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Blazing goIITian

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2 Jan 2010 11:22:22 IST

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A screen is placed 90 cm from an object.The image of the object on the screen is formed by a convex

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A screen is placed 90 cm from an object.The image of the object on the screen is formed by a convex lens at TWO deffierent locations separated by 20 cm. what is the focal length of the lens used? (NCERT PART ii)

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The HellBlaizer

Hot goIITian

Joined: 31 Jan 2009

Posts: 156

2 Jan 2010 13:21:31 IST

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Distance between the image (screen) and the object, D = 90 cm

Distance between two locations of the convex lens, d = 20 cm

Focal length of the lens = f

Focal length is related to d and D as:

Therefore, the focal length of the convex lens is 21.39 cm.

Abhishek S

Blazing goIITian

Joined: 10 Nov 2008

Posts: 658

3 Jan 2010 14:07:35 IST

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thanks..that formula wasnt there in ncert book..

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