Question

Two thin lenses of power 4D and -1.5 D are placed in contact. Find the power and focal length of the lens formed after combination? State the nature of each lens?

Answer 1

Answer:

P1 = +4 D            P2 = -1.5 D

P = P1 + P2                  (Note: P is the combined lens power)

P = (+4 -1.5) D

P = +2.5 D

P = 1/F

+2.5 =  1/F

F = 0.4m   OR  F = 40cm.           (F is the focal length of the combined lens)

HOPE THIS HELPS

Answer 2

Answer: $P_{eq}=2.5D$ and $F_{eq}$=0.4 m

Explanation:

• Given, $P_{1}$=4 D and $P_{2}$=(-1.5 D).
• To find the resultant power of the lens we will add the power of two lenses.
• $P_{eq}$=$P_{1}+P_{2}$

     ⇒$P_{eq}=(4-1.5)D$

     ∴ $P_{eq}=2.5D$

• We know $P=\frac{1}{F}$

         ⇒ $F_{eq}$=$\frac{1}{2.5}$

         ∴ $F_{eq}$= 0.4 m

• Hence, the required power is 2.5D and focal length is 0.4 m.

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