Question

Two lens of power 15d and +5d are placed is contact co-axially .the focal length of equivalent lens is:

Answer 1

D1=+15D

D2=+5D

Effective power=+15+5=+20D

D=1/f(in meters)

20=1/f

f=1/20

f=0.05m

Answer 2

The focal length of equivalent lenses is 5 cm.

Explanation:

Given that,

Power of lens 1, $P_1=15 D$

Power of lens 2, $P_2=5D$

To find,

The focal length of equivalent lens.

Solution,

In case of combination of lens, the equivalent power is given by :

$P=P_1+P_2$

$P=15+5$

P = 20 D

Let f is the focal length of equivalent lens. The relation between the focal length and the power of lens is given by :

$f=\dfrac{1}{P}$

$f=\dfrac{1}{20}$

f = 0.05 m

or

f = 5 cm

So, the focal length of equivalent lens is 5 cm.

Learn more,

Combination of lenses

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