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Question Bank 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. Class-X 1 Question Bank 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. Class-X 2 Question Bank 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. Class-X 3 Question Bank 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. Class-X 4 Question Bank 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 . Name the wave. Ans. The wave is called 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 (b) X-rays : 0.1 to 100 (c) Ultra-violet rays : 100 to 4000 (d) Visible light : 4000 to 8000 Class-X 5 Question Bank (e) Infrared radiation : 8000 to 3 107 (f) Microwaves : 3 107 to 1011 (g) Radiowaves : 11 1011 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 and 9000 respectively. Name the waves. Compare the speeds of these waves when they travel in vacuum. Ans. The wave A of wavelength (0.01 ) is gamma radiation. The wave B of wavelength (9000 ) 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. Class-X 6 Question Bank 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 to 400,000 . (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. Class-X 7 Question Bank 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. Class-X 8 Question Bank (b) From 4000 to 100 . (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 Class-X 9 Question Bank 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 Class-X 10 Question Bank
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