Question

What is the equivalent focal length of coaxial contact combination of two lenses of power-15D and+5D

Answer 1

Given that

P1=-15D

P2=+5D

P=P1+P2

P= -15+5

P=-10D

Equivalent focal length ,f= 1/P

f = 1/-10

f = -0.1m

Answer 2

The equivalent power of the combination of two lenses is 10 cm.

Explanation:

It is given that,

Power of lens 1, $P_1=-15\ D$

Power of lens 2, $P_2=+5\ D$

To find,

The equivalent focal length of combination of two lenses.

Solution,

The equivalent power of the combination of two lenses is given by :

$P=P_1+P_2$

$P=-15+5$

P = -10 D

Let f is the equivalent power of the combination of two lenses. It is given by in terms of focal length as :

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

$f=\dfrac{1}{-10}$

f = -0.1 m

or

f = -10 cm

So, the equivalent power of the combination of two lenses is 10 cm.

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Equivalent power of the combination

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