Question

equivalent focal length of two lenses in contact having power -15D and +5D will be​

Answer 1

p= p1 +p2

p= -15+5 = -10D

                           P=1/f (in m)

                           -10D  = 1/f

                            f= 1/-10

                            f =  - 0.1 m          

                            f= -10 cm

Answer 2

The equivalent focal length of two lenses in contact will be (-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,

Equivalent focal length of two lenses.

Solution,

When two lenses are in contact, the resultant power of the lens is the sum of power of other lenses such that :

$P=P_1+P_2$

$P=-15+5$

P = -10 D

If f is the focal length of the combination of lens, it is given by :

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

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

f = -0.1 m

or

f = -10 cm

So, the equivalent focal length of two lenses in contact will be (-10 cm)

Learn more,

Power of lens

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