The focal length of convex lens 20 cm. It is kept in contact with a concave lens of focal length 25cm. Determine the focal length and power of the combined lens.

Asked by imranbharveli | 7th Mar, 2020, 07:47: PM

Expert Answer:

Let f1 be the focal length of convex lens and f2 be the focal length of concave lens.
 
If F is focal length of comined lenses, then we have, 1/F = ( 1/f1 ) + ( 1/f2 )
 
1/F = (1/20) - (1/25) = 1/100
 
hence focal length F of combined lenses is 100 cm
 
power of convex lens P1= 1/0.2 = +5
power of concave lens P2= 1/(-0.25) = - 4
 
power of combined lens = P1 + P2 = +5 -4 = +1

Answered by Thiyagarajan K | 7th Mar, 2020, 08:55: PM

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