too curved
• ★Correction: A concave (diverging) lens is used. It spreads out the light rays so the image forms correctly on the retina.

B. Hypermetropia (Far-sightedness)

A person with hypermetropia can see distant objects clearly but finds it difficult to see nearby objects.

• ★The image of a nearby object is formed behind the retina.
• ★Causes: – Eye becomes too short – Lens becomes too flat
• ★Correction: A convex (converging) lens helps bring the image onto the retina.

C. Presbyopia

This defect occurs in older people. With age, the lens loses flexibility and the ciliary muscles weaken. Such people cannot see nearby objects clearly.

• ★Corrected using convex lenses or bifocal glasses.

A prism is a transparent solid with triangular sides. When light enters a prism, it bends because of refraction. When it leaves, it bends again.

• ★Light bends towards the normal when entering a denser medium (air → glass).
• ★Light bends away from the normal when moving back to a rarer medium (glass → air).

This bending of light causes the ray to emerge at an angle called the angle of deviation.

When white light passes through a prism, it splits into seven colours — Violet, Indigo, Blue, Green, Yellow, Orange and Red. This splitting of white light into colours is called dispersion.

• ★Red bends the least.
• ★Violet bends the most.

Rainbow

A rainbow is a natural example of dispersion. It is formed by water droplets in the atmosphere which act like tiny prisms.

Twinkling of Stars

Stars appear to twinkle because their light passes through different layers of the atmosphere. These layers have different densities, so the direction of light keeps changing.

Why Planets Do Not Twinkle

Planets are much closer to Earth and appear as discs, not points. The variations cancel out, so planets do not twinkle.

Early Sunrise and Delayed Sunset

We see the Sun about 2 minutes before it actually rises and about 2 minutes after it sets. This is because the atmosphere bends sunlight towards us.

Why is the Sky Blue?

Air molecules scatter shorter wavelengths of light (blue) more than longer wavelengths (red). Because blue light scatters in all directions, the sky appears blue.

Why Are Sunsets Red?

During sunset, sunlight travels a longer path through the atmosphere. Most of the blue light is scattered away, leaving mainly red and orange.

Why Are Danger Signals Red?

Red light has the longest wavelength and is scattered the least. This makes it easy to see even from far away or in fog.

Eye donation can help people who suffer from corneal blindness.

• ★Eyes should be removed within 4–6 hours after death.
• ★One donor can help restore sight to up to 4 people.
• ★People of any age can donate.