Question

Obtain expression for the effective focal length,when convex lens and concave lens of f1 and f2 placed co axially with each other....

Answer 1

Answer:

PHYSICS

When two thin lenses of focal lengths f

1

and f

2

are kept coaxially and in contact, prove that their combined focal length "f" is given by

f

1

=

f

1

1

+

f

2

1

October 15, 2019avatar

Naseha Keluth

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VIDEO EXPLANATION

ANSWER

Let a point object O is placed on the principal axis

By lens formula for image formation by lens L

1

v

′

1

−

u

1

=

f

1

1

....... (1)

I' serves as a virtual object for the second lens L

2

which forms a final image I at a distance 'v'. Then for L

2

v

1

−

v

′

1

=

f

2

1

........ (2)

Adding equation (1) and (2)

v

1

−

u

1

=

f

1

1

+

f

2

1

.......... (3)

If we replace these two lenses by a single lens such that if an object is kept at u image is formed at v then its focal length f is given by

v

1

−

u

1

=

f

1

....... (4)

From equation (3) and (4) we get

f

1

=

f

1

1

+

f

2

1

Explanation:

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