Q#1
Light passes through a closed cylindrical tube containing gas. If the gas is gradually pumped out, the speed of light inside the tube will(a) increase
(b) decrease
(c) remain constant
(d) first increases and then decreases.
Answer: (a)
When the gas is gradually pumped out, a vacuum is being created in the tube. The refractive index for vacuum is 1. The refractive index of any other medium is more than 1. The relation between the speed of light in two mediums is given as
v₁/v₂ = µ₂/µ₁
Here µ₁, for vacuum = 1; for gas, µ₂ > 1, Hence
v₁/v₂> 1
v₁ > v₂
So, the speed of the light in the vacuum will increase. Thus the option (a) is true.
Q#2
The speed of red light and yellow light are exactly the same
(a) in a vacuum but not in the air
(b) in the air but not in a vacuum
(c) in a vacuum as well as in the air.
(d) neither in a vacuum nor in air.
Answer: (a)
The speed of light depends on the refractive index of a medium. In the vacuum, the refractive index is 1 for all wavelengths (colors) and they travel exactly with the same velocity = c. In any other medium, the refractive indices for different colors of light are different. Hence in the air µᵣ and µᵧ are different. Therefore the speed of red and yellow lights are not the same in the air.
Q#3
An illuminated object is placed on the principal axis of a converging lens so that a real image is formed on the other side of the lens. If the object is shifted a little
(a) the image will be shifted simultaneously with the object.
(b) the image will be shifted a little later than the object.
(c) the image will be shifted a little earlier than the object.
(d) the image will not shift.
Answer: (b)
The image is formed by the rays emitted from the object after refraction from the lens. So the rays have to travel a distance from the object to the image. It takes a little time for the light rays to travel this distance from the new position of the object to the image. Hence the image will be shifted a little later than the object. Option (b).
But this time will be very very small because the speed of the light is very high and may not be felt by human eyes.
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