Question

Two lens of powers +1.5D and - 1D are kept in contact. What is the focal length of the combination? ​

Answer 1

Explanation:

Combined power of lens

⇒P = P1 + P2

⇒P = P1 + P2

⇒ +1.5 – 1D

⇒+0.5D

∴

Power of lens = +0.5D

⇒Power of lens = 1/focal length(in meters)

⇒0.5=1/focal length

⇒focal length=1/0.5

⇒focal length= 10/5

⇒focal length= 2meters

∴

focal length = 2×100cm=200cm

Answer 2

Answer: f = + 2.00 m

Explanation:

We know,

P(net) = Sum of power of all lenses kept in contact ( with sign convention )

=> P(net) = + 1.5 D - 1.0 D = + 0.5 D

=> 1/f = ½ m^-1

=> f = 2 m.