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Electromagnetic Waves and Spectrum
1. Define the term dispersion of light.
Ans. The phenomenon due to which white light splits up into seven colours on
passing through a prism is called dispersion of light.
2. Explain the cause of dispersion of white light through a prism.
Ans. White light is made of seven colour bands (VIBGYOR), such that the rays in
each band have different wavelength and different frequencies. When the white
light is incident on the refracting face of the prism each coloured ray deviates
through different angle, depending upon its wavelength. The red rays deviate
least, whereas the violet rays deviate most. It is this differential deviation of rays,
which causes dispersion.
3. Draw a diagram showing the dispersion of white light. Can a glass slab disperse
white light? If not, why?
Ans. Dispersion of white light is shown below.
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A glass slab cannot disperse white light.
Reason : A glass slab acts as a double prism as illustrated in diagram. The rays
dispersed by prism A are recombined by prism B. Thus, on the whole no
dispersion takes place.
4. Briefly describe an experiment to show that a prism itself produces no colour,
during dispersion of white light.
Ans. White light passing through a slit, is made to fall on an equilateral prism A, in a
dark room. The dispersed light is then received on a white screen ST, having a
small hole in it. The light passing out of the hole (say, green light), is made to
pass through another prism B, placed in an inverted position. It is observed that
the light on passing through the prism B only gets refracted, but does not suffer
any dispersion.
By moving the screen ST, up or down, experiment can be conducted for any
other colour of light. It is seen that in each case, only refraction takes place.
See figure below.
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Thus, experiment proves that white light gets dispersed at the first face of prism
only. Afterwards it gets refracted. Hence, prism does not colour the white light.
5. Which colour of white light is deviate (i) the most (ii) the least ?
Ans.(i) Violet colour deviates the most.
(ii) Red colour deviates the least.
6. Copy the diagram given below and complete ray diagram to show the nature of
light produced on the screen.
Or
(i) Draw a diagram to show how white light splits into constituent colours.
(ii) Draw another diagram to show how constituent colours reunite to form white
light.
Ans.
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7. What do you understand by the term visible spectrum? State the order of
colours starting from the colour having maximum wavelength.
Ans. A band of seven colours obtained on the screen, when white light is dispersed is
called visible spectrum.
The order of colours in the increasing wavelength is :
Red, orange, yellow, green, blue, indigo and violet.
8. (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 the same way as in (i) above, what change is expected
in the emergent beam?
Ans. (i) The beam of monochromatic light travels parallel to the base of the
equiangular prism.
(ii) The white light will split into seven colours, such that the red colour deviates
the least and the violet colour deviatesmost.
9. The wavelengths for the light of red and blue colours are roughly 7 × 10–7 m
and 4 × 10–7 m respectively. Which colour will have greater speed in vacuum?
Which colour has greater speed in glass?
Ans. (i) In vacuum, both red and blue colours have same speed.
(ii) In glass, the speed of red colour is more than blue colour.
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10. Give the list of at least five radiations in the order of their increasing
frequencies, which make up the complete electromagnetic spectrum. Which of
these has the highest penetrating power :
Ans. (i) Radiowaves
(ii) Infra-red rays
(iii) Visible light
(iv) Ultraviolet rays,
(iv) X-rays.
X-rays have the highest penetrating power.
11. Arrange the following in the order of their increasing wavelength :
X-rays,infra-red rays, radiowaves, gamma rays and microwaves.
Ans. Gamma rays, X-rays, infra-red rays, microwaves, radiowaves.
12. A wave has a wavelength of 0.01 Å. Name the wave.
Ans. The wave is called γ radiation.
13. What is the range of the wavelength of the following electro- magnetic waves?
(a) gamma rays
(b) x-rays
(c) ultra-violet rays
(d) visible
(e) infra-red
(f) microwaves
(g) radiowaves
Ans. (a) Gamma rays : shorter than 0.1 Å
(b) X-rays : 0.1 Å to 100 Å
(c) Ultra-violet rays : 100 Å to 4000 Å
(d) Visible light : 4000 Å to 8000 Å
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(e) Infrared radiation : 8000 Å to 3 × 107 Å
(f) Microwaves : 3 × 107 Å to 1011 Å
(g) Radiowaves : 11 × 1011 Å and above.
14. Name the radiations of wavelength : (i) longer than 8 × 10–7m,
(ii) shorter than 4 × 10–7 m.
Ans.(i) Infra-red radiations have wavelength longer than 8 × 10–7 m.
(ii) Ultra-violet radiations have wavelength less than 4 × 10–7 m.
15. Two waves A and B have wavelengths 0.01 Å and 9000 Å respectively. Name the
waves. Compare the speeds of these waves when they travel in vacuum.
Ans. The wave A of wavelength (0.01 Å) is gamma radiation.
The wave B of wavelength (9000 Å) is infra-red radiation.
Speed of wave A in vacuum : Speed of wave B in vacuum = 1 : 1.
16. Name three properties of ultra-violet radiations which are similar to white light.
Ans. (i) Both obey the laws of reflection.
(ii) Both obey the laws of refraction.
(iii) Both travel with the speed of 3 × 108 ms–1.
17. Give two properties of ultra-violet radiations which differ from visible light.
Ans. (i) They get readily scattered as compared to visible light.
(ii) They are chemically more active than visible light.
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18. Name three properties of infra-red radiations, which similar to white light.
Ans. (i) Both obey the laws of reflection.
(ii) Both obey the laws of refraction.
(iii)Both travel with the speed of 3 × 108 ms–1.
19. State two properties of infra-red radiations, which differ from visible light.
Ans. 1. Infra-red radiations produce heating effect, but not the visible radiation.
2. Infra-red radiations do not get scattered easily as compared to visible light.
20. Answer the following questions regarding infra-red spectrum :
(i) On which side of visible spectrum is it found’?
(ii) State the range of infra-red spectrum.
(iii) Why is rock salt prism used in experiments for infra-red spectrum?
(iv) How will you detect infra-red spectrum?
(v) State four properties of infra-red radiation.
(vi) Name two natural sources which emit infra-red radiations.
(vii) State two important uses of infra-red radiations.
Ans. (i) It is found beyond red band of visible spectrum.
(ii) From 8000 Å to 400,000 Å.
(iii) It is because it does not absorb infra-red radiations, unlike glass prism.
(iv) Slowly move a thermometer, whose bulb is blackened beyond red part of
spectrum. It is observed that thermometer registers sudden rise in temperature.
This proves the presence of infra-red radiation.
(v) 1. They travel with the speed of 3 × 108 ms–1.
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2. They obey laws of reflection and refraction.
3. They heat the material, which absorbs them.
4. They do not easily get scattered and can pass through fog.
(vi) 1. Sun
2. Active volcanoes.
(vii) 1. They are used by doctors to relieve pain from swollen joints of patients.
2. They are used in heat seeking missiles.
21. Answer the following questions regarding ultraviolet spectrum.
(a) On which side of visible spectrum is it formed?
(b) What is the range of ultraviolet spectrum?
(c) Name the material of prism which absorbs least ultraviolet radiation.
(d) How can you detect ultraviolet radiations?
(e) What is the effect of these radiations on :
(i) atmosphere, (i) human skin, (iii) human eyes?
(f) In which way are ultraviolet radiation beneficial to :
(i) Human being; (ii) Jewellers; (iii) Bankers?
(g) Give four important properties of ultraviolet radiations.
(h) Amongst the sources listed below, name two most powerful sources of
ultraviolet radiation :
(i) Room heater, (ii) Arc welding, (iii) Electric lamp, (iv) Sun.
Ans. (a) It is found beyond violet band of visible spectrum.
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(b) From 4000 Å to 100 Å.
(c) Quartz prism.
(d) Place a zinc sulphide crystal before violet part of spectrum. The crystal
glows with a greenish blue glow, showing the presence of ultraviolet
radiations.
(e) (i) They are absorbed by the oxygen molecules present in air. The molecules
of oxygen then combine chemically to form ozone.
(ii) They can cause skin cancer on prolonged exposure.
(iii) They can damage retina and can cause permanent blindness.
(f) (i) Controlled exposure of ultraviolet radiations stimulates the body to
produce vitamin D. Vitamin D is then used by body for strengthening
bones and teeth.
(ii) Ultraviolet radiations can be used to distinguish between the fake and
the real diamonds.
(iii) Ultraviolet radiations can detect any alteration in the bank cheques and
the forged currency notes.
(g) 1. They travel with the velocity of 3 × 108 ms–1.
2. They obey laws of reflection and refraction.
3. They cause fluorescence in zinc sulphide, barium platinocyanide etc.
4. They are easily scattered and ionise the gases.
(h) (i) Sun (ii) Arc welding or electric welding
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22. Fill in the blank spaces.
(a) ................... are the waves of highest frequency [Gamma rays / X-rays]
(b) ..................... are used for taking photographs of objects in dark [infra-red
rays / ultraviolet rays]
(c) ................. are electro magnetic waves of wavelength nearly 10–8 cm.
[cosmic rays/X-rays]
Ans. (a) Gamma rays (b) Infra-red rays
(c) X-rays
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