Question

An object 2.4 m in front of a lens forms a sharp image on a film 12 cm behind the lens. A glass plate 1cm thick, of refractive index 1.50 is interposed between lens and film with its plane faces parallel to film. At what distance (from lens) should object be shifted to be in sharp focus on film?

7.2 m

2.4 m

3.2 m

5.6 m

Solution

5.6 m

Case I: u = –240cm, v = 12, by Lens formula
$1 over straight f space equals space 7 over 80 Case space II colon space 12 minus 1 third space equals space 35 over 3 space open parentheses normal space shift space equals space 1 minus 2 over 3 space equals space 1 third close parentheses straight f equals space 7 over 80 straight u space equals space 5.6$

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