3 Marks Question

Question 13 Marks

In Fizeau method of measuring the speed of light, the toothed wheel is placed in the focal plane of a converging lens. How would the experiment be affected if the wheel is slightly away from the focal plane?

Answer

If the wheel is placed away from the focal plane the light returning light rays will fall in an extended area of the wheel, this will let image to appear even when the light ray is blocked by one of the teeth of the wheel.

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Question 23 Marks

In the original Fizeau method, the light travelled 8.6km and then returned. What could be the difficulty if this distance is taken as 8.6m?

Answer

There is no difficulty if the distance travelled by light is decreased. In this method, light has to travel a large distance of 8.6km. So, the intensity of the light decreases considerably and the final image becomes dim. If somehow this distance is decreased, the final image is dark due to the increased light intensity.

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Question 33 Marks

The speed of sound in air is 332m/s. Is it advisable to define the length 1m as the distance travelled by sound in $\frac{1}{332}\text{s}$?

Answer

No, it is not advisable to define the length 1m as the distance travelled by sound in $\frac{1}{332}\text{s}$ because the speed of sound is affected by various factors such as temperature, humidity and nature of medium. So, it cannot be said that the distance travelled by sound in $\frac{1}{332}\text{s}$ will be 1m, less than 1m or more than 1m.

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Question 43 Marks

Consider Galileo's method of measuring the speed of light using two lanterns. To get an accuracy of about 10%, the time taken by the experimenter in closing or opening the shutter should be about one tenth of the time taken by the light in going from one experimenter to the other. Assume that it takes $\frac{1}{100}$ second for an experimenter to close or open the shutter. How far should the two experimenters be to get a 10% accuracy? What are the difficulties in having this separation?

Answer

We have speed of light = 299792458m/sTo have a accuracy of 10% the light has to travel $\frac{1}{10}\text{th}$ of a second between the observers so,
Distance travelled by the light in 0.1s = 0.1 × 299792458 = 29979km.
The difficulty in separation of that distance will be the curvature of earth. As the earth’s surface is curved, light from one of the experimenters won’t reach the other.

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