Question

a concave lens of focal length 25 cm and convex lens of focal length 20cm placed in contact with each other what is the power and focal length of their combination​

Answer 1

Question :-

a concave lens of focal length 25 cm and convex lens of focal length 20cm placed in contact with each other what is the power and focal length of their combination

Answer :-

First we know that what is concave and convex lens ,

Concave lens :-

• A concave lens is a lens that possesses at least one surface that curves inwards. It is a diverging lens, meaning that it spreads out light rays that have been refracted through it. A concave lens is thinner at its centre than at its edges, and is used to correct short-sightedness (myopia).

Convex lens :-

• A convex lens is also known as a converging lens. A converging lens is a lens that converges rays of light that are traveling parallel to its principal axis. They can be identified by their shape which is relatively thick across the middle and thin at the upper and lower edges.

Now we goes to the question ,

Given :-

• A concave lens of focal length 25 cm

• Convex lens of focal length 20cm placed in contact with each other

To find :-

• what is the power and focal length of their combination

Now,

Focal length of convex lens ( f₁ ) = -25cm

Focal length of convex lens ( f₂ ) = +20cm

Power of combination P = ?

Focal length of combination F = ?

Power of combination is ,

$\: \: \: \: \: \: \: \: \: \: P \: = \dfrac{100}{ f_{1}(cm) }$

$\: \: \: \: \: \: \: \: \: \: P \: = \dfrac{100}{ - 25 } = - 4$

$\: \: \: \: \: \: \: \: \: \: P_{2} \: = \dfrac{100}{ f_{2}(cm) } = \dfrac{100}{20} = + 5$

$\: \: \: \: \: \: \: \: \: \: P = P _{1} + P _{2}$

$\: \: \: \: \: \: \: \: \: \: P = - 4 + 5 = 1$

Now focal length of the combination is ,

$F = \dfrac{100}{ P} = 100cm = 1m$

Therefore,

☞ Power of combination is 1

☞Focal length of combination is 1m