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

Correct option is

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

Q1

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

Q2

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

Q3

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 2o at the objective, the angular width of the image is:       

Q4

Opera glasses have a minimum length of 20 cm and a magnifying power of 5 when viewing distant objects. The focal lengths of lenses used are: 

Q5

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:       

Q6

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:

Q7

The diameter of the moon is 3.5 × 103 km and its distance from the earth is  seen by a telescope, having the focal lengths of the objective and the eye-piece as 4m and 10 cm respectively; the diameter of the image of the moon will be approximately:

Q8

The aperture of the largest telescope in the world is 5 m. if the separation between the moon and earth is 4 × 105 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:  

Q9

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:

Q10

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: