Question

The minimum light intensity that can be perceived by the eye is about 10–10 watt per metre2. The number of photons of wavelength  metre that must enter the pupil of area 10–4 m2 per sec for vision is approximately equal to:    

Solution

Correct option is

 

Let n be the required number of photons. Then   

                    

                   

                 

SIMILAR QUESTIONS

Q1

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:       

Q2

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:

Q3

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:

Q4

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:  

Q5

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:

Q6

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: 

Q7

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:   

Q8

A film projector magnifies a 100 cm2 film strip on a screen. If the linear magnification is 4, the area of the magnified film on the screen is:

Q9

The resolution limit of the eye is 1 minute. At a distance x km from the eye, two persons stand with a lateral separation of 3 metre. For the two persons to be just resolved by the naked eye, x should be:

Q10

An observer looks at a tree of height 15 metres with a telescope of magnifying power 10. To him the tree appears: