## Question

A good photographic print is obtained by an exposure of two seconds at a distance of 20 cm from the lamp. The time of exposure required to get an equally good result at a distance of 40 cm is:

### Solution

8 second

We know that the intensity of light varies inversely as the (distance)^{2}. When distance is doubled, the intensity becomes one-fourth. So, the time of exposure should be four times. Hence, time of exposure = 2 × 4 = 8 sec.

#### SIMILAR QUESTIONS

The length of a telescope is 36 cm. The focal lengths of its lenses can be

An astronomical telescope of ten-fold angular magnification has a length of 44 cm. The focal length of the objective is:

A simple telescope, consisting of an objective of focal length 60 cm and a single eye lens of focal length 5 cm is focused on a distant object in such a way that parallel rays emerge from the eye lens. If the object subtends an angle of 2^{o} at the objective, the angular width of the image is:

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An astronomical telescope having an objective of focal length 100 cm is focused on the moon. Find the distance through which the eye-piece should be pulled back to focus an object situated at a distance 80 m from the objective:

In a terrestrial telescope the focal length of erecting lens is 2 cm. The length of the telescope is 96 cm. If the magnifying power of the telescope is 10, then the focal lengths of eye-piece and objective are respectively:

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The aperture of the largest telescope in the world is 5 m. if the separation between the moon and earth is 4 × 10^{5} km and the wavelength of visible light is 5000 Å, then the minimum separation between the objects on the surface of the moon which can be just resolved is approximately:

The focal length of the objective lens of a telescope is 30 cm and that of its eye lens is 3 cm. It is focused on a scale 2 metres distant from it. The distance of the objective lens from the eye lens to see with relaxed eye is:

A camera objective has an aperture diameter *d*. If the aperture is reduced to diameter *d*/2, the exposure time under identical conditions of light should be made: