Question

A combination of lenses for a camera contains two converging lenses of focal lengths 20 cm and 40 cm and a diverging lens of focal length 50 cm. Find the power and focal length of the combination.

Answer 1

Answer:

0.055 D.

Explanation:

From the question we get that the focal lengths of two convex lenses are given as  x = 20 cm  and y = 40 cm.  

And, also the focal length of a concave lens  or the diverging lens is given as  

c = – 50 cm.  

Hence, the equivalent focal length of the combination  will be (F):-  

1/F = 1/a + 1/b + 1/c.  

1/F = 1/20 + 1/40 – 1/50 .

1/F = 11/200.  

S, on solving we will get that.

F = 200/11 = 18.18 cm  .

So, the power of the combination will be.  

1/F .

1/18.18 .

Which on solving.

0.055 D.

Answer 2

Answer:

given,

in the convex lens

f1=+20cm

f2=+40cm

in the concave lens

f3= -50cm

focal lenth of combination=

1/f1 + 1/f2 +1/f3

1/20 + 1/40 - 1/50

=11/200 so, f= 200/11 = +18.18cm

p=1/f

=1/18.18×100

=10000/1818

=+5.5D(ans)