The Human Eye and the Colourful World Class 10 Notes

The Human Eye and the Colourful World Class 10 Notes

In this chapter, we apply the concepts of optics to investigate some natural optical phenomena. We’ll also talk about rainbow formation, the splitting of white light, and the blue colour of the sky. Read along for complete notes of the class 10 chapter, the human eye and colourful world.

The Human Eye

The human eye is a very treasurable and sensitive sense organ. It enables us to appreciate the beautiful world and colours that surround us. When we close our eyes, we can identify objects based on their smell, sound, taste, or touch to some extent. It is, however, impossible to identify colours while closing one’s eyes. As because it allows us to see beauty, the human eye is the most important sense organ. The human eye works in the same way that a camera does. The cornea, a thin membrane that forms the transparent bulge on the front of the eyeball, allows light to enter the eye. 

The eyeball has a diameter of around 2.3 cm and is roughly spherical in shape. Almost all of the light rays entering the eye undergo refraction on the cornea’s outer surface. The crystalline lens merely offers the finer focal length adjustment needed to focus things at various distances on the retina. Behind the cornea, we discover a structure known as the iris. The black, muscular diaphragm known as the iris regulates pupil size. The amount of light entering the eye is regulated and controlled by the pupil.

On the retina, the eye lens creates an inverted real image of the object. Many light-sensitive cells make up the delicate membrane known as the retina. When illuminated, the light-sensitive cells become active and produce electrical signals. The optic nerves transmit these signals to the brain. The brain decodes these impulses, which then analyses the data to allow us to see things as they are.

Power of Accommodation 

Accommodation refers to the ability of the eye lens to adjust its focal length.

The lens of the eye is made of fibrous material. The ciliary muscles have some control over their curvature. When they relax, the lens thins, increasing the focal length. This allows us to see distant objects clearly. However, the focal length of the eye lens cannot be increased beyond a certain limit.

People’s crystalline lenses can become milky and cloudy as they age. This is referred to as cataracts. This results in partial or complete vision loss. Cataract surgery can help you regain your vision.

The eye’s capacity to focus on both close and far objects by adjusting the eye’s focal length is referred to as accommodation.

The eye’s near point, or the least distance of distinct vision, is the shortest distance at which the eye can see objects clearly without strain. It is approximately 25 cm for a young adult with normal vision.

Visual Deficiencies and Their Corrections

There are three types of common refractive vision defects, and spherical lenses are used to correct these flaws. There are three types of myopia: i) Presbyopia, ii) hypermetropia, iii) and myopia.


Myopia is also known as short-sightedness. It is a condition in which which the picture of a distant object forms in front of the retina rather than on it. This flaw is fixed by using a concave lens with sufficient power to return the image to the retina.


A person with hypermetropia can clearly see distant objects but not nearby objects. It occurs because rays of light from nearby objects focus at a point behind the retina.


The ability of the eye to accommodate decreases with age. Presbyopia is the medical term for this defect. Concave and convex lenses are used in bifocal lenses. The upper portion is made up of a concave lens, and the lower part is a convex lens, which aids in distant vision.

Light Refraction Through a Prism

A triangular prism made of glass has three rectangular lateral surfaces and two triangular bases. These surfaces are slanted toward one another. The angle of the prism is formed by its two lateral faces.

A Glass Prism Disperses White Light

A light beam’s spectrum is the band of its coloured components. All of the colours might not be visible on their own. The colour sequence will be easier to remember if you utilise the initials VIBGYOR. White light is a general term for any light with a spectrum similar to that of sunlight. The dividing of light into its component colours is known as light dispersion.

Atmospheric Refraction

A star’s twinkle is caused by the atmospheric refraction of starlight. When starlight enters the earth’s atmosphere, it undergoes continuous refraction before reaching the earth.

Atmospheric refraction occurs in a medium with a varying refractive index.

Planets are much closer to Earth and thus considered extended sources. Let’s consider a planet to be a collection of many point-sized light sources. The total variation in the amount of light entering our eye from all of the individual point-sized sources will average out to zero, effectively eliminating the twinkling effect.

The Early Sunrise and the Late Sunset

Due to atmospheric refraction, we can see the Sun about 2 minutes before sunrise and 2 minutes after sunset. We mean the Sun’s actual crossing of the horizon when we say “actual sunrise.” The observations depict the actual and apparent positions of the Sun with respect to the horizon. The difference in time between the actual and apparent sunsets is approximately 2 minutes. This occurrence also causes the Sun’s disc to flatten at sunrise and sunset.

Scattering of Light

The interaction of light with the objects around us causes a variety of spectacular natural phenomena. Some of the wonderful occurrences we are familiar with include the blue colour of the sky, the colour of deep-sea water, and the reddening of the Sun at sunrise and sunset. 

The path of a light beam passing through a true solution is invisible. However, its path becomes clear in a colloidal mix where the particle size is relatively large.

Tyndall Effect

The Earth’s atmosphere is a complex mixture of minute particles. The Tyndall effect can be described as the dispersion of light by colloidal particles. This phenomenon has been observed when a smooth beam of sunlight arrives in a smoke-filled space through a small hole. After being sparsely reflected by these particles, the light reaches us.

A scattering of light causes the particles to become visible. The Tyndall effect is visible when light travels through some heavily forested canopy. Light is dispersed here by tiny water drops in the mist.

The size of the scattering particles influences the colour of the scattered light. Blue light is mainly scattered by very small particles, whereas bigger particles disperse visible light wavelengths.

Why Is the Clear Sky Blue in Colour?

Small particles in the air spread blue (shorter wavelengths) extra strongly than red when the Sun’s rays travel through the atmosphere (longer wavelengths). If the earth’s surface does not have an atmosphere, there would be no scattering. Even though scattering is minimal at such high altitudes, travellers interpret the sky as dark.

Red is the least affected by fog or smoke. As a result, it seems to have the same colour from a distance.

Colour of the Sun at Sunrise and Sunset

the Sun’s rays close to the horizon travel through thicker layers of air and a considerable distance in the earth’s atmosphere before reaching our eyes.

Because only a small portion of blue and violet light is spread thinly at noon, the Sun appears white. Near the horizon, the particles scatter most of the blue light and shorter wavelengths. Consequently, the light of longer wavelengths reaches our eyes. the Sun thus appears reddish.

We hope these human eye class 10 notes helped you understand better the various optical phenomena in nature. We also discussed rainbow formation, the splitting of white light and the blue colour of the sky.