Question

A thin convex lens of focal length 20 cm is placed in closed contact with a thin concave lens of focal length 5 cm. The power of combination is​

Answer 1

Focal of convex lens(f)=+20cm=20/100=+.20m

44P=1/f=1/.20=100/20=+5D

focal length of concave lens(f)=−10cm=−10/100=−1m

P=1/−1=−10D

power of combination=5−10=−5D

# MysteriousShine

Answer 2

Given:-

Focal length of convex lens = 20cm

and, focal length of concave lens = 5cm

To find:-

The power of combination of lenses.

Solution:-

Let the focal length of convex mirror be f and focal length of convave mirror be F.

Let p and P be the power of convex and concave lens respectively.

Now, f = 20cm

therefore,

p = 100/20 = 5D

And, F = -5cm

therefore,

P = 100/-5 = -20D

Now, power of combination of lenses

= p+ P

= 5 +(-20)

= -15D

Hence, power of combination is -15D.

Formula used:-

P = 1/f(m) = 100/f(cm)

where, P = Power of lens

and, f = focal length