Question .1. Write down the following and fill in the blanks spaces with appropriate colours :
(a) Green + Magenta = ___________.
(b) Red + _________ = Yellow.
(c) Red + Blue = _____________.
(d) Blue + _________ = Cyan.
(e) Yellow + Blue = _________ .
(f) Magenta + ___________ = White.
(g) _______ + Red = White.
(h) Red + ________ + Green = White.
(i) A red rose appears ________ when seen in green light.
(j) A piece of red cloth appears red in white light because it _________ blue and green and ______ only red.
Solution:
(a) White
(b) Green
(c) Magenta
(d) Green
(e) White
(f) Green
(g) Cyan
(h) Blue
(i) Black
(j) absorbs and reflects.
Question .2. Write down the colour spectrum produced when white light is passed through a prism. Which of these colours is deviated most ?
Solution:
The colour of the spectrum are violet, indigo, blue, green, yellow, orange and red.
The violet colour is deviated the most.
Question .3. What is the wavelength of the electromagnetic wave whose frequency is 1012 Hz ? Name this electromagnetic wave.
Solution:
Wavelength of the wave = 3 × 10-4 m. The electromagnetic wave is microwave.
Question .4. Show the effect of mixing two primary colours through labelled colour triangle.
Solution:
Blue + Green = Cyan
Green + Red = Yellow
Red + Blue = Magenta
Cyan + Magenta + Yellow = White.
Question .5. Give one useful and one harmful effect of ultra violet radiation on the human body.
Solution:
(i) Useful Effect : When human body is exposed to ultraviolet radiation, vitamin D is formed in the body.
(ii) Harmful effect : Overexposed of ultraviolet radiation to the body causes skin cancer. It is also harmful to the human eye
Question .6. Define a pair of complementary colours. Name the colours in one such pair.
Solution:
Two colours are said to be complementary to each other if their addition produces white. Red and Cyan or Green and Magenta or Blue and Yellow are three pairs of complementary colours.
Question .7. Define dispersion of light.
Solution:
The splitting of white light into its seven constituents colours is known as dispersion of light.
Question .8. Explain briefly how white light gets dispersed by a prism.
Solution:
The refractive index of glass in different colour of light is different. White light is a mixture of seven colour. When white light is incident on the surface of a prism, different colours are deviated differently, as deviation depends upon refractive index. Thus all the colours of light come out of the prism from different points.
Question .9. Draw a diagram to show that white light can be split up into different colours. Draw another diagram to show how the colours can be combined to give the effect of white light .
Solution:
For diagram consult a Text Book.
Question .10. Name the extreme colours in a pure spectrum of light.
Solution:
The two extreme colours in the spectrum of white light are violet and red.
Question .11. State the equation for the relation between frequency and wavelength of light in vacuum.
Solution:
The required equation is c = ν λ.
Question .12. Name any four regions of electromagnetic spectrum (other than visible light) in increasing order of wavelength.
Solution:
(i) Gamma rays
(ii) X – rays
(iii) UV rays and
(iv) Infra red rays.
Question .13. Name any two electromagnetic waves which have a frequency higher than that of violet light. State one use of each.
Solution:
X – rays and Gamma rays. X – rays are used in X – ray crystallography. Gamma rays are used in radiotherapy.
Question .14. Give one use each of the electromagnetic radiation given below :
(i) microwave
(ii) ultra violet radiation
(iii) infrared radiation.
Solution:
(i) Microwaves are used in satellite communication.
(ii) Ultraviolet radiations are used in sterilizing purposes.
(iii) Infrared radiations are used for therapeutic purposes.
Question .15. How would you show the presence of UV and IR rays in the spectrum?
Solution:
If a blackened bulb thermometer is moved from violet end to red end of the spectrum, first an steady rise in temperature is observed . When thermometer is moved beyond the red end, rapid rise in temperature is seen. This proves the presence of IR rays. If the radiation from red end to violet end are passed through silver chloride solution it almost remains unaffected beyond the violet end proving presence of UV rays.
Question .16. When yellow paint and blue paint are mixed we get green colour. When yellow light and blue light are mixed, white light is obtained. Give reasons.
Solution:
Yellow light is made up of red and green light. When this is mixed with blue light, we get white light. The effect of mixing of two paints of different colours is different from mixing of two pure spectral light. Yellow paint reflects red, green and yellow lights and blue paint reflects blue and green lights. Hence, when these two paints are mixed, the mixture reflects only green light.
Question .17. A green shirt is observed in blue light. What colour it will appear to be and why ?
Solution:
The green shirt appear black in blue light. The reason is that, green shirt appear green in white light because it reflects green light and absorbs red and blue. In blue light, which is a primary colour, does not contain green and hence does not reflects any colour. Hence, it appear black.
Question .18. Explain why in day light an object appear red when seen through a red glass and black when seen through a blue glass ?
Solution:
An object of red or yellow (red + green) colour can reflect red light and not blue colour. Hence it seems red through a red glass and black through a blue glass.
Question .19. Write the names of a pair of colours which combine to give white light. What is the name given to such a pair of colours ?
Solution:
Pair of colours, which combine to give white light, is known as complementary colours. The complementary colours are :
(i) Red and Cyan
(ii) Green and Magenta
(iii) Blue and Yellow.
Question .20. A flag is made up of three strips of cloth of yellow, white and cyan colours. Name the colour of a particular light in which this flag will appear to be of a single colour.
Solution:
As yellow, cyan and white strips of cloth can all reflect green colour only. Therefore, the flag will appear to be of the same colour in green light.
Question .21. (i) White light is passed through a yellow filter. What colour is (colours are) seen on a screen plated at the end ?
(ii) If the light emerging from the yellow filter is then passed through a red filter, what will be seen on the screen placed at the end ?
Solution:
(i) When white light is passed through yellow filter, the colour seen on the screen are yellow, red and green.
(ii) If light emerging from the yellow filter is passed through red filter, only red colour will be seen on the screen
Question .22. What will be the colour of an object which appears green in white light and black in red light ?
Solution:
The colour of this object would be green.
Question .23. Give the approximate range of wavelengths in vacuum associated with UV rays and visible light.
Solution:
(i) For UV rays λ = 3 × 10-8 m to 4 × 10-7 m
(ii) For visible light λ = 4 ×10-7 m to 8 × 10-7 m.
Question .24. A TV station transmits waves of frequency 200 MHz. Calculate the wavelength of the waves if their speed in air is 3.0 × 108 m s-1.
Solution:
Here, ν = 200 MHz = 200 × 106 Hz, c = 3.0 × 108 m s-1.
Using, c = ν λ we have
Λ = c / ν = (3.0 × 108) / (200 × 106) = 1.5 m.
Question .25. Why are infrared radiations preferred over ordinary visible light for taking photograph in fog ?
Solution:
Infrared radiation can pass through fog without being absorbed. Hence, infrared radiation is preferred over visible light.
Question .26. (i) What will be the colour of a blue flower when it is seen in magenta coloured light?
(ii) Name another secondary colour of light in which the flower will show the same colour as it shows in the magenta coloured light.
Solution:
(i) Magenta is composed of red and blue. Hence blue coloured flower will look blue as it will reflect blue part of magenta colour.
(ii) Cyan is a combination of blue and green. Hence it will seen blue in cyan colour also.
Question .27. (i) A particular type of high energy invisible electromagnetic rays help us to study the structure of crystals. Name these rays and give another important use of these rays.
(ii) How does the speed of light in glass change on increasing the wavelength of light ?
Solution:
(i) The high energy invisible electromagnetic rays used in the study of crystals are X – rays. X – rays are widely used in locating the presence of foreign bodies in human body.
(ii) The speed increases with increase in wavelength
Question .28. A room has window pans made of a special glass which can reflect green light,
transmit red light, scatter blue light and absorb all the other colours of light.
The room is illuminated with white light from inside. What colour will the
window pane appear when seen from
(i) inside the room,
(ii) outside the room ?
Solution:
(i) From inside window pane will appear cyan colour as it reflects green light and scatters blue light and green + blue = cyan.
(ii) Form outside window pane will appear red as it transmits red light.
