Question

To print a photograph from a negative, the time of exposure to light from a lamp placed 60  cm away is 2.5 s. What exposure time is required if the lamp is placed 1.2 m away?

Solution

Correct option is

10 s

 

Let I be the luminous intensity of the lamp. The illuminance E1 at a distance r1 (60 cm = 0.60 m) is

                       

If t1 is the exposure time, the total amount of light falling on the negative is

                      

When the lamp is at a distance r2 = 1.2m, the illuminance will be

                     

Let t2 be the exposure time at this distance. The amount of light required is

                    

To obtain the same quality of print, the amount of light falling on the negative must be the same in the two cases, i.e.

                 

Using (i) and (ii) we have

                  

Given r1 = 0.60 mt1 = 2.5 s and r2 = 1.2 m. Hence  

                

SIMILAR QUESTIONS

Q1

A ray of light incident normally on face AB of an isosceles prism travels as shown in fig. The least value of the refractive index the prism must have should be    

                                                                        

Q2

A beaker is filled with water to a height of 8.00 cm. The apparent depth of a needle fixed at the bottom of the beaker as measured by a travelling microscope is found to be 6.00 cm. If the height of water in the beaker is increased to 12.0 cm, by what distance would the microscope have to be moved to focus on the needle again?       

Q3

When a thin wedge-shaped film is illuminated by a parallel beam of light of wavelength 6000 Ã…, 7 fringes are observed in a certain region of the film. How many fringes will be observed in the same region of the film if light of wavelength 4200 Ã… is used?

Q4

A parallel beam of white light is reflected from a thin wedge-shaped film. The colour of the fringe at the edge of the wedge will be 

Q5

When a ray of light is incident on a glass slab at an angle of 60o, the angle between the reflected and refracted rays is 90o. The refractive index of glass is  

Q6

The polarising angle for glass is 

Q7

What is the SI unit of luminous intensity?

Q8

What are the dimensions of luminous intensity?   

Q9

What is the luminous intensity of a lamp which produces an illuminance of 12 lux at a distance of 5.0 m from it?  

Q10

A lamp placed at a distance of 50.0 cm from a screen on one side produces the same illumination as a standard 60 cd candle placed 100 cm away on the other side of the screen. What is the luminous intensity of the lamp?