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?

Why Kaysons ?

Video lectures

Access over 500+ hours of video lectures 24*7, covering complete syllabus for JEE preparation.

Online Support

Practice over 30000+ questions starting from basic level to JEE advance level.

Live Doubt Clearing Session

Ask your doubts live everyday Join our live doubt clearing session conducted by our experts.

National Mock Tests

Give tests to analyze your progress and evaluate where you stand in terms of your JEE preparation.

Organized Learning

Proper planning to complete syllabus is the key to get a decent rank in JEE.

Test Series/Daily assignments

Give tests to analyze your progress and evaluate where you stand in terms of your JEE preparation.

SPEAK TO COUNSELLOR ? CLICK HERE

Download JEE Question paper & Solution Download JEE Important questions Download Complete syllabus of Maths, Physics, Chemistry

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?

5 s
10 s
15 s
20 s

diffcult

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 m, t₁ = 2.5 s and r₂ = 1.2 m. Hence

Download JEE Question paper & Solution Download JEE Important questions Download Complete syllabus of Maths, Physics, Chemistry

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

easy View solution

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?

medium View solution

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?

easy View solution

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

easy View solution

Q5

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

easy View solution

Q6

The polarising angle for glass is

easy View solution

Q7

What is the SI unit of luminous intensity?

easy View solution

Q8

What are the dimensions of luminous intensity?

easy View solution

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?

easy View solution

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?

easy View solution