Class 10 Science Notes Chapter 10 The Human Eye and the Colourful World: Complete Study Notes

The human eye is one of the most important sense organs, allowing us to see the colourful world around us. Have you ever wondered how your eyes help you read a book, recognize faces, or enjoy the beautiful colours of a rainbow? Have you noticed that stars twinkle at night, while planets shine steadily, or wondered why the sky appears blue? These are all interesting phenomena related to light and the human eye. 

In Class 10 Science Notes Chapter 10 The Human Eye and the Colourful World, you will find a complete explanation of the structure and working of the human eye, the common defects of vision and their correction, and important optical phenomena such as refraction through a prism, dispersion of light, rainbow formation, atmospheric refraction, and scattering of light. 

A clear understanding of this chapter helps students build a strong foundation in optics and prepares them for higher studies as well as competitive examinations. 

Topics Covered in Class 10 Science Notes Chapter 10 The Human Eye and the Colourful World

The Human Eye	Dispersion of White Light by a Glass Prism
Structure of the Human Eye	Spectrum of White Light (VIBGYOR)
Power of Accommodation	Newton's Experiment
Defects of Vision and Their Correction	Rainbow Formation
Myopia	Atmospheric Refraction
Hypermetropia	Twinkling of Stars
Presbyopia	Advanced Sunrise and Delayed Sunset
Refraction of Light Through a Prism	Scattering of Light

Class 10 Science Chapter 10 The Human Eye and the Colourful World Complete Notes

The Human Eye

The human eye is one of the most important sense organs. It allows us to see the shape, size, colour, and movement of objects around us. While our other senses help us identify objects through touch, smell, taste, or sound, only our eyes enable us to observe colours and details clearly.

Structure of the Human Eye 

Important Parts of the Human Eye and Their Functions

Part	Function
Cornea	The transparent front part of the eye. It allows light to enter and performs most of the refraction.
Aqueous Humour	A clear fluid between the cornea and the lens. It nourishes the eye and helps maintain its shape.
Iris	The coloured muscular part of the eye that controls the size of the pupil.
Pupil	The opening in the iris through which light enters the eye.
Crystalline Lens (Eye Lens)	Focuses light onto the retina. Its shape changes with the help of ciliary muscles to view objects at different distances.
Ciliary Muscles	Change the curvature of the eye lens, allowing the eye to focus on near and distant objects.
Vitreous Humour	A transparent jelly-like substance that fills the space between the lens and the retina and helps maintain the eyeball's shape.
Retina	The light-sensitive layer where the image is formed. It contains photoreceptor cells that convert light into electrical signals.
Optic Nerve	Carries electrical signals from the retina to the brain, where the image is interpreted.

Power of Accommodation

The power of accommodation is the ability of the eye lens to change its focal length with the help of ciliary muscles, allowing us to see both nearby and distant objects clearly.

• Distant objects: Ciliary muscles relax, the lens becomes thin, and the focal length increases.
• Nearby objects: Ciliary muscles contract, the lens becomes thicker, and the focal length decreases.

Near Point and Far Point

• Near Point: The closest distance for clear vision without strain (25 cm for a normal eye).
• Far Point: The farthest distance for clear vision (Infinity for a normal eye).

Cataract

With age, the crystalline lens may become cloudy, causing cataract, which can lead to blurred vision. It is usually treated through cataract surgery.

Defects of Vision and Correction

Myopia 

Myopia is a defect in which a person can see nearby objects clearly but distant objects appear blurred. In a myopic eye, the image of a distant object is formed in front of the retina.

Myopia and Its Correction

Causes: Excessive curvature of the eye lens. Elongation of the eyeball

Correction: Corrected using a concave (diverging) lens, which shifts the image back onto the retina.

Hypermetropia

Hypermetropia is a defect in which a person can see distant objects clearly but nearby objects appear blurred. In this condition, the image of a nearby object is formed behind the retina.

Hypermetropia and Its Correction 

Causes: Focal length of the eye lens is too long. Eyeball is shorter than normal.

Correction: Corrected using a convex (converging) lens, which helps focus the image on the retina.

Presbyopia

Presbyopia is an age-related vision defect in which the power of accommodation decreases, making it difficult to see nearby objects clearly. It occurs due to the weakening of the ciliary muscles and reduced flexibility of the eye lens.

Presbyopia and Bifocal Lens

Correction: Corrected using convex lenses for near vision. People with both myopia and hypermetropia may require bifocal lenses:

• Upper part (Concave lens): For distant vision
• Lower part (Convex lens): For near vision

Refraction of Light through a Prism 

A triangular glass prism refracts light because its two refracting surfaces are inclined to each other. The angle between these two surfaces is called the angle of the prism. Unlike a rectangular glass slab, the emergent ray from a prism is not parallel to the incident ray.

When a light ray passes through a prism:

• At the first surface (air to glass), the ray bends towards the normal.
• At the second surface (glass to air), the ray bends away from the normal.
• Due to the shape of the prism, the emergent ray deviates from its original path.

Angle of Deviation: The angle of deviation (∠D) is the angle between the direction of the incident ray and the emergent ray.

• A prism has two triangular bases and three rectangular lateral faces.
• The angle between the two refracting surfaces is called the angle of the prism (∠A).
• Light bends towards the normal while entering the prism and away from the normal while leaving it.
• The emergent ray is deviated from the original path due to the prism's shape.

Dispersion of White Light by a Glass Prism 

When white light passes through a glass prism, it splits into seven colours. This phenomenon is called dispersion of light. The band of colours formed is known as the spectrum.

Spectrum of White Light (VIBGYOR)

The seven colours of the spectrum are: V (Violet), I (Indigo), B (Blue), G (Green), Y (Yellow), O (Orange) and R (Red)

Why Does Dispersion Occur?

Different colours of light bend by different amounts while passing through a prism.

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

As a result, the colours separate and form a spectrum.

Newton's Experiment

Isaac Newton showed that white light is made up of seven colours. He passed the spectrum through a second inverted prism, which recombined the colours to produce white light again.

Rainbow Formation

A rainbow is a natural spectrum formed after rainfall due to the dispersion of sunlight by tiny water droplets in the atmosphere.

• Water droplets act like tiny prisms.
• Sunlight is refracted, dispersed, internally reflected, and refracted again before reaching the observer.
• A rainbow is always seen opposite to the Sun.

Atmospheric Refraction

Atmospheric refraction is the bending of light as it passes through different layers of the Earth's atmosphere, which have varying densities and refractive indices. This causes the apparent position of objects to change.

Twinkling of Stars

Stars appear to twinkle because their light is continuously refracted by the Earth's atmosphere before reaching our eyes. Since stars are very far away and act as point sources of light, small changes in the atmosphere make their brightness appear to fluctuate.

Advance Sunrise and Delayed Sunset

Due to atmospheric refraction, the Sun appears about 2 minutes earlier than the actual sunrise and remains visible for about 2 minutes after the actual sunset. The same phenomenon also causes the apparent flattening of the Sun's disc near the horizon

Scattering of Light

Scattering of light is the phenomenon in which light is redirected in different directions when it strikes tiny particles present in a medium. It is responsible for many natural phenomena, such as the blue colour of the sky, reddening of the Sun at sunrise and sunset, and the colour of deep sea water.

Tyndall Effect 

The Tyndall effect is the scattering of light by colloidal particles, making the path of a light beam visible. It can be observed when sunlight enters a smoke-filled room or passes through a dense forest with tiny water droplets. 

Why is the colour of the clear sky is blue? 

The atmosphere contains tiny air molecules and dust particles that scatter shorter wavelengths of light (blue) more than longer wavelengths (red). As a result, blue light reaches our eyes from all directions, making the sky appear blue.

If there were no atmosphere, there would be no scattering, and the sky would appear dark, as seen from space.

Why are danger signals red?

Red light has the longest wavelength among visible colours, so it is scattered the least by fog, smoke, or dust. This allows it to travel longer distances and remain clearly visible, which is why danger and stop signals are red.