# Selina ICSE Class 10 Physics Solutions Chapter 6 Spectrum

Question 1. Name three factors on which the deviation produces by a prism depends and state how does it depend on the factors stated by you.
Solution:

The deviation produced by the prism depends on the following four factors:
(a) The angle of incidence – As the angle of incidence rises, the angle of deviation initially falls and, for a certain angle of incidence, achieves a minimal value. The angle of deviation is seen to grow as the angle of incidence is increased further.
(b) The prism’s composition, or refractive index: For a given angle of incidence, a prism with a higher refractive index will deviate more than a prism with a lower refractive index.
(c) Prism angle: As the prism angle grows, so does the angle of deviation.
(d) The color or wavelength of the light employed. As the wavelength of light decreases, the angle of departure increases. The light’s hue or wavelength Angle of deviation rises as light’s wavelength decreases. As the wavelength of light incoming on a prism decreases, so does the deviation it produces.

Question 2. How does the deviation produced by a triangular prism depend on the colors (or wavelengths) of light incident on it?
Solution:

As the wavelength of light incoming on a prism decreases, so does the deviation it produces.

Question 3. How does the speed of light in glass change on increasing the wavelength of light?
Solution:

With an increase in wavelength, light moves quickly.

Question 4. Which color of white light travels (a) fastest (b) slowest, in glass?|
Solution:

a) Red color travels fastest in glass.
b) Blue color travels slowest in glass.

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

Subjective property of light related to its wavelength:
(i) The wavelength for blue light approximate is 4800 Å
(ii) The wavelength for red light approximate is 8000 Å

Question 6. What is the range of wavelength of the spectrum of white light in (i) Å, (ii) nm?
Solution:

(i) 4000 Å to 8000 Å
(ii) 400 nm to 800 nm

Question 7. (a) Write the approximate wavelengths for (i) blue and (ii) red light.
(b) The wavelengths of violet and red light are 4000 Å respectively. Which of the two has the higher frequency?
Solution:

(a) (i) The approximate wavelength of blue light is around 4800.
(ii) The approximate wavelength of red light is around 8000.
(b) Violet light has a wavelength of 4000.

Question 8. Write the seven prominent colors present in white light spectrum in order of their increasing wavelength.
Solution:

In order of increasing frequency, these are the seven most noticeable hues of the white light spectrum: red, orange, yellow, green, blue, indigo, and violet.

Question 9. Name the seven prominent colors of the white light spectrum in order of their increasing frequencies.
Solution:

The seven colors are red, orange, yellow, green, blue, indigo, and violet, in descending order of increasing frequency.

Question 10. Name four colors of the spectrum of white light which have wavelength longer than blue light.
Solution:

The spectrum of white light which have wavelength longer than blue light are Red, orange and yellow.

Question 11. Which color of white light is deviated by a glass prism the most and which the least?
Solution:

The violet light is most and the red light is least distorted by a glass prism.

Question 12. The wavelengths for the light of red and blue colors are roughly 7.8×107 m and 4.8× 107 m respectively.
(a) Which color has the greater speed in vacuum?
(b) Which color has the greater speed in glass?
Solution:

(a) Red and Blue both color move at the same speed in a vacuum.
(b) Red light travels more quickly through a glass.

Question 13. Define the term dispersion of light.
Solution:

Dispersion of light is the process by which a prism divides white light into its individual color.

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

When white light strikes the initial surface of a prism and enters the glass, light of various hues is refracted or diverted through various angles as a result of the varying speeds at which the glass moves. Thus, the first surface of the prism is where white light is divided into its component colors. Therefore, the alteration in light speed with wavelength or frequency is the root cause of dispersion.

Question 15. Explain briefly, with the help of a neat labeled diagram, how white light gets dispersed by a prism.
Solution:

When white light strikes the initial surface of a prism and enters the glass, light of various colors is refracted or diverted through various angles as a result of the varying speeds at which the glass moves. Thus, the first surface of the prism is where white light is divided into its component colors. Only refraction occurs on the second surface, where various colors are bent at various angles. As a result, upon refraction at the second surface, the colors become even further divided (violet being deviated the most and red the least)

Question 16. What do you understand by the term spectrum?
Solution:
The spectrum is the color band that appears on a screen when white light is sent through a prism.

Question 17. A ray of white light is passed through a glass prism and spectrum is obtained on a screen.
(a) Name the seven colors of the spectrum in order.
(b) Do the colors have the same width in the spectrum?
(c) Which of the color of the spectrum of white light deviates (i) the most, (ii) the least?
Solution:

(a) The seven colors of the spectrum are Violet, Indigo, Blue, Green, Yellow, Orange, Red.
(b) No, the widths of the spectrum differ based on the colors.
(c) (i) The most deviate color is violet.
(ii) The least deviate color is blue.

Question 18. The following diagram shows the path taken by a narrow beam of yellow light is replaced by a narrow beam of white light incident at the same angle. Draw another diagram to show the passage of white light through the prism and label it to show the effect of prism on the white light.

Solution:

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

(i) Complete the diagram to show effect of prism on the beam and to show what is seen on the screen.
(ii) A slit is placed in between the prism and the screen to pass only the light of green color. What will you then observe on the screen?
(iii) What conclusion do you draw from the observation in part (b) above?
Solution:

(i) White light’s component colors may be seen on the screen following prism dispersion.

(ii) Only green light is seen on the screen when a slit is placed between the prism and screen to allow only the passage of green light.
(iii) We deduce from the observation that the prism itself does not create color.

Question 20 (i) if a monochromatic beam of light undergoes minimum deviation through an equiangular prism, how does the beam pass through the prism, with respect to its base?
(ii) If white light is used in same way as in part (a) above, what change is expected in the emergent beam?
(iii) What conclusion do you draw about the nature of white light in part (b)?
Solution:

(i) A monochromatic light beam flows parallel to the base of the prism if it passes through an equi-angular prism with the least amount of deviation.
(ii) The spectrum is created when white light separates into its individual colors.
(iii) From this, we deduce that white light has several colors.

Exercise-6a
Multiple Choice Types

Question 1. 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
Solution: d) Both deviation and dispersion

Question 2. In the spectrum of white light by a prism, the color at the extreme end opposite to the base of prism is:
a) Violet
b) Yellow
c) Red
d) Blue
Solution: Red

Question 3. 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
Solution: c) 400 nm to 800 nm

Exercise-6a
Numerical

Question 1. Calculate the frequency of yellow light of wavelength 550nm. The speed of light is 3×108 m/s-1
Solution:
Given, wavelength λ = 550 nm = 550 × 10-9
Speed of light, c = 3 × 108

Question 2. The frequency range of visible light is from 3.75 x 1014  hz→7.5×1014 Hz. Calculate its wavelength range. Take speed of light = 3×108 m/s-1
Solution:

Speed of light, c = 3×108
Frequency range = 3.75 x 1014
Speed of light = frequency x wavelength
For frequency = 3.75 x 1014 Hz

Exercise-6b

Question 1. (a) Give a list of at least five radiations, in order of their increasing frequencies, which make up the complete electromagnetic spectrum.
(b) Which of the radiation mentioned by you in part (a) has the highest penetrating power
Solution:

(a) There are five types of radiation, listed in increasing order of frequency: X-rays, gamma rays, visible light, ultraviolet, and infrared waves.
(b) Gamma rays have the greatest penetration power

Question 2. Arrange the following radiations in the order of their increasing wavelength:
(i) X-rays, infrared rays, ratio waves, gamma ray and micro waves?
(ii) Which radiation is used for satellite communication?
Solution:

(i)Radio waves, microwaves, X-rays, infrared rays, and microwaves
(ii) Satellite communication employs microwave technology.

Question 3. A wave has a wavelength of 10-3 nm.
(i) Name the wave.
(ii) State its one property different from light.
Solution:

(i) The wavelength of a gamma ray is 10-3 nm.
(ii) Gamma rays have a significant penetrating power.

Question 4. A wave has wavelength 50Å.
(i) Name the wave.
(ii) State the speed in vacuum.
(iii) State its one use.
Solution:

(i) The wave’s wavelength is 50. Consequently, it is an X-ray.
(ii) In a vacuum, the wave will move at a speed of 3 108 m/s.
(iii) Complex molecules and crystals’ atomic arrangements may both be studied using X-rays.

Question 5. (a) Name the high energetic invisible electro-magnetic wave which helps in the study of structure of crystals.
(b) State one more use of the wave named in part (a).
Solution:

(a) The study of crystals makes use of X-rays.
(b) It is further used to identify bone fractures.

Question 6. State the name and the range of wavelength of the invisible electromagnetic waves beyond the red end of the visible spectrum.
Solution:

Infrared radiations are the electromagnetic waves that extend past the red end of the spectrum. Between 8000 and 107 (or 800 nm to 1 mm)

Question 7. Name three radiations and their wavelength range which are invisible and beyond the violet end of the visible spectrum.
Solution:

The three radiations beyond the violet end of the visible spectrum are:
1) Ultraviolet radiations, which range from 100 to 4000 nm.
2) From 0.1 to 100 X-rays

Question 8. Give the range of wavelength of the electromagnetic waves visible to us.
Solution:

The ranges of wavelength of the electromagnetic waves are 4000 Å to 8000 Å.

Question 9. Name the region beyond the
(i) Red end of the spectrum.
(ii) Violet end of the spectrum called.
Solution:

(i) Red end of the spectrum – Infrared region.
(ii) Violet end of the spectrum called – Ultraviolet region.

Question 10. What do you understand by the invisible spectrum?
Solution:

The invisible spectrum is that portion of the electromagnetic spectrum that is between the red and violet ends and is inaccessible to human eyes.

Question 11. Name the radiation which can be detected by
(i) Thermopile.
(ii) Solution of silver chloride.
Solution:

Question 12. State the approximate range of wavelength associated with the (i
(i) Ultraviolet rays.
(ii) Visible light
(iii) Infrared rays.
Solution:

(i) Ultraviolet rays-wavelength range 100 Å to 4000 Å
(ii) Visible light-wavelength range 4000 Å to 8000 Å
(iii) Infrared radiations-wavelength range 8000 Å to 107 Å

Question 13. Name the radiations of wavelength just
(i) Longer than 8 x 10-7 m
(ii) Shorter than 4 x 10-7 m
Solution:

(i) Infrared radiations – longer than 8 x 10-7 m
(ii) Ultraviolet radiations – shorter than 4 x 10-7 m

Question 14. Name two electromagnetic waves of frequency smaller than that of violet light. State one use of each
Solution:

1) Ultraviolet rays, which range from 100 to 4000 nm
Use: To determine whether jewels, eggs, ghee, etc. are pure.
2) From 0.1 to 100 X-rays
Use: To find fractures in teeth, bones, etc.

Question 15. Give one use of
(i) Microwaves
(iv) Gamma rays
Solution:

(i) Communications through satellite employ microwaves.
(ii) Ultraviolet rays are employed to determine the quality of jewels, eggs, ghee, and other materials.
(iii) Infrared radiations are utilized in the television and other devices’ remote controls.
(iv) Cancer cells are killed by gamma rays in the field of medicine.

Question 16.(i) Name the waves of lowest wavelength.
(ii) Name the rays or waves used for taking photographs in dark
(iii) Name the rays or waves produced by the changes in the nucleus of an atom
(iv) Name the rays or waves of wavelength nearly 0.1 nm.
Solution:

(i) Gamma rays have the lowest wavelength.
(ii) To take pictures in the dark, infrared light is employed.
(iii) Modifications to an atom’s nucleus result in the production of gamma rays.
(iv) The wavelength of X-rays is close to 0.1 nm.

Question 17. Two waves A and B have wavelength 0.01 Å and 9000 Å respectively.
(a) Name the two waves.
(b) Compare the speeds of these waves when they travel in vacuum.
Solution:

(b) Due to the fact that all electromagnetic waves move at the speed of light in a vacuum, the ratio of these waves’ speeds there is 1:1.

Question 18. Name two sources, each of infrared radiations and ultraviolet radiations.
Solution:
Infrared radiation is produced by all heated objects, such as a hot iron ball, flame, fire, etc.
UV radiations are produced by the electric arc and sparks.

Question 19. What are infrared radiations? How are they detected? State one use of these radiations
Solution:

The electromagnetic waves with infrared radiation have wavelengths between 8000 and 107 nm. When a blackened-bulb thermometer is pushed from the violet end to the red end, a modest rise in temperature is seen, but when it is moved past the red area, a quick rise in temperature is seen. It implies that there are certain radiations in the part of the spectrum beyond the red end that have a high heating impact but cannot be seen. The infrared radiations are what are known as these radiations. Medical professionals employ infrared radiation for medicinal effects.

Question 20. What are ultraviolet radiations? How are they detected? State one use of these radiations.
Solution:

The term “ultraviolet radiations” refers to electromagnetic radiation with wavelengths between 100 and 4000.
Detection: When various radiations from the red to the violet ends of the spectrum and beyond are impacted on a silver-chloride solution, it is found that the solution is unaffected from the red to the violet ends. However, it first turns violet just before the violet end before changing to dark brown at the last end. As a result, radiations that are chemically more active than visible light and occur beyond the violet end of the spectrum are known as UV radiations.
Use: Sterilization is accomplished using ultraviolet light.

Question 21. Name three properties of ultraviolet radiations which are similar to visible light.
Solution:

(a) In air, ultraviolet radiation moves at a speed of 3 x 10-8 mm in a straight line (or vacuum).
(b) They abide with the reflection and refraction rules.
(c) They have an impact on the picture plate.

Question 22. Give two properties of ultraviolet radiations which differ from the visible light.
Solution:

(a) When ultraviolet rays hit a zinc sulphide screen, they cause fluorescence.
(b) They endanger human health by spreading diseases like cancer.

Question 23. Mention three properties of infrared radiations similar to the visible light.
Solution:

(a) Infrared radiation moves at a speed of 3 x 108 m/s in vacuum and moves in a straight line like light.
(b) They abide with the reflection and refraction rules.
(c) They don’t make the zinc sulphate screen glow.

Question 24. Give two such properties of infrared radiations which are not true for visible light.
Solution:

(a) The infrared spectrum is invisible.
(b) They have no impact on common photographic film.

Question 25. Name the material prism required for obtaining the spectrum of
(i) Ultraviolet light.
Solution:

(i)Radiation is passed through a quartz prism to produce ultraviolet light.
(ii) By passing radiation through a rock salt prism, infrared radiations are produced.

Question 26. Name the radiations which are absorbed by the greenhouse gases in the earth’s atmosphere.
Solution:

The main greenhouse gases in the atmosphere of the Earth are ozone, carbon dioxide, methane, and water vapour. The infrared radiations in the Earth’s atmosphere are absorbed by these greenhouse gases.

Question 27. State one harmful effect each of the ultraviolet and infrared radiation
Solution:

The harmful effect of ultraviolet exposure is health risks include skin cancer. The harmful effect of infrared radiation is skin burns.

Question 28. (i) Infrared radiations are used for photography in fog.
Why infra-red radiation it’s preferred over ordinary visible light for taking photographs in the fog?
(ii) Infrared radiations are used as signals during war
(iii) The photographic darkrooms are provided with infrared lamps.
(iv) A rock salt prism is used instead of a glass prism to obtain the infrared spectrum
(v) A quartz is required for obtaining the spectrum of the ultraviolet light
(vi) Ultraviolet bulbs have a quartz envelope instead of glass
Solution:

(i) Because infrared radiations are not greatly dispersed by the atmosphere and may thus pass noticeably through it, they are employed in fog photography.
(ii) Because infrared radiations are invisible to the human eye and are not heavily absorbed by the environment, they are utilized as signals in times of conflict.
(iii) Infrared lights are utilized in darkrooms for picture development since they don’t chemically alter the photographic film while yet offering some visibility.
(iv) Only a rock-salt prism may be used to get the infrared spectrum since a glass prism absorbs infrared light, but a rock-salt prism does not.
(v) Since regular glass absorbs ultraviolet light, it is necessary to utilize a quartz prism to get the spectrum of UV radiations.
(vi) Because ultraviolet light is not absorbed by quartz, UV bulbs have a quartz envelope rather than a glass one.

Exercise-6b

Question 1. The most energetic electromagnetic radiations are :
a) Microwaves
b) Ultraviolet waves
c) X-rays
d) Gamma rays
Solution: gamma rays

Question 2. The source of ultraviolet light is:
a) Electric bulb
b) Red hot iron ball
c) Sodium vapor lamp
d) Carbon arc-lamp
Solution: Carbon arc-lamp

Question 3. A radiations A is focused by a proper device on the bulb of a thermometer. Mercury in the thermometer shows a rapid increase. The radiations A are:
b) Visible light
d) X-rays

Exercise-6b
Numerical

Question 1. An electromagnetic wave has a frequency of 500 MHz and a wavelength of 60 cm
(i) Calculate the velocity of the wave.
(ii) Name the medium through which it is travelling
Solution:

(i) Frequency = 500MHz = 500 x 106
Wavelength = 60 cm = 0.6 m
Velocity of wave = frequency x wavelength
Velocity of wave = 500 x 106
Velocity of wave = 3 x 10m/s
(ii) Electromagnetic wave is travelling through air.

Question 2. The wavelength of X-rays is 0.01 Å. Calculate its frequency.
Solution:

Wavelength = 0.01Å = 0.01 x 10-10
Speed of X-rays = 3 x 108
Speed of light = frequency x wavelength
C = 𝛎ℷ

Exercise-6c

Question 1. What is meant by scattering of light?
Solution:

When white light from the sun enters the earth’s atmosphere, the light is dispersed, or spreads out in all directions, by airborne dust particles, unbound water molecules, and gas molecules. The term for this occurrence is light scattering.

Question 2. How does the intensity of scattered light depend on the wavelength of incident light? State conditions when this dependence holds.
Solution:

It is discovered that the intensity of dispersed light is inversely related to the fourth power of the light’s wavelength. When the wavelength of the light incident is substantially shorter than the size of air molecules, this connection holds.

Question 3. When sunlight enters the earth’s atmosphere, state which colour of light is scattered the most and which the least.
Solution:

As the intensity of dispersed light is determined to be inversely related to the fourth power of wavelength of light, violet is the hue that scatters the most and red the least.

Question 4. A Beam of Blue, Green, and Yellow Light Passes through The Earth’s Atmosphere.
(i) Name the Color which is scattered the Least.
(ii) Name the color which is scattered the most
Solution:

(i) Of the three radiations, yellow light has the longest wavelength. As a result, it disperses the least.
(ii) Of the three radiations, blue light has the shortest wavelength. As a result, it disperses the most.

Question 5. Which color of white light is scattered the least? Give reason.
Solution:

Largest wavelength light is least likely to be dispersed. Red light is therefore the least dispersed.

Question 6. The danger signal is red. Why?
Solution:

Red light may go a greater distance because its wavelength is the longest of all visible light wavelengths and is thus least dispersed by the air molecules in the atmosphere. As a result, when compared to other hues with the same intensity, red light may be seen from the greatest distance. As a result, it is employed as a hazard signal so that it may be seen from a great distance.

Question 7. How would the sky appear when seen from the space (or moon)? Give reason for your answer.
Solution:

Since the moon has no atmosphere, sunlight that strikes its surface is not scattered in any way. Therefore, only the light coming directly from the sun may be seen by an observer in space on the surface of the moon. As a result, the sky will be colorless and seem black to someone standing on the surface of the moon.

Question 8. What characteristic property of light is responsible for the blue color of the sky?
Solution:
The sky’s blue tint results from light’s scattering properties because blue light, with its short wavelength, scatters more than any other color.

Question 9. The color of sky, in direction other than of the sun, is blue. Explain.
Solution:

The air molecules along its route cause the light to be dispersed in various directions as it passes through the atmosphere. Compared to red light, which has a large wavelength, blue light is more diffused because of its short wavelength. In contrast to the dispersed blue light that enters our eyes from all other ways, the light that comes directly from the sun is rich in red hue. As a result, the sky appears blue when viewed from an angle other than the direction of the sun.

Question 10. Why does the sun appear red at sunrises and sunset?
Solution:

The sunlight needs to travel the most distance through the atmosphere to reach the observer during sunrise and dusk. Short-wavelength blue light is lost during transit from the sun owing to scattering, whereas long-wavelength red light is very slightly dispersed and hence not significantly lost. As a result, sunlight that reaches the viewer is mostly devoid of blue light and abundant in red.

Question 11. The sky at noon appears white. Give reason.
Solution:

When the sun is above our heads at midday, we get light beams directly from the sun without much color dispersion. The sky seems white because light needs to travel through fewer atmospheres

Question 12. The clouds are seen white. Explain
Solution:

The clouds are closer to the surface of the planet and they are made up of aggregates of water molecules larger than the wavelength of visible light and dust particles. Since all hues of incoming white light from the sun are equally scattered by the dust and water molecules present in clouds, when the scattered light reaches our eyes, the clouds seem white.

Question 13. Give reason why the smoke from fire looks white.
Solution:

Because the particle’s size is greater than the light’s wavelength, it scatters light of all colors, giving the smoke from the fire its white appearance.

Exercise-6c
Multiple Choice Types

Question 1. In white light of sun, maximum scattering by the air molecular present in the earth’s atmosphere is for:
a) Red color
b) Yellow color
c) Green color
d) Blue color
Solution: d) Blue color

Question 2. To an astronaut in a space-ship, the earth appears:
a) White
b) Red
c) Blue
d) black
Solution: c) Blue