The Human Eye and The Colorful world Class 10th Notes- Free NCERT Class 10 Science Chapter 11 Notes - Download PDF

Ever wondered how we see the world around us or why the sky looks blue and sunsets appear red? All these answers lie in Class 10 Science Chapter 10: The Human Eye and the Colourful World. This chapter explains how the human eye works, how we see objects and what are the reasons for beautiful phenomena such as rainbows, twinkling stars and the colours in the sky. It is a very important chapter for board exams and science olympiads like NSO and NSEJS etc. These NCERT notes for class 10 science are prepared by subject experts as per the latest CBSE syllabus.

These Class 10 NCERT notes are written in easy to understand language and cover important topics such as the structure and functioning of the human eye, power of accommodation, defects of vision and their correction using lenses. These NCERT Notes also explain concepts like refraction through a prism, dispersion of light, and scattering of light using real-life examples.

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• NCERT Solutions for Class 10 Science Chapter 10 Human Eye and Colourful World
• NCERT Exemplar Class 10 Science chapter 10 Human Eye and Colourful World

The Human Eye

• It is one of the most sensitive sense organs that is present in living organisms
• The image is formed on a screen that is light sensitive, known as the Retina.
• A thin membrane in the eye through which light enters the eye is known as the Cornea.
• Eyeballs are spherical.
• The presence of the lens helps in adjusting objects at different distances on the retina.
• The dark muscular structure which controls the size of the pupil is Iris. Hence, the pupil regulates the amount of light entering the eye.
• Real, inverted images are formed on the retina. It contains light-sensitive cells Rods and Cons. These cells send signals to the brain with the help of the optic nerve

Power of Accommodation

The focal lens of the lens in the eye can be decreased or increased. The eye lens gets thin when muscles are relaxed. This results in increased focal length. It helps to see distant objects. And when the eye lens becomes thick, the focal length is decreased and we see nearby objects.

Power of accommodation is the ability of the eye lens to adjust the focal length.

• The near Point of the Eye is at 25 cm and the Far Point of the Eye is infinity.

• In old age, the lens becomes milky and cloudy, this is called a Cataract. This leads to complete or partial loss of vision.

Defects of Vision and their Correction

The three common defects of vision are:

Myopia (Nearsightedness): In this, distant objects cannot be seen clearly, but nearby objects are visible.

Light from a far object forms an image before it reaches the retina.

Corrected using concave lens.

Hypermetropia (Far-sightedness): Nearby objects are seen clearly whereas distant objects can not be seen clearly.

Image of a nearby object is formed behind the retina.

Corrected using a convex lens.

Presbyopia: The power of accommodation decreases with ageing. They have difficulty in near vision as well as far vision. Bifocal lenses are used to correct the defects.

Refraction of Light Through a Prism

A glass prism is a refracting medium with two or more non-parallel refracting surfaces. It is named according to the shape of its base. The prism shown below is a triangular prism which has two triangular bases and three rectangular lateral surfaces. For a triangular glass prism, the angle between the non-parallel (lateral) refracting surfaces is known as the angle of the prism denoted by $A$.

When light passes through a prism as shown in figure below, it gets deviated and the angle between the emergent ray and the incident ray is called angle of deviation ( $\delta$ ).

Dispersion of White Light By A Prism

When light falls on the prism, the prism splits the incident light into a band of seven colours. The colours observed are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). The band of colour is Spectrum. Thus, the splitting of incident light into different colours is Dispersion. The splitting is because of the bending of light rays at different angles. Violet light bends the most, whereas red light bends the least.

Recombination of spectrum of white light-

1 more prism is placed in an inverted position to get back the white light

The phenomenon of a rainbow is also because of the dispersion of light.

Atmospheric Refraction

With height, the layers of the atmosphere changes, so does its optical density, hence light on entering the earth's atmosphere undergoes refraction continuously. The refraction of light by the earth's atmosphere is termed as atmospheric refraction. Now, let us see few phenomena occurring in nature due to the atmospheric refraction of light.

Twinkling of Stars

When a star is lit in the atmosphere, it has to undergo refraction. The refractive index changes and the light bends towards the normal. As a result, the actual position of the star is a little bit different from its apparent position. We also know that the atmosphere of the earth is not stationary, so changes in the apparent position of the stars occur. Thus, we see them twinkling in the sky or the Earth’s atmosphere.

Advance Sunlight and Delayed Sunset

Due to the phenomenon of atmospheric refraction, the sun is visible 2 minutes before the actual sunrise, which is known as advanced sunlight. Just like the sunrise, the Sun is also visible 2 minutes after the actual sunset has taken place. This is known as Delayed Sunset.

Scattering of Light

When sunlight falls on a mixture of minute particles (like smoke, tiny water droplets, suspended particles of dust and molecules of air in the earth's atmosphere), it gets scattered in all directions, if the size of the particle is comparable to the wavelength of light. These particles are more effective in scattering light of shorter wavelength at the blue end than light of longer wavelength at the red end.

Tyndall Effect

When a beam of light strikes tiny water droplets and suspended particles of dust the path of the beam becomes visible. This is the Tyndall Effect. Fine particles scatter, and particles of larger size scatter light of longer wavelengths.

Why does the sky appear blue?

The colour of the sky appears blue because of the scattering of light. When the sunlight passes through the atmosphere, the fine particles present in the air will scatter the blue colour since it has a shorter wavelength.

Why does the sky appear red during sunrise and sunset?

When the Sun rises and sets, its light which is present near the horizon passes through thicker layers of atmosphere and travels larger distances before reaching our eyesight. When the light of the Sun present overhead travels a comparatively shorter distance than light from the horizon, it results in the white appearance of the sun. Near the horizon, most of the blue light and shorter wavelengths are scattered by the particles. Therefore, the red light that reaches our eyes as it has the longest wavelength, hence the sky appears reddish.

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