Spectrum Solutions for ICSE Board Class 10 Physics

1. Name three factors on which the deviation produced by a prism depends and state how it depends on the factors stated by you.

Ans: The deviation produced by the prism depends on the four factors such as follows:

The angle of incidence: As the angle of the incidence increases first the angle of deviation decreases and reaches a minimum value for a certain angle of the incidence. Due to a further increase in the angle of incidence, there is an increase in the angle of deviation.

The material of the prism: For a given angle of incidence, the higher refractive index of prism produces a greater deviation than the prism with the lower refractive index.

The angle of prism: The angle of deviation increases when there is an increase in the angle of the prism.

The color or the wavelength of light used: The angle of deviation increases when there is a decrease in the wavelength of light.

2. How does the deviation produced by a triangular prism depend on the colour (or wavelength) of light incident on it? 

Ans: The deviation which is produced by a prism increases when there is a decrease in the wavelength of light incident on it.

3. How does the speed of light in glass change by increasing the wavelength of light?

Ans: When there is an increase in the wavelength the speed of light in glass also increases.

4. Which colour of white light travels (a) fastest (b) slowest, in glass?

Ans: The colour red travels fastest while the blue colour travels slowest in glass.

5. Name the subjective property of light related to its wavelength.

Ans: The subjective property of light is its colour which is related to its wavelength.

6. What is the range of wavelength of the spectrum of white light in 

(i) Å

Ans: The range of wavelength in Å is 4000  to 8000.

(ii) nm

Ans: The range of wavelength in nm 400 nm to 800 nm.

7. (a) Write the approximate wavelengths for (i) blue and (ii) red light.

Ans:  For blue light the approximate wavelength is 4800 Å and for red light the approximate wavelength is 8000 Å

(b) The wavelengths of violet and red light are 4000 Å respectively. Which of the two has the higher frequency?

Ans: The red light has a higher frequency than that of violet light.

8. Write the seven prominent colours present in white light in the order of the increasing wavelength

Ans: The seven prominent colours which are present in the white light in order to increase the wavelength are: Violet, indigo, blue, green, yellow, orange and red.

9. Name the seven prominent colours of the white light spectrum in order of their increasing frequencies.

Ans: The seven prominent colours of white light spectrum in order to increase their frequencies are: red, orange, yellow, green, blue, indigo and violet. 

10. Name four colours of the spectrum of white light which have wavelengths longer than blue light.

Ans: The colours of the spectrum of white light which have a wavelength longer than blue light are green, yellow, orange and red.

11. Which colour of the white light is deviated by a glass prism (i) the most and, (ii) the least?

Ans: When the white light falls on a glass prism, each colour present in it is refracted by a different angle, from which the red colour least deviates and the violet colour is the most one.

12. The wavelengths for the light of red and blue colours are nearly $7.8 \times 1{0^{ - 7}}m$ and $4.8 \times 1{0^{ - 7}}m$ respectively.

(a) Which colour has the greater speed in a vacuum?

Ans: Both of the given colours have the same speeds in a vacuum.

(b) Which colour has the greatest speed in glass?

Ans: The red colour has a greater speed in glass.

13. Define the term dispersion of light.

Ans: The dispersion of light is the phenomenon of splitting a beam of the white light into its seven constituent colours when it passes through a transparent medium.

14. Explain the cause of dispersion of white light through a prism.

Ans: When the white light enters the first surface of a prism the light of different colours due to their different speeds in glass, gets deviated through different angles towards the base of the prism. Therefore the cause of the dispersion of white light is the change in speed of light with respect to its wavelength.

15: Explain briefly, with the help of a neat labelled diagram, how does white light gets dispersed by a prism. On which surface of the prism, there is both the dispersion and deviation of light, and on which surface of the prism, there is only the deviation of light?

Ans: When the white light enters into the first surface of a prism the light of different colours due to their different speeds in glass, gets deviated through the different angles towards the prism. Therefore at the first surface of the prism, the dispersion of white light into its constituent colours takes place.

On the second surface of the prism, only refraction takes place and the different colours deviate through different angles, Therefore the colours get further separated on refraction at the second surface.

16. What do you understand about the term spectrum?

Ans: The spectrum is the band of colours seen on a screen when it is passed through a prism.

17. A ray of white light is passed through a glass prism and a spectrum is obtained on a screen.

(a) Name the seven colours of the spectrum in order.

Ans: The seven colours of spectrum are: violet, Indigo, Blue, Green, Yellow, Orange and Red.

(b) Do the colours have the same width in the spectrum?

Ans: No, different colours have different widths in the spectrum.

(c) Which of the colours of the spectrum of white light deviates?

(i) The most? 

Ans: The violet colour is the most deviated in the spectrum. 

(ii) The least?

Ans: The red colour is the least deviated in the spectrum.

18. The diagram shown below shows the path taken by a narrow beam of yellow monochromatic light passing through an equiangular glass prism. If the yellow light is replaced by a narrow beam of white light incident at the same angle, draw another diagram to show the passage of the beam through the prism and label it to show the effect of a prism on the white light.

Ans:

19. The figure shows a thin beam of white light from a source S striking on one face of a prism.

(a) Complete the diagram to show the effect of the prism on the beam and to show what is seen on the screen.

Ans: After the dispersion through the prism the constituent colours of white light are seen on the screen.

(b) A slit is placed in between the prism and the screen to pass only the light of green colour. What will you then observe on the screen?

Ans: When a slit is introduced in between the prism and screen to pass only the light of green colour then only green light is observed on the screen.

(c) What conclusion do you draw from the observation in part (b) above?

Ans: From the observation, we conclude that the prism itself produces no colour.

20. (a) A beam of monochromatic light undergoes minimum deviation through an equiangular prism, how does the beam pass through the prism, with respect to its base?

Ans: The light beam inside the Prism will be parallel to the base.

Under the minimum deviation,

Angle of incidence = Angle of emergence

(b) If white light is used in the same way as in part (a) above, what change is expected in the emergent beam?

Ans: The white light splits into its constituent colours. That is the spectrum is formed.

(c) What conclusion do you draw about the nature of white light in part (b)?

Ans: We conclude that the nature of white light is polychromatic.

21. When a white light ray falls on a prism, the ray at its first surface suffers:

(a) no refraction

(b) only dispersion

(c) only deviation

(d) both deviation and dispersion

Ans: When a white light ray falls on a prism, the ray at its first surface suffers both deviation and dispersion. Hence the correct answer is option d.

22. In the spectrum of white light by a prism, the colour at the extreme end opposite to the base of a prism is:

(a) Violet

(b) Yellow

(c) Red

(d) Blue

Ans: The colour at the extreme end opposite to the base of a prism is red. Hence the correct answer is option c.

23. The wavelength range of white light is

(a) 4000 nm to 8000 nm

(b) 40 nm to 80 nm

(c) 400 nm to 800 nm

(d) 4 nm to 8 nm

Ans: The wavelength range of white light is 400 nm to 800 nm. Hence the correct answer is option c.

24. Calculate the frequency of yellow light of wavelength 550nm. The speed of light is $3 \times 1{0^8}m{s^{ - 1}}.$

Ans: Given,

Wavelength $\lambda  = 550{\text{nm}}$

$ = 550 \times {10^{ - 9}}{\text{m}}$

Speed of light, ${\text{C}} = 3 \times {10^8}{\text{m}}/{\text{s}}$

We know that

Frequency = Speed of light / Wavelength

Frequency $ = \left( {3 \times {{10}^8}} \right)/\left( {550 \times {{10}^{ - 9}}} \right)$

Frequency $ = 5.4 \times {10^{14}}{\text{Hz}}$

25. The frequency range of visible light is from $3.75 \times 1{0^{14}}Hz$ to $7.5 \times 1{0^{14}}Hz$. Calculate its wavelength range. Take speed of light $ = 3 \times 1{0^3}m{s^{ - 1}}$

Ans: Given,

Speed of light, $c = 3 \times {10^8}{\text{m}}/{\text{s}}$

Frequency range $ = 3.75 \times {10^{14}}{\text{Hz}}$ to $7.5 \times {10^{14}}{\text{Hz}}$

Speed of light = frequency $ \times $ wavelength

For frequency $ = 3.75 \times {10^{14}}{\text{Hz}}$

$\lambda  = {\text{c}}/{\text{v}}$

$\lambda  = \left( {3 \times {{10}^8}{\text{m}}/{\text{s}}} \right)/\left( {3.75 \times {{10}^{14}}{\text{Hz}}} \right)$

$\lambda  = 8 \times {10^{ - 7}}$

$\lambda  = 8000{Å}$

For frequency $ = 7.5 \times {10^{14}}{\text{Hz}}$

$\lambda  = c/v$

$\lambda  = \left( {3 \times {{10}^8}m/s} \right)/\left( {7.5 \times {{10}^{14}}Hz} \right)$

$\lambda  = 4 \times {10^{ - 7}}m$

$\lambda  = 4000{Å}$

Therefore the wavelength range is $4000{Å}$ to $8000{Å}$  

26. (a) Give a list of at least five radiations, in the order of their increasing wavelength, which make up the complete electromagnetic spectrum.

Ans: The complete electromagnetic spectrum in the increasing order of their wavelength is:

Gamma rays

X – rays

Ultraviolet rays

Visible light

Infrared radiations

(b) Name the radiation mentioned by you in part (a) which has the highest penetrating power.

Ans: The gamma rays have the highest penetrating power.

27. (a) Arrange the following radiations in the order of their increasing wavelength:

X-rays, infrared rays, radio waves, gamma rays and microwaves.

Ans: The radiations in the order of their increasing wavelength are:

Gamma rays

X- rays

Infrared rays

Microwaves

Radio waves

(b) Name the radiation mentioned by you in part (a) which has the highest penetrating power.

Ans: The gamma rays have the highest penetrating power.

28. A wave has a wavelength of $1{0^{ - 3}}nm$;

(a) Name the wave

Ans: The gamma rays have a wavelength of ${10^{ - 3}}{\text{nm}}$.

(b) State its one property different from light.

Ans: The gamma rays have a large penetrating power.

29. A wave has a wavelength 50 Å

(a) Name the wave.

Ans: An electromagnetic wave having wavelength $50{Å}$ is $X$ -rays.

(b) State its speed in a vacuum.

Ans: The speed of an electromagnetic wave in vacuum is $3 \times {10^8}{\text{m}}/{\text{s}}$.

(c) State its one use.

Ans: Since, they are stopped by bones, they are used for the detection of fracture in bones, teeth etc, and for diagnostic purposes such as CAT scan in medical science.

30. (a) Name the high energetic invisible electromagnetic wave which helps in the study of the structure of crystals.

Ans: The x-rays are used in the study of crystals.

(b) State one more use of the wave named in part (a).

Ans: It is also used in the detection of fractures in bones, teeth.

31. State the name and the range of wavelengths of the invisible electromagnetic waves beyond the red end of the visible spectrum.

Ans: The electromagnetic waves beyond the red extreme of the visible light have certain radiations which produce a strong heating effect which are called infrared radiations. The range of wavelength is $8000{Å}$ to ${10^7}Å$. 

32. Name three radiations and their wavelength range which are invisible and beyond the violet end of the visible spectrum.

Ans: The three rays which are invisible beyond the violet end of the visible spectrum are as follows:

Gamma rays

X-rays

Ultraviolet rays

The wavelengths are as follows:

Gamma rays - below $0.01{\text{nm}}$

X-rays - 0.01 - $10{\text{nm}}$

Ultraviolet rays $ - 10 - 400{\text{nm}}$

33. Give the range of wavelengths of the electromagnetic waves visible to us. 

Ans: The range of wavelength $4000{Å}$ to $8000{Å}$ of the electromagnetic waves visible to us.

34. Name the region beyond,

(i) The red end

Ans: Infrared is the region that is beyond the red end of the spectrum.

(ii) The violet end of the spectrum.

Ans: Ultraviolet is the region that is beyond the violet end of the spectrum.

35. What do you understand about the invisible spectrum?

Ans: The invisible spectrum is the part of the spectrum beyond the red extreme and the violet extreme to which our eyes do not respond.

36. Name the radiation which can be detected by,

(a) a thermopile

Ans: The infrared radiations can be detected by the thermopile.

(b) a solution of silver chloride.

Ans: The ultraviolet radiations can be detected by silver chloride because in their presence silver chloride becomes dark.

37. State the approximate range of wavelength associated with,

(a) The ultraviolet ray

Ans: Ultraviolet rays - wavelength range 100Å to 4000Å.

(b) The visible light

Ans: Visible light - wavelength range 4000Å to 8000Å

(c) Infrared rays 

Ans: Infrared radiations - wavelength range 8000Å to ${10^7}Å$.

38. Name the radiations of wavelength just,

(a) Longer than $8 \times 1{0^{ - 7}}m$

Ans: The infrared radiation is longer than $8 \times {10^{ - 7}}{\text{m}}$.

(b) Shorter than $4 \times 1{0^{ - 7}}m$

Ans: The ultraviolet radiations are shorter than $4 \times {10^{ - 7}}{\text{m}}$

39. Name two electromagnetic waves of wavelength smaller than that of violet light. State one use of each.

Ans: The ultraviolet radiations and X-rays are the electromagnetic waves of wavelength smaller than that of the violet light. The wavelength of ultraviolet radiation is 100Å to 40000Å. The wavelength of X-rays is 0.1Å to 100Å. The ultraviolet radiations are used for detecting purity of things like gems, eggs, ghee etc. The x-rays are used for detecting fractures in bones, teeth, etc.

40. Give one use each of,

(a) Microwaves

Ans: Microwaves are used in radar communication.

(b) Ultraviolet radiations

Ans: The ultraviolet radiations are used in producing the vitamin D in the food of plants and animals.

(c) Infrared radiations

Ans: The infrared radiations are used in the remote control of the television as well as in other gadgets.

(d) Gamma rays.

Ans: The gamma rays are used in the industry to check the welding.

41. Name the two waves,

(a) Of lowest wavelength

Ans: The gamma rays have the lowest wavelength.

(b) Used for taking photographs in a dark

Ans: The infrared rays are used for taking photographs in the dark.

(c) Produced by the changes in the nucleus of an atom

Ans: The gamma rays are the waves produced by the changes in the nucleus of an atom.

(d) Of wavelength nearly 0.1nm

Ans: The $x$-rays have wavelengths nearly to $0.1{\text{nm}}$.

42. Two waves A and B have wavelengths of 0.1Å to 9000Å respectively.

(a) Name the two waves. 

Ans: Wave A is Gamma rays and wave B is Infrared radiations.

(b) Compare the speeds of these waves when they travel in a vacuum.

Ans: All electromagnetic waves travel with the speed of light in a vacuum. Therefore the ratio of speeds of these waves in a vacuum is 1: 1.

43. Name two sources, each of infrared radiations and ultraviolet radiations.

Ans: All the red hot bodies such as a heated iron ball, flame, fire, etc. are the sources of infrared radiations. The electric arc and the sparks give ultraviolet radiations.

44. What are infra-red radiations? How are they detected? State one use of these radiations.

Ans: The electromagnetic waves of wavelength in the range of 8000Å to ${10^7}Å$ are called infrared radiations

Detection: If a thermometer with a blackened bulb is moved from the violet end towards the red end of the spectrum, it is observed that there is a very slow rise in the temperature, but when it is moved beyond the red extreme, a rapid rise in the temperature is seen, which means that the region of the spectrum beyond the red extreme has certain radiations which produce a strong heating effect which is invisible. These radiations are known as infrared radiations. The infrared radiations are used as signals during the war as they are not visible and are not absorbed much in the medium.

45. What are ultraviolet radiations? How are they detected? State one use of these radiations

Ans: The ultraviolet radiations are electromagnetic radiations of wavelength from 100Å to ${1000}Å$

Detection: If the silver chloride solution is exposed to electromagnetic waves starting from the red to the violet end and then beyond it. From the red end to the violet end it is observed that the solution remains almost unaffected. But beyond the violet end, the solution first turns violet, and finally it becomes dark brown. Therefore there exist certain radiations beyond the violet extreme of the spectrum which are chemically more active than that of the visible light. These radiations are known as ultraviolet radiations.

The ultraviolet radiations are used for detecting the purity of things like gems, eggs, ghee etc.

46. Name three properties of ultraviolet radiation which are similar to visible light. 

Ans: The three properties of ultraviolet radiation which are similar to visible light are as following:

1. The ultraviolet radiations travel in a straight line with a speed of $3 \times 10^{8} \mathrm{~ms}^{-1}$ in air or in vacuum.

2. They obey the laws of reflection as well as of refraction.

3. They strongly affect the photographic plate as they are chemically more active than that of visible light.

47. Give two properties of ultraviolet radiations which differ from the visible light. 

Ans: Two properties of ultraviolet radiations different from the visible light are as follows:

1. The ultraviolet radiations can pass through quartz, but they are absorbed by the glass.

2. They are usually scattered by the dust particles that are present in the earth's atmosphere.

48. Mention three properties of infrared radiations similar to visible light.

Ans: The properties are follows:

1. The infrared radiations travel in straight lines like light, with a speed almost equal to $3 \times {10^8}{\text{m}}{{\text{s}}^{ - 1}}$ in vacuum.

2. They obey the laws of reflection as well as of refraction.

3. They are unaffected by the electric and magnetic fields.

49. Give two properties of infrared radiations which differ from visible light.

Ans: Two properties of infrared radiation different from the visible light are as follows:

1. They are absorbed by the glass, but they are not absorbed by the rock salt.

2. They are detected by their heating properties using a thermopile or a blackened bulb thermometer.

50. Name the material of prism required for obtaining the spectrum of,

(a) Ultraviolet light

Ans: A quartz prism is required for obtaining the spectrum of ultraviolet light.

(b) Infrared radiations

Ans: The infrared radiations are obtained by passing radiations through a rock salt prism.

51. Name the radiations which are absorbed by the greenhouse gases in the earth's atmosphere.

Ans: The major greenhouse gases which are present in the earth's atmosphere are water vapors, carbon dioxide, methane and ozone. The infrared radiation is absorbed by the greenhouse gases in the earth's atmosphere.

52. State one harmful effect each of the,

(a) Ultraviolet radiations

Ans: Ultraviolet radiation can cause health hazards like skin cancer if the human body is exposed to them for a long period.

(b) Infrared radiations.

Ans: The infrared radiations can cause skin burns.

53. Give a reason for the following:

(a) Infrared radiations are used for photography in fog 

Ans: The infrared radiations are used for photography in fog because they are not much scattered, so they can penetrate appreciably through it.

(b) Infrared radiations were used for signals during the war.

Ans: The infrared radiations are used as signals during the war as they are not visible and are not absorbed much in the medium.

(c) The photographic darkrooms are provided with infrared lamps.

Ans: The infrared lamps are used in darkrooms for developing photographs as they provide some visibility without affecting the photographic film.

(d) A rock salt prism is used instead of a glass prism to obtain the infrared spectrum.

Ans: The infrared spectrum can be obtained only by using a rock salt prism because the rock salt prism does not absorb the infrared radiations, whereas a glass prism absorbs them.

(e) A quartz prism is required for obtaining the spectrum of ultraviolet light. 

Ans: A quartz prism is used for obtaining the spectrum of the ultraviolet light as they can pass through quartz whereas ordinary glass absorbs the ultraviolet light.

(f) Ultraviolet bulbs have a quartz envelope instead of glass.

Ans: The ultraviolet bulbs have a quartz envelope instead of glass as they are not absorbed by quartz whereas ordinary glass absorbs ultraviolet light.

54. The most energetic electromagnetic radiations are:

(a) Microwaves

(b) Ultraviolet waves

(c) X-rays

(d) Gamma rays

Ans: The most energetic electromagnetic radiations are gamma rays. Hence the correct answer is option d.

55. The source of ultraviolet light is:

(a) electric bulb

(b) red hot iron ball

(c) sodium vapour lamp

(d) carbon arc-lamp

Ans: The source of ultraviolet light is carbon arc-lamp. Hence the correct answer is option d.

56. A radiation P is focused by a proper device on the bulb of a thermometer. Mercury in the thermometer shows a rapid increase. The radiation P is

(a) infrared radiation

(b) visible light

(c) ultraviolet radiation

(d) X-rays

Ans: The radiation ${\text{P}}$ is infrared radiation. Hence the correct answer is option a. 

57. An electromagnetic wave has a frequency of 500MHz and a wavelength of 60cm.

(a) Calculate the velocity of the wave.

Ans: Frequency $ = 500{\text{MHz}} = 500 \times {10^6}{\text{Hz}}$

Wavelength $ = 60{\text{cm}} = 0.6{\text{m}}$

Velocity of wave = frequency $ \times $ wavelength

$ = 500 \times {10^6} \times 0.6 = 3 \times {10^8}{\text{m}}/{\text{s}}$

(b) Name the medium through which it is travelling.

Ans: The electromagnetic wave travels through the air medium.

58. The wavelength of X-rays is 0.01 Å. Calculate its frequency.

Ans: Wavelength $= 0.01{Å} = 0.01 \times {10^{ - 10}}{\text{m}}$

Speed of X-rays $= 3 \times {10^8}{\text{m}}/{\text{s}}$

Speed of light = frequency $ \times $ wavelength

$c = v/\lambda $

$v = c\lambda $

$v = \left( {3 \times {{10}^8}{\text{m}}/{\text{s}}} \right)/\left( {0.01 \times {{10}^{ - 10}}{\text{m}}} \right)$

$v = 3 \times {10^{20}}{\text{Hz}}$

Hence the frequency is $3 \times {10^{20}}{\text{Hz}}$.

59. What is meant by scattering of light?

Ans: The scattering of light is the phenomenon when the white light from the sun enters the earth's atmosphere, it gets scattered or the light spreads in all directions by the dust particles and air molecules present in the atmosphere.

60. How does the intensity of scattered light depend on the wavelength of incident light? State conditions when this dependence holds.

Ans: The intensity of the scattered light is found to be inversely proportional to the fourth power of the wavelength. This relation holds when the air molecules are smaller in size than that of the wavelength of incident light.

61. When sunlight enters the earth's atmosphere, a state which colour of light is scattered the most and which the least.

Ans: As the intensity of the scattered light is inversely proportional to the fourth power of wavelength of light. Therefore the violet color is scattered the most while red the least.

62. A beam of blue, green and yellow light passes through the Earth's atmosphere. Name the colour which is scattered,

(a) The least

Ans: Out of three radiations the yellow light has the highest wavelength. Therefore it gets scattered the least.

(b) The most 

Ans: Out of three radiations the blue light has the lowest wavelength. Therefore it gets scattered the most.

63. Which colour of white light is scattered the least? Give a reason.

Ans: The light having the longest wavelength is scattered the least. Therefore the red coloured light is scattered the least.

64. The danger signal is red. Why?

Ans: The wavelength of red light is longest in the visible light. Therefore the light of red color is scattered least by the air molecules of the atmosphere. Thus when compared to other colours the light of red colour can penetrate to a longer distance. Hence red light can be seen from the farthest distance in comparison to the light of other colors with the same intensity. Therefore the red light is used for danger signals so that the signal may be visible from a far distance even in fog.

65. The sun will appear black to an observer on the moon's surface. Explain. 

Ans: Since there is no atmosphere on the moon, therefore there is no scattering of light on the moon's surface. Hence to an observer on the surface of the moon, except for the light reaching directly from the sun, no light reaches the eyes of the observer. Thus the sun will appear black to an observer on the moon's surface.

66. What characteristic property of light is responsible for the blue colour of the sky?

Ans: The blue colour is scattered the most due to its short wavelength. The scattering property of the light is responsible for the blue colour of the sky.

67. The colour of the sky, in the direction of the sun, is blue. Explain. 

Ans: As the light travels through the atmosphere, it gets scattered in different directions by the air molecules present in its path. Due to its short wavelength, the blue light is scattered more than that of the red light of long wavelength. Therefore the light directly from the sun reaching our eye is rich in red colour while the light from all the other directions reaching our eye is the scattered blue light. Hence the sky in the direction of the sun is blue.

68. Why does the sun appear red at sunrise and sunset?

Ans: The light from the sun has to travel the longest distance of the atmosphere to reach the observer at the time of sunrise and sunset. The blue light of short wavelength is lost due to scattering while the red light of long wavelength is scattered a little and is not lost much. Therefore the blue light is almost absent in the sunlight reaching the observer while only the red light reaches us. 

69. The sky at noon appears white. Give a reason.

Ans: At noon the sun is overhead and the light travels a relatively shorter distance through the atmosphere to reach the earth. Because of the short distance of light, only blue light is scattered and most of the light is not scattered. Therefore the sun appears white.

70. The clouds are seen as white. Explain.

Ans: The clouds are nearer the earth's surface and contain dust particles. The water droplets present in clouds are bigger than that of the wavelength of all seven colors of light. Therefore the water droplets scatter the entire spectrum. Thus the clouds appear white.

71. Give a reason why the smoke from a fire looks white.

Ans: The smoke appears white in color due to the size of the smoke particles being bigger than that of the wavelength of the light. The white light is a mixture of a large number of wavelengths but they scatter at the same extent since the condition of scattering is that the size of particles should be smaller than that of the wavelength of light. Therefore the scattered light appears white.

72. In the white light of the sun, maximum scattering by the air molecules present in the earth's atmosphere is for:

(a) red colour

(b) yellow colour

(c) green colour

(d) blue colour

Ans: In the white light of the sun, maximum scattering by the air molecules present in the earth's atmosphere is for blue color. Hence the correct answer is option d.

73. To an astronaut in a space-ship, the earth appears:

(a) white

(b) red

(c) blue

(d) black

Ans: To an astronaut in a spaceship, the earth appears blue. Hence the correct answer is option c.

Selina Solutions Concise Physics Class 10 Chapter 6 Spectrum discusses in detail the electromagnetic spectrum and its composition. This chapter covers topics such as the distortion produced by the triangular prism, the colors in white light and its wavelength and frequency range, the scattering of white light by a prism, and the formation of the spectrum. To gain a deeper understanding of the chapter, students are advised to practice Selina Solutions regularly.

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List of Subtopics Covered Under the Chapter Spectrum of Selina Solutions Physics Class 10

1. Triangular Prism leading to deviation

2. Wavelength and frequency range of colors in white light

3. Prism and formation of a spectrum leading to dispersion of white light.

4. Electromagnetic spectrum

5. Different radiations of the electromagnetic spectrum and their properties and uses.

6. Difference between the ultraviolet, visible, and infrared radiations

7. Scattering of light

8. Some applications related to scattering.