An 90mm focal-length lens is used to focus an image on the film of a camera....

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Physics

An 90mm focal-length lens is used to focus an image on the filmof a camera. The maximum distance allowed between the lens and thefilm plane is 130mm .
A. How far ahead of the film should the lens be if the object tobe photographed is 2.9
m away?
B. How far ahead of the film should the lens be if the object tobe photographed is 1.1
m away?
C. What is the closest object this lens could photographsharply?

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4.1 Ratings (799 Votes)

1/fÂ Â Â =Â Â Â 1/vÂ Â Â -Â Â Â 1/u

Â Â Â Â Â vÂ Â Â =Â Â Â image distance from lens

Â Â Â uÂ Â Â =Â Â Â object distance from lens

Â Â Â uÂ Â Â will be taken -ve following sign convention.

Â Â Â a.Â Â Â u1Â Â Â =Â Â - 2.9 mÂ Â Â =Â Â Â - 2900Â Â Â mm

Â Â Â 1/90.0Â Â Â =Â Â Â 1/v1Â Â Â +Â Â Â 1/(-2900)

Â Â Â v1Â Â Â = 90.0 * 2900 / (2900 + 90)

Â Â Â Â Â Â Â Â Â =Â Â Â 87.29 mm

u3Â Â Â =Â Â - 1.1Â Â Â mÂ Â Â =Â Â Â - 1100Â Â Â mm

Â Â Â 1/90.0Â Â Â =Â Â Â 1/v3Â Â Â +Â Â Â 1/(- 1100)

Â Â Â v3Â Â Â = 90.0 * 1100/ (1100 + 90)

Â Â Â Â Â Â Â Â = 83.19 mm

The image formed will be real as long as the object lies beyond the focus of the lens. once the object is at focus, the image will be virtual and can not be taken on the film.

Â Â Â Minimum distance over which clear photograph could be takenÂ Â = 90.0 mm

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