The Human Eye and the Colourful World — 15 Most Important Questions

Explanation

The power of accommodation is the ability of the human eye to adjust its focal length by changing the curvature of the eye lens, so as to clearly see objects placed at different distances.

This adjustment is done by the ciliary muscles, which control the thickness of the eye lens. When we look at a nearby object, the ciliary muscles contract and make the lens thicker (more curved), reducing the focal length. When we look at a distant object, the muscles relax, making the lens thinner, increasing the focal length.

Explanation

The blue colour of the sky is due to Tyndall Effect — the scattering of sunlight by the fine particles present in the atmosphere.

Among all the colours of white light, blue light has a shorter wavelength and is scattered much more strongly by atmospheric particles (tiny dust and gas molecules) than red light. This scattered blue light reaches our eyes from all directions across the sky, making it appear blue.

Explanation

The near point (also called the least distance of distinct vision) of a normal human eye is 25 cm.

This means a person with normal vision can see an object clearly when it is at least 25 cm away from the eye. Objects closer than 25 cm appear blurred because the ciliary muscles cannot make the lens curved enough to focus at a shorter distance.

Explanation

The iris controls the amount of light entering the eye by adjusting the size of the pupil.

In bright light, the iris contracts, making the pupil smaller to reduce the amount of light entering. In dim light, the iris expands, making the pupil larger to allow more light in. This helps protect the retina from excessive light and helps us see in low-light conditions.

Explanation

At sunrise and sunset, sunlight travels through a longer path in the atmosphere to reach our eyes. During this longer journey, most of the shorter wavelength colours (blue, violet) are scattered away.

Only the longer wavelength red and orange light remains and reaches our eyes directly, making the sun appear reddish or orange. This is also due to the scattering of light phenomenon.

Explanation

Myopia (Nearsightedness): A person with myopia can see nearby objects clearly but cannot see distant objects clearly. The image of a distant object is formed in front of the retina instead of on it.

Cause: The eyeball is elongated (too long) or the eye lens is too thick (too curved), causing the focal point to fall before the retina.

Corrective Measure: Myopia is corrected using a concave (diverging) lens of suitable focal length. The concave lens diverges the incoming light rays before they enter the eye, so the eye lens can then focus them correctly on the retina.

Explanation

• Dispersion of light is the splitting of white light into its constituent seven colours (VIBGYOR) when it passes through a medium like a prism or water droplet. It occurs due to different wavelengths bending by different amounts (refraction).
• Scattering of light occurs when light strikes very small particles (like dust or gas molecules in the atmosphere) and gets deflected in different directions. Shorter wavelengths scatter more than longer ones.

Dispersion produces a spectrum (rainbow), while scattering explains phenomena like the blue sky and red sunset.

Explanation

Presbyopia is a defect of vision that occurs due to ageing. With age, the ciliary muscles become weak and the eye lens loses its flexibility. As a result, the person cannot see nearby objects clearly (the near point shifts farther away). Presbyopia is also called old-age long-sightedness.

Correction: Presbyopia is corrected using convex (converging) lenses for reading. Since the person also often has myopia with age, bifocal lenses are commonly used — the upper part has concave lens (for distance) and the lower part has convex lens (for near vision).

Explanation

Defect: The person is suffering from Myopia (Nearsightedness) because they cannot see distant objects clearly. Their far point is 40 cm.

Corrective Lens: A concave lens is required. The lens should form the image of a distant object (at infinity) at the person's far point (40 cm = 0.40 m).

Power of lens: P = 1/f = 1/(−0.40 m) = −2.5 D

The negative sign confirms it is a concave (diverging) lens.

Explanation

Structure of the Human Eye: The human eye is a roughly spherical organ. Light enters through the cornea, passes through the pupil (controlled by iris), is refracted by the lens, and falls on the retina.

• Cornea: The transparent, curved front surface of the eye. It is responsible for most of the refraction (bending) of light entering the eye. About 75–80% of refraction occurs at the cornea.
• Iris: The coloured, ring-shaped muscular diaphragm behind the cornea. It controls the size of the pupil and thereby regulates the amount of light entering the eye.
• Eye Lens (Crystalline lens): A transparent, biconvex, flexible lens made of layers of protein fibres. It fine-tunes the focus and helps in accommodation — changing shape to focus objects at different distances.
• Retina: The light-sensitive screen at the back of the eye. It contains photoreceptor cells called rods (sensitive to dim light/black-white vision) and cones (sensitive to colour/bright light). The retina converts light into electrical signals sent to the brain via the optic nerve.

Note: Draw a cross-section diagram of the eye labelling cornea, aqueous humour, iris, pupil, lens, vitreous humour, retina, optic nerve and blind spot for full marks in the exam.

Explanation

Formation of Rainbow: A rainbow is formed by the combined effect of refraction, total internal reflection, and dispersion of sunlight by water droplets suspended in the atmosphere after rain.

• Sunlight enters a spherical water droplet and gets refracted (dispersed into colours).
• The dispersed light undergoes total internal reflection at the back inner surface of the droplet.
• When the light exits the droplet, it is again refracted, which further spreads out the colours.
• Violet light exits at about 40° and red light at about 42° to the incoming sunlight. This angular separation creates the arc of colours we see as a rainbow.

Conditions for observing a rainbow:

• The sun must be behind the observer.
• Water droplets (rain, mist) must be present in the sky in front of the observer.
• The sun must be at a low angle (usually during morning or afternoon).

Explanation

• (a) Definition: Myopia — cannot see distant objects clearly. Hypermetropia — cannot see nearby objects clearly.
• (b) Cause: Myopia — eyeball too long or lens too convex. Hypermetropia — eyeball too short or lens too flat (less convex).
• (c) Image formation: Myopia — image of distant object forms in front of retina. Hypermetropia — image of nearby object forms behind the retina.
• (d) Correction: Myopia — corrected by concave (diverging) lens. Hypermetropia — corrected by convex (converging) lens.

Explanation

Tyndall Effect: When a beam of light passes through a colloidal solution or a medium containing tiny suspended particles, the path of light becomes visible due to the scattering of light by the particles. This phenomenon is called the Tyndall effect. It is named after the scientist John Tyndall.

Daily life examples:

• Blue colour of the sky: The atmosphere contains tiny gas molecules and dust particles. Sunlight passing through it gets scattered. Blue light (shorter wavelength) is scattered much more than red light, making the sky appear blue when we look at it from any direction.
• Visible beam of headlights in fog: When a car's headlight shines through fog, the path of light becomes visible because the fine water droplets in the fog scatter the light in all directions, making the beam visible — a classic example of the Tyndall effect.

Explanation

Dispersion through a Prism: When white light passes through a glass prism, it splits into a band of colours called the spectrum. The colours formed are — Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR). Violet is deviated the most; red is deviated the least.

Why it happens: Different colours of light have different wavelengths and hence different speeds in glass. Violet (shortest wavelength) slows down the most and bends the most; red (longest wavelength) bends the least. This difference in bending is called dispersion.

Newton's Experiment:

• Newton passed sunlight through a prism and obtained a spectrum on a screen — proving white light is composed of seven colours.
• He then placed an inverted second prism in the path of the spectrum. The second prism recombined all seven colours back into white light — proving the colours were originally part of white light, not added by the prism.

Angle of Deviation: The angle between the incident ray and the emergent ray (after passing through the prism) is called the angle of deviation. It is different for different colours of light — maximum for violet and minimum for red. This difference in the angle of deviation is what causes the spreading out (dispersion) of colours.

Explanation

There are three main defects of vision:

1. Myopia (Short-sightedness):

• Cause: Excessive curvature of eye lens or elongated eyeball.
• Image: Image of a distant object is formed in front of the retina.
• Correction: Concave (diverging) lens of suitable power. It diverges parallel rays so the eye lens can focus them on the retina.

2. Hypermetropia (Long-sightedness / Far-sightedness):

• Cause: Eye lens is too flat or eyeball is too short.
• Image: Image of a nearby object is formed behind the retina.
• Correction: Convex (converging) lens of suitable power. It converges rays before entering the eye so they can be focused on the retina.

3. Presbyopia (Age-related):

• Cause: Weakening of ciliary muscles and reduced flexibility of eye lens due to ageing.
• Image: Cannot focus on nearby objects; near point moves farther away.
• Correction: Convex lens for near vision. Bifocal lenses are used if the person also has myopia.

Note: For the board exam, draw two separate ray diagrams for myopia (showing image in front of retina and correction with concave lens) and hypermetropia (showing image behind retina and correction with convex lens). Label all parts clearly.