Chapter Notes

THE HUMAN EYE

The human eye is a remarkable and sensitive sense organ that allows us to perceive the world in colour. While other senses like smell, taste, or touch can help us identify objects, our eyes are uniquely capable of identifying colours, making them one of our most significant sense organs.

Functionally, the human eye works much like a camera. It has a lens system that focuses light to form an image on a light-sensitive screen called the retina. Let's explore the key parts and their functions:

• Cornea: This is a thin, transparent membrane that forms a bulge on the front surface of the eyeball. When light enters the eye, most of the refraction (bending of light) happens at the outer surface of the cornea.
• Eyeball: The eyeball is roughly spherical, with a diameter of about $2.3 \text{ cm}$.
• Iris: Located behind the cornea, the iris is a dark, muscular diaphragm. Its primary job is to control the size of the pupil.
• Pupil: The pupil is the opening in the center of theiris. It regulates and controls the amount of light that enters the eye. In bright light, the iris makes the pupil smaller, and in dim light, it makes it larger.
• Eye Lens: This is a crystalline lens located behind the pupil. While the cornea does most of the heavy lifting for refraction, the eye lens provides the fine adjustments to the focal length needed to focus objects at different distances onto the retina.
• Retina: The retina is a delicate membrane at the back of the eye that acts like the film in a camera. It contains a huge number of light-sensitive cells. When light hits these cells, they become activated and generate electrical signals.
• Optic Nerves: These nerves transmit the electrical signals from the retina to the brain. The brain then interprets these signals, allowing us to perceive the image of the object as it is.

The image formed by the eye lens on the retina is an inverted, real image. The brain processes this inverted image so that we see the world upright.

Power of Accommodation

The eye lens is made of a flexible, jelly-like material. Its shape, and therefore its focal length, can be changed by the action of the ciliary muscles. This ability of the eye lens to adjust its focal length to see both near and distant objects clearly is called accommodation.

• Viewing Distant Objects: When you look at something far away, the ciliary muscles are relaxed. This allows the eye lens to become thinner, which increases its focal length. This change enables the eye to focus the distant object clearly on the retina.
• Viewing Nearby Objects: When you look at an object close to you, the ciliary muscles contract. This contraction increases the curvature of the eye lens, making it thicker. As a result, the focal length of the eye lens decreases, which allows you to see the nearby object in sharp focus.

Near Point: The minimum distance at which an object can be seen clearly without any strain is called the least distance of distinct vision, or the near point of the eye. For a young adult with normal vision, the near point is about $25 \text{ cm}$.

Far Point: The farthest point up to which the eye can see objects clearly is called the far point. For a normal eye, the far point is infinity.

Therefore, a normal eye can see objects clearly anywhere between $25 \text{ cm}$ and infinity.

Sometimes, in old age, the crystalline lens can become milky and cloudy. This condition is called a cataract, and it can cause partial or even complete loss of vision. Fortunately, vision can often be restored through cataract surgery.

DEFECTS OF VISION AND THEIR CORRECTION

Sometimes, the eye can lose its power of accommodation, leading to blurred vision. These issues are known as refractive defects of the eye. The three most common defects are myopia, hypermetropia, and presbyopia. These can usually be corrected by using suitable lenses.

(a) Myopia

Myopia is also known as near-sightedness.

• Symptom: A person with myopia can see nearby objects clearly but struggles to see distant objects distinctly.
• Cause: This defect occurs because the image of a distant object is formed in front of the retina, not on it. This can happen for two reasons:
  1. Excessive curvature of the eye lens (it's too powerful).
  2. Elongation of the eyeball.
• Correction: Myopia is corrected by using a concave lens of a suitable power. The concave (diverging) lens spreads out the light rays before they reach the eye's lens, effectively moving the final image backward onto the retina.

(b) Hypermetropia

Hypermetropia is also known as far-sightedness.

• Symptom: A person with hypermetropia can see distant objects clearly but finds it difficult to see nearby objects distinctly. Their near point is farther than the normal $25 \text{ cm}$.
• Cause: This defect occurs because the light rays from a nearby object are focused at a point behind the retina. This happens because:
  1. The focal length of the eye lens is too long (it's too weak).
  2. The eyeball has become too small.
• Correction: Hypermetropia is corrected by using a convex lens of an appropriate power. The convex (converging) lens provides the additional focusing power needed to bend the light rays more sharply, bringing the image forward onto the retina.

(c) Presbyopia

Presbyopia is a defect that typically occurs with ageing.

• Symptom: Most people find their near point gradually recedes as they get older, making it difficult to see nearby objects comfortably without corrective glasses.
• Cause: This happens due to the gradual weakening of the ciliary muscles and the diminishing flexibility of the eye lens. The eye simply loses its power of accommodation.
• Correction: Sometimes, a person may suffer from both myopia and hypermetropia. In such cases, bi-focal lenses are required.
  • A common type of bi-focal lens has two parts. The upper portion is a concave lens to correct for myopia (facilitating distant vision).
  • The lower portion is a convex lens to correct for hypermetropia (facilitating near vision).

Today, refractive defects can also be corrected with contact lenses or through surgical procedures.

REFRACTION OF LIGHT THROUGH A PRISM

A triangular glass prism is a transparent object with two triangular bases and three rectangular lateral surfaces that are inclined to each other. The angle between its two lateral faces is called the angle of the prism.

When a ray of light passes through a prism, it is refracted (bent) twice: once when it enters the prism from air, and again when it leaves the prism back into the air.

• When the light ray enters the prism (from air to glass), it bends towards the normal.
• When the light ray exits the prism (from glass to air), it bends away from the normal.

Because of the prism's peculiar shape, the emergent ray (the ray that comes out) is bent at an angle to the direction of the incident ray (the ray that went in). This angle is known as the angle of deviation ($\angle D$).

DISPERSION OF WHITE LIGHT BY A GLASS PRISM

One of the most exciting phenomena shown by a glass prism is the splitting of white light into a band of colours. This process is called dispersion.

When a beam of white light (like sunlight) is passed through a glass prism, it splits into its seven component colours. This band of colours is called a spectrum. The sequence of colours, often remembered by the acronym VIBGYOR, is:

• Violet
• Indigo
• Blue
• Green
• Yellow
• Orange
• Red

Why does dispersion happen? Dispersion occurs because different colours of light bend through different angles as they pass through the prism.

• Violet light bends the most.
• Red light bends the least. As a result, the rays of each colour emerge along different paths and become distinct, forming the spectrum.

Sir Isaac Newton was the first to demonstrate this. He used a second, identical prism placed in an inverted position to intercept the spectrum from the first prism. He observed that the colours recombined, and a beam of white light emerged from the second prism. This observation led him to conclude that sunlight is made up of seven colours. Any light that produces a spectrum similar to sunlight is called white light.

ATMOSPHERIC REFRACTION

Atmospheric refraction is the refraction of light caused by the Earth's atmosphere. The air in our atmosphere is not uniform; its density and temperature vary at different altitudes. Hotter air is less dense and has a slightly lower refractive index than cooler, denser air.

When light travels through layers of air with changing refractive indices, it bends. This effect causes several interesting phenomena.

Twinkling of stars

The twinkling of stars is a direct result of atmospheric refraction.

• Starlight travels through space and enters the Earth's atmosphere, where it undergoes continuous refraction as it passes through layers of air with fluctuating densities.
• This refraction causes the light path to vary slightly. Since stars are so far away, they act as point-sized sources of light.
• As the path of the starlight changes, the apparent position of the star fluctuates, and the amount of light entering our eye flickers. Sometimes the star appears brighter, and other times fainter. This flickering is what we perceive as twinkling.

Why don't the planets twinkle? Planets are much closer to Earth and are seen as extended sources, not point sources. You can think of a planet as a collection of a large number of point-sized light sources. While the light from each individual point flickers due to atmospheric refraction, the total amount of light entering our eye from all the points averages out to zero. This nullifies the twinkling effect, so planets appear to shine with a steady light.

Advance sunrise and delayed sunset

Atmospheric refraction makes the Sun visible to us about 2 minutes before the actual sunrise and for about 2 minutes after the actual sunset.

• Actual sunrise is when the Sun is at the horizon.
• When the Sun is slightly below the horizon, its light travels through the atmosphere and is refracted downwards towards our eyes.
• Because of this bending, we see an apparent image of the Sun that is above the horizon, even when the actual Sun is still below it.
• The same phenomenon occurs at sunset, making the Sun appear to stay above the horizon for about two minutes after it has actually set. This effect also causes the Sun's disc to appear flattened at sunrise and sunset.

SCATTERING OF LIGHT

When light interacts with particles in its path, it can be redirected in various directions. This phenomenon is called scattering of light. The Earth's atmosphere is a mixture of tiny particles like smoke, water droplets, dust, and air molecules, which are perfect for scattering light.

Tyndall Effect

The Tyndall effect is the scattering of light by colloidal particles, which makes the path of a light beam visible.

• You can see this when a fine beam of sunlight enters a smoke-filled room through a small hole. The smoke particles scatter the light, making the beam visible.
• Another common example is when sunlight filters through the canopy of a dense forest. Tiny water droplets in the mist scatter the light, creating visible rays.

The colour of the scattered light depends on the size of the scattering particles:

• Very fine particles scatter shorter wavelengths (like blue light) more effectively.
• Larger particles scatter light of longer wavelengths.
• If the particles are large enough, the scattered light may even appear white.

Why is the colour of the clear Sky Blue?

The blue colour of the sky is a result of light scattering.

• The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light.
• These particles are much more effective at scattering light of shorter wavelengths (the blue and violet end of the spectrum) than light of longer wavelengths (the red end). In fact, red light has a wavelength about $1.8$ times greater than blue light.
• When sunlight passes through the atmosphere, the fine particles scatter the blue light in all directions. This scattered blue light then enters our eyes from all over the sky, making the sky appear blue.

This same principle explains why danger signals are red. Red light is scattered the least by fog or smoke, so it can travel the longest distance without being scattered away. This allows it to be seen clearly from a distance.