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61. Why are “blues” (NEER) added to clothes when washed?
 
When clothes are washed frequently, they get scars and discolorations, either due to hard water phenomena or some others. And the scars are generally yellowish in color. Among the "VIBGYOR" pattern of the components of light, yellowish color has to be countered with bluish to preserve its original white color. So "blues" (NEER) is added.
 
 
62. Why is the color of the sky blue?
 
When the sunlight passes through atmosphere, they are scattered in all possible directions. The blue color is scattered more than the other dispersed colors because wavelength of blue light is minimum as compared to rest of colors. When people look at the sky (not in the direction of the Sun), the light ray reaching the observer's eye is the reflected or the scattered one, never the direct light coming from the Sun. Such light is more likely to possess light radiations of the blue group, rather than the red or the yellow group. So, the color of the sky, as it appears to the observer is blue.
 
 
63. Why is the color of the sky reddish during sunset (in the evening)?
 
This phenomena is more observed more around the region of the Sun or when looking at the Sun directly. When the observer looks at the Sun directly or looks at the regions around it, the light rays reaching the person's eyes is more or less the direct light. The blue color due to its high scattering capacity get dispersed and get finished up from the direct light, so can't reach the observer. So, the color of the sky is reddish during sunset (in the evening).
 
 
64. When white light passes through a flat piece of window glass it is not broken down into colors as a prism does it. Explain why?
 
When a ray of light is passed through a prism it get dispersed because the different colors of light are deviated in different amount. But in case of a flat piece of window glass, the light ray are only displaced but not deviated. Hence, light do not get dispersed into its component colors. So, when the light is passed through a flat piece of window glass it is not broken down into colors as does by the prism.
 
 
65. Dispersion is caused by refraction but not by reflection. Why?
 
This happens because in refraction light radiations 'get through' a different medium (i.e. a medium of different density), whereas in reflection, radiations do not even enter the different medium. When a light ray passes from one medium into another, refraction takes place where the velocities of lights having different wavelength are different so, they are deviated in different amount. Hence a light ray is dispersed due to refraction. But in the reflection phenomena, there is no change in the medium by which a light ray do not get dispersed. Hence, dispersion is cause by refraction but not by reflection.
 
 
66. What is the reason for dispersion of light?
 
The phenomenon of the splitting of white light into its constituent colors, when the light ray is passed through a prism is called "dispersion of light". When a ray of light is passed through a prism it gets dispersed because the different colors of light are deviated in different amount so they cannot be focused on the same point and its constituent colors can be seen.
 
 
67. When a ray of white light is passed through a glass prism, it is dispersed into different colors. But when it passes through a rectangular glass slab no color is seen. Explain why?
 
When a ray of light is passed through a glass prism it get dispersed because the different colors of light are deviated in different amount. But when it is passed through a rectangular glass slab no color is seen as it behaves as combination of two prism of same refractive index attached in opposite manner to each other. When white light enters into a rectangular glass slab, it is dispersed at the first face by first prism. But at the second face of the prism, the dispersion is opposite to that of the first face and equal in magnitude, so they cancel each other and the light emerging from second face of slab is white. Hence, when a ray of white light is passed through a glass prism, it is dispersed into different colors. But when it passes through a rectangular glass slab no color is seen.
 
 
68. Why is fluorescent tube light considered better than tungsten filament bulb?
 
As the maximum current energy is change into light energy in the fluorescent tube light than the tungsten filament (where the other energy is lost in the form of heat due to high resistivity), the fluorescent tube light gives more efficiency of light than tungsten filament i.e. electricity is wasted in the form of heat in tungsten filament. Hence, fluorescent tube light is considered better than tungsten filament bulb. (In a tungsten lamp, light is the result of excessive heating, so heat is produced first, then light.
 
 
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70. Why does a photographer use a red bulb in his dark room?
 
Red light has larger wavelength and so less frequency. Therefore it is scattered very less. When a photographer is in the dark room, the negatives should not As the red light scatters less as compared to other colors because its wavelength is maximum,
 
 
71. Why a 100 W bulb appears brighter than the 40W bulb placed at the same distance?
 
The luminous efficiency of a 100 W bulb is greater than that of 40 W bulb. So, 100 W bulb gives more luminosity than 40 W bulb. This is the main reason behind the glowing of 100 W bulb more brightly than 40 W bulb placed at the same distance.
 
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