Question

A convex lens of focal length 25cm and a concave lens of focal length 10cm are placed in close contact with each other. Calculate the lens power of this combination.

Answer 1

See this

First divide power of convex lens then power of concave lens

100/25 + (-100/10) =P

Answer 2

The lens power of this combination is $-6D$.

Explanation:

Given the focal length of the convex lens, F = 25 cm.

The focal length of the concave lens, f = - 10 cm.

so power of the convex lens, $P = \frac{1}{0.25 m} =4D$

The power of concave lens, $p =\frac{1}{-0.10m} =-10D$

Now the combined power is the algebraic sum of the individual powers of the lenses, thus

$P_t=P+p$

$P_t=4D+(-10D)$

    $= -6D$

Hence,  the lens power of this combination is $-6D$.

#Learn More: Power of the lens.

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