Question

.What is the focal length of double convex lens for which radius of curvatureof the surface is 40cm?(Given refractive index of glass w.r.t air is 1.5) a)50cm b)40cm c) -30cm d)-40cm​

Answer 1

Answer:

60cm

Explanation:

R

1

= 60 cm, R

2

= -60 cm, n = 1.5

[∵ For a double convex lens, R

2

=−1R

1

]

∴

f

1

=(n−1)[

R

1

−R

2

1

]

⇒

f

1

=(1.5−1)[

60

1

+

60

1

]

⇒

f

1

=0.5×

60

2

=0.5×

30

1

=

60

1

⇒ f= 60 cm