Human Eye

The eyeball is located in our eye socket, and its average diameter is about 2 to 2.5 cm. The human eye helps us see and understand the world around us.

Different Parts of the Human Eye and Their Functions

1. White Circle / Cornea:

• It is a transparent and curved membrane located at the front part of the eye.
• Light first enters the eye through the cornea.
• Most of the refraction of light entering the eye takes place on the outer surface of the cornea.
• The cornea not only helps bend light but also protects the eye.

2. Lens:

• It is a convex lens that converges light rays onto the retina.
• The lens is made of a fibrous, jelly-like transparent substance.
• The most important function of the lens is to make fine adjustments in focal length to see objects clearly at different distances.

3. Iris:

• Located behind the cornea, it is a dark-colored muscular diaphragm.
• The function of the iris is to control the size of the pupil, thus regulating the amount of light entering the eye.

4. Pupil:

• The pupil is a small aperture located at the center of the eye.
• It controls the amount of light that enters the eye.
• In dim light, the pupil dilates, and in bright light, it contracts.

 

5. Retina:

• The retina is a thin and delicate membrane located in the inner part of the eye.
• It contains light-sensitive cells (photoreceptor cells) that get activated when light falls on them.
• These cells generate electrical signals that travel to the brain through the optic nerve.
• The brain interprets these signals, enabling us to see objects.

 

Far Point

The farthest point up to which an eye can see objects clearly is called the far point of the eye.For a normal eye, it is at infinity.

 

Near Point

The minimum distance at which an object can be seen clearly without strain is called the near point of the eye.

• For an adult with normal vision, the near point is about 25 cm from the eye.
• It is also known as the least distance of distinct vision.

 

Accommodation

The ability of the eye lens to adjust its focal length to see objects clearly at different distances is called accommodation.

• The curvature of the lens is controlled by the ciliary muscles.

Action of Ciliary Muscles:

1. When ciliary muscles relax:

• The lens becomes thin.
• The focal length increases.
• This process helps in viewing distant objects.

2. When ciliary muscles contract:

• The lens becomes thick.
• The focal length decreases.
• This process helps in viewing nearby objects.

 

Defects of Vision and Their Correction

Cataract

In some elderly people, the crystalline lens of the eye becomes cloudy or milky, making it difficult for light to pass through. This condition is called a Cataract.

Effect:

• Vision may become blurred or completely lost.

Correction:

• The affected lens is removed by surgery and replaced with an artificial lens.
• After surgery, normal vision can be restored.
• Timely treatment of cataract is important to prevent vision loss.

 

Myopia (Near-Sightedness)

A person with myopia can see nearby objects clearly but not distant ones.

The far point of such a person is not at infinity but closer to the eye.

Causes:

(i) Excessive curvature of the eye lens.

(ii) Elongation of the eyeball.

Correction:

This defect can be corrected using a concave lens of suitable power.

 

Hypermetropia (Long-Sightedness)

A person with hypermetropia can see distant objects clearly but not nearby ones.

The near point of such a person lies farther than the normal near point (25 cm).

Causes:

(i) The focal length of the eye lens becomes too long.

(ii) The eyeball becomes shorter.

Correction:

This defect can be corrected using a convex lens of suitable power.

 

Presbyopia

With advancing age, the accommodation power of the human eye decreases.

In most people, the near point moves farther away.

This defect is known as Presbyopia.

Cause:

It occurs due to the weakening of the ciliary muscles and the loss of elasticity of the crystalline lens.

Correction:

By using convex lenses.

Sometimes, a person suffers from both myopia and hypermetropia. Such people require bifocal lenses, whose upper part is concave and lower part is convex.

 

Refraction of Light Through Prism

A prism has two triangular bases and three rectangular lateral surfaces.

Angle of Prism: The angle between two refracting surfaces of a prism.

Angle of Deviation: The angle between the incident ray and the emergent ray.

Dispersion of White Light by Glass Prism

When sunlight passes through a glass prism, it splits into a band of seven colors.

These seven colors are Violet, Indigo, Blue, Green, Yellow, Orange, and Red.

This band of colors is called a Spectrum.

Dispersion: The splitting of white light into its component colors is called dispersion.

• VIBGYOR : VIBGYOR is an abbreviation that helps remember the seven colors of the spectrum:
  
  • V – Violet
  • I – Indigo
  • B – Blue
  • G – Green
  • Y – Yellow
  • O – Orange
  • R – Red

 

• After passing through a prism, different colors bend at different angles relative to the incident ray.
• Red light bends the least, while violet light bends the most.
• Isaac Newton was the first to obtain the spectrum of sunlight using a glass prism. He then used a second similar prism in an inverted position relative to the first one and observed that white light emerged again. He concluded that sunlight is composed of seven colors.

 

Rainbow Formation

• A rainbow is a natural spectrum visible in the sky after rainfall due to refraction, dispersion, and internal reflection of sunlight in water droplets present in the atmosphere.
• It always appears in the direction opposite to the Sun.
• Tiny water droplets act like small prisms. Sunlight entering these droplets is refracted, dispersed, and then internally reflected before emerging out again.

As a result, different colors reach the observer’s eyes.

 

Twinkling of Stars

A distant star appears as a point source of light. As the star’s light passes through the Earth’s atmosphere, it undergoes continuous refraction. Due to variations in the refractive index of air layers, the apparent position of the star keeps changing. Hence, the amount of light entering our eyes fluctuates, and the star appears to twinkle.

 

Advanced Sunrise and Delayed Sunset

Due to atmospheric refraction, the Sun becomes visible about 2 minutes before the actual sunrise and remains visible for about 2 minutes after the actual sunset.

Atmospheric Refraction

The refraction of light caused by the Earth’s atmosphere due to varying air densities is called atmospheric refraction.

Effects of Atmospheric Refraction:

• Twinkling of stars
• Early sunrise and delayed sunset
• Stars appearing slightly higher than their actual position
• Apparent shifting of objects seen through hot air
  • The air just above a flame is hotter and less dense than the air above it, having a lower refractive index. Because air is not a uniform medium, the apparent position of objects changes when seen through hot air.

 

Scattering of Light

Tyndall Effect

When a beam of light passes through fine particles such as dust, smoke, water droplets, or air molecules, its path becomes visible.

This phenomenon of light scattering by colloidal particles is called the Tyndall Effect.

Examples:

• When a beam of light enters a smoky room through a small hole, the Tyndall effect can be observed.
• Sunlight streaming through dense forest foliage also shows the Tyndall effect.

 

Rayleigh’s Law

The scattering of light is inversely proportional to the fourth power of its wavelength (λ⁴).

Thus, Scattering ∝ 1 / λ⁴.

• Colors with shorter wavelengths (blue and violet) scatter more.
• Colors with longer wavelengths (red) scatter less.

Dependence on Particle Size:

(i) Very fine particles scatter mainly blue light.

(ii) Larger particles scatter light of longer wavelengths.

(iii) If particles are very large, the scattered light appears white.

 

Question: Why is the sky blue?

Answer:

When sunlight passes through the atmosphere, tiny gas molecules scatter shorter wavelengths (blue light) more than longer ones. Hence, the sky appears blue.

 

Question: Why does the sky appear black to astronauts at high altitude?

Answer:

At high altitudes, the atmosphere is very thin, so there is almost no scattering.

Light travels directly to the eyes, and the sky appears black.

 

Question: Why do clouds appear white?

Answer:

Clouds consist of tiny water droplets larger than the wavelength of visible light.

These droplets scatter all colors equally, making the light appear white.

 

Question: Why do planets not twinkle?

Answer:

Planets do not twinkle because they do not emit light themselves but reflect sunlight.

They are much closer to Earth and appear as extended sources of light, not point sources.

Due to this, atmospheric refraction has a negligible effect on them, and they do not twinkle.

 

Question: Why is the danger signal red in color?

Answer:

According to Rayleigh’s law, red light has the longest wavelength and hence is least scattered.

• Because of its long wavelength: Red light scatters the least.
• It is visible clearly even through fog, mist, or smoke.

Therefore, red color is used for danger signals.