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Physics 1 Marks Question

Speed of Light · Gujarat Board (GSEB) STD 12 Science (GSEB - English Medium). 6 questions.

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Question 11 Mark
Light passes through a closed cylindrical tube containing a gas. If the gas is gradually pumped out, the speed of light inside the tube will:
  1. Increase.
  2. Decrease.
  3. Remain constant.
  4. First increase and then decrease.
Answer
  1. Increase.

Explanation:

If the gas is gradually pumped out, a vacuum will be created inside the closed cylindrical tube, and experimentally, light travels at the fastest speed in vacuum as compared to any other medium.

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Question 21 Mark
Which of the following methods can be used to measure the speed of light in laboratory?
  1. Roemer method.
  2. Fizeau method.
  3. Focault method.
  4. Michelson method.
Answer
  1. Focault method.

Explanation:

Foucault gave the first laboratory method to find the velocity of light. He obtained a value of 2.98 × 108m/s 2.98 × 108m/s from his measurements.

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Question 31 Mark
The speed of light is 299,792,458m/s.
  1. With respect to the earth.
  2. With respect to the sun.
  3. With respect to a train moving on the earth.
  4. With respect to a spaceship going in outer space.
Answer
  1. With respect to the earth.
  2. With respect to the sun.
  3. With respect to a train moving on the earth.
  4. With respect to a spaceship going in outer space.

Explanation:

The speed of light is a fundamental constant, and with respect to any inertial frame, it is independent of the motion of the light source.

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Question 41 Mark
The speeds of red light and yellow light are exactly same:
  1. In vacuum but not in air.
  2. In air but not in vacuum.
  3. In vacuum as well as in air.
  4. Neither in vacuum nor in air.
Answer
  1. In vacuum but not in air.

Explanation:

Different wavelengths travel at different speeds through different media. In vacuum, the speeds of both the red light and yellow light are same but are different in air due to some optical density of air. Both wavelengths act in a different way in the air.

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Question 51 Mark
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,
  1. The image will be shifted simultaneously with the object.
  2. The image will be shifted a little later than the object.
  3. The image will be shifted a little earlier than the object.
  4. The image will not shift.
Answer
  1. The image will be shifted a little later than the object

Explanation:

Light rays emitting from a source have to cover some optical distance to form an image of the source on the other side of the lens. So, when a light source is shifted by some distance on the principal axis, then the light rays emitting from the new position of the source take some time to form a shifted image of the object on the other side of the lens. However, this delay is very small because the speed of light has a very larger value.

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Question 61 Mark
Which of the following methods can be used to measure the speed of light in water?
  1. Roemer method.
  2. Fizeau method.
  3. Focault method.
  4. Michelson method.
Answer
  1. Focault method.

Explanation:

Foucault method can be used to measure the speed of light in water. One of the advantage of this method is that one can put some transparent medium (or water) between two mirrors to measure the speed of light in that medium (or water). Foucault observed that the velocity of light in water is less than that in the air.​

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