A screen is placed at a distance of 100 cm from an object. The image of the object isformed on the screen by a convex lens for two different locations of the lens separated by 20 cm. Calculate the focal length of the lens used.
Asked by Raj
20th February 2018,
5:54 AM
Answered by Expert
Answer:
The given problem is illustrated in figure. As we fix the object and screen only move the lens to form images, we can see that object to lens distance u and image to lens distance are interchanged. In a former case object is placed beyond 2F to get diminished image. In later case object is placed between F and 2F and we get a magnified image.
From figure it can be seen that u+v = 100; and u-v = 20;
By solving we get u =60 cm and v = 40 cm.
hence focal length f is given by 
Answered by Expert
20th February 2018,
1:33 PM
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