a convex lens of focal length 25 cm is placed in close contact with a concave lens of focal length 20 cm.find the power and focal length of this combination.

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Solution

For two or more thin lenses close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: P = P₁ + P₂

_{$T h e c o n c a v e l e n s s h o u l d h a v e a p o w e r g i v e n b y : - P = \frac{- 1}{f} = \frac{- 1}{. 20 m} = - 5 D T h e p o w e r o f t h e c o n v e x m i r r o r w i l l b e P = \frac{1}{. 25} = 4 D T h e r e s u l \tan t p o w e r w i l l b e t h e s u m o f t h e 2 p o w e r s W h i c h w i l l b e - 1 D, i . e ., i t w i l l a c t a s a c o n c a v e l e n s o f f o c a l l e n g t h - 100 c m o r - 1 m .$}

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