The Colourful World CBSE Class 10 Science

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Last Updated on September 9, 2024 by XAM CONTENT

The world around us is vibrant and full of colours, which not only makes life beautiful but also helps us perceive and understand our surroundings better. In Class 10 Science, the chapter “The Colourful World” delves into the fascinating phenomena that explain how we see colours, how light behaves, and the various optical effects caused by the interaction of light with objects and the atmosphere.

In this chapter, students explore key concepts such as the dispersion of light, the formation of rainbows, the scattering of light, and why the sky appears blue. Understanding these phenomena not only enhances our appreciation of natural beauty but also deepens our grasp of the fundamental principles of physics that govern the behaviour of light.

By learning about the splitting of white light into its constituent colours and the cause of atmospheric phenomena, this chapter helps students connect everyday experiences with scientific explanations. This knowledge is not only crucial for academic purposes but also sparks curiosity about the world around us.

Chapter	Human Eye and The Colourful World
Topic	The Colourful World
Covers	Learn about the topic Notes Important Questions FAQs
Board	CBSE
Class	10
Subject	Science
Useful for	Class 10 Studying Students
Important Link	Science Topics for CBSE Class 10

Understanding The Colourful World

Prism: A transparent material that has two triangular bases and three rectangular lateral surfaces which are inclined to each other at an angle lesser than 90°.

Angle of prism: The angle between two of the lateral surfaces of a prism is called angle of prism. ∠A or ∠BAC is the angle of prism in the figure shown.

Incident ray: PE is the incident ray.

Refracted ray: EF is the refracted ray.

Emergent ray: FS is the emergent ray.

Angle of incidence: It is the angle between the incident ray and normal to the surface of prism. ∠i is the angle of incidence.

Angle of refraction: It is the angle between refracted ray EF, and normal to the point of incidences. ∠r is the angle of refraction.

Angle of emergence: It is the angle between ray, FS and normal to the surface. ∠e is the angle of emergence.

Angle of deviation: It is angle between the incident ray and emergent ray. ∠d is the angle of deviation.

Refraction of light by a prism:

• Ray PE is entering from rarer to denser medium at surface AB, it undergoes refraction and bend towards the normal.
• EF is refracted ray, and FS travels from denser medium to rarer medium, therefore it bends away from the normal.
• ∠r is less than ∠i and ∠e is more than ∠r.

Dispersion of white light by glass prism:

The splitting of white light into its seven constituent colours is called dispersion. The colour sequence is Violet, Indigo, Blue, Green, Yellow, Orange and Red. This colour pattern is called visible spectrum. Red colour has the longest wavelength, while, violet colour has shortest wavelength.

Cause of dispersion:

The white light is made up of seven colours. Each of these colours travel at different speeds. So when they pass through the prism, they bend at different angles. So all the colours are visible. Red colour deviates the least and violet colour deviated the most because of the shortest wavelength of violet light.

Recombination of spectrum of white light:

• Newton used prism for the first time to get the spectrum of sunlight.
• He tried to split the colours of spectrum of white light or sunlight by using another prism, as shown in the figure.
• He observed that when all the colours were passed through the second prism, a beam of white light emerged out. This observation shows that light is made up of seven colours.

Rainbow Formation:

• It is a natural phenomenon based upon the dispersion of white light appearing in the sky after rain.
• It is caused by dispersion of sunlight by tiny water droplets, present in the atmosphere.
• Rainbow is always formed in the direction opposite to the sun.
• Small water droplets act like a small prism which refract and disperse the incident sunlight, then refract internally and finally refract the light again, when it comes out of the rain drop.
• It is due to internal reflection and dispersion of light, we observe different colours in a rainbow.

Atmospheric refraction:

The refraction of light caused by earth’s atmosphere, having different layers of different optical densities and refractive indices is called atmospheric refraction.

Twinkling of Stars:

• Stars emit their own light due to which they shine at night.
• Stars appear very bright for a moment and then becomes dim again and again; this phenomenon is called twinkling of stars.
• Twinkling of a star is caused by the scattering of light in the earth’s atmosphere
• Stars appear as twinkling point source of light because they are very far away from the earth
• Planets are closer to the earth as compared to the distance of stars. They appear as small disc of light. Each point on the disc can be considered as a point source of light. The different points produces either brighter or dimmer effect in such a way that the overall intensity of the light remains almost the same. Thus, the planets do not have twinkling effect.
• Due to atmospheric refraction, the stars also appear to be higher in the sky than they actually are.

Tyndall effect:

• The earth’s atmosphere contains minute particles like smoke, water droplets, suspended dirt and dust particles and other molecules in air, forming a colloidal solution.
• The phenomenon of scattering of light by colloidal particles is called Tyndall effect. The path of light becomes clearly visible due to this effect.
• The colour of the scattered light depends upon the size of colloidal particles.

• Blue light is scattered by very tiny particles of colloidal solution.
• Colours of higher wavelength are scattered by bigger sized colloidal particles

Blue colour of sky:

When sunlight passes through the atmosphere, fine particles in air scatter the blue colour (shorter wavelength) more than the red colour, that is why the sky appears blue in colour.

Red light:

Danger signals are red in colour because red colour is least scattered by fog or by smoke. It can be easily seen from a distance.

Colour of Sun at sunrise and sunset:

• Light from Sun near the horizon passes through thick layers of air and larger distance in earth’s atmosphere before reaching our eyes.
• Light from Sun overhead will travel shorter distance. At noon, Sun appears white as only a little portion of blue and violet colours are scattered.
• Near the horizon most of the blue light is scattered away by the particles, thus making sky blue in colour. Light which reaches our eyes is of longer wavelength, i.e. gives reddish appearance of Sun.

The Colourful World Important Questions

Multiple Choice Questions:

(i) At noon the sun appears white as
(a) light is least scattered
(b) all the colours of the white light are scattered away
(c) blue colour is scattered the most
(d) red colour is scattered the most

(ii) Which of the following phenomena of light are involved in the formation of a rainbow?
(a) Reflection, refraction and dispersion
(b) Refraction, dispersion and total internal reflection
(c) Refraction, dispersion and internal reflection
(d) Dispersion, scattering and total internal reflection

(iii) Twinkling of stars is due to atmospheric
(a) dispersion of light by water droplets
(b) refraction of light by different layers of varying refractive indices
(c) scattering of light by dust particles
(d) internal reflection of light by clouds

Answers:

(i) (b) It is due to scattering of all colours of light
(ii) (c) Refraction, dispersion and internal reflection
(iii) (b) refraction of light by different layers of varying refractive indices

Assertion-Reason Type Questions:

Select the correct answer to the question from the codes (i), (ii), (iii) and (iv) as given below:

(i) Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the assertion.
(ii) Both ‘A’ and ‘R’ are true but ‘R’ is not correct explanation of the assertion.
(iii) ‘A’ is true but ‘R’ is false.
(iv) ‘A’ is false but ‘R’ is true.

Question:

Assertion: Sun appears reddish in the morning. It will not be observed on moon.
Reason: It is because moon does not have atmosphere, therefore, light will not scatter.

Ans. (a) (i) Both ‘A’ and ‘R’ are true and ‘R’ is correct explanation of the assertion.

Very Short Answer Type Questions

1. What is prism?
2. Define dispersion of light. Name the scientist who discovered it.
3. On what factors the colour of scattered light by a colloid depends upon?
4. Which colour of white light suffers: (i) least deviation and (ii) maximum deviation when a beam of white light is passed through the prism?
5. Name the two phenomena involved in the formation of rainbow.
6. What is the cause of dispersion of light by prism?
7. Why do stars twinkle?
8. Explain why the planets do not twinkle.
9. Why does the Sun appear reddish early in the morning?
10. Why does the sky appear dark instead of blue to an astronaut?
11. How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light?
12. What is Tyndall effect?
13. Which phenomenon is responsible for making the path of light visible?

Answers:

1. Prism is a glass object with two triangular bases and three rectangular lateral surfaces inclined at an angle.
2. The splitting of white light into its constituent seven colours is called dispersion of light. Sir Issac Newton discovered dispersion of light.
3. It depends upon the size of colloidal particles which scatters light.
4. (i) Red, (ii) Violet
5. (i) Dispersion, (ii) Internal reflection
6. It is because different colours move with different speed in the prism and emergent ray bend to different extent towards the base of prism.
7. Stars twinkle because their light is coming from very far away and undergoes atmosphere refraction, while passing through different layers of the atmosphere having different optical density and refractive indices.
8. Planets are closer to the earth and we consider them as a collection of large number of point sized sources of light, so that the total variation in light is almost zero. Therefore, planets do not twinkle.
9. Sun is near the horizon, most of the blue light is scattered away by the atmospheric particles, while red light is least scattered and reaches our eyes, early in the morning.
10. At higher altitudes in the space, there is no (or least) atmosphere and therefore no (or least) scattering of light takes place, that is why sky appears dark.
11. When a second prism is placed inverted to the first prism showing dispersion of white light, recombines the constituents of the light emerges out from the second prism is white light as shown in the figure:

12. The scattering of light by colloidal particles is called Tyndall effect. The path of light is clearly visible when it passes through colloidal solution.

13. Tyndall effect

Read more

• Human Eye and The Colourful World Numerical Problems
• Human Eye and The Colourful World Assertion Reasoning

Also check

• Electricity Class 10 Case Study Questions Science Chapter 12
• The Human Eye and the Colourful World Class 10 Case Study Questions Science Chapter 11
• Metals and Non-metals Class 10 Case Study Questions Science Chapter 3
• Light – Reflection and Refraction Class 10 Case Study Questions Science Chapter 10
• Life Processes Class 10 Case Study Questions Science Chapter 6
• Acids Bases and Salts Class 10 Case Study Questions Science Chapter 2
• Chemical Reactions and Equations Class 10 Case Study Questions Science Chapter 1

Mind Map – The Colourful World

Quick Notes

• Refraction takes place through prism in a specific way.
• The angle between two lateral faces of a prism is called angle of prism.
• When white light passes through a prism, it splits into its constituent seven colours. Splitting of white light is called dispersion of light.
• VIBGYOR is an acronym that will help to remember the sequence of colours.
• The combination of light of different wavelength forms the spectrum. Newton was the first to get the spectrum of sunlight using a prism.
• Any light that gives spectrum similar to sunlight is called white light.
• Rainbow is a natural phenomenon, appearing in the sky after rain. It is caused by dispersion of sunlight by tiny water droplets present in the atmosphere, which act like a prism.
• The twinkling of stars is due to atmospheric refraction.
• Sun is visible to us about 2 minutes before the actual sunrise and about 2 minutes after the actual sunset due to atmospheric refraction.
• The blue colour of sky, colour of deep-sea water, red colour of sun at sunrise and at sunset is due to scattering of light.
• The path of light becomes visible in colloidal solution due to scattering of light by the bigger sized colloidal particles. It is called Tyndall effect.
• Very fine particles scatter blue light, while large sized particles scatter longer wavelengths of light.

Frequently Asked Questions (FAQs) on The Colourful World Class 10