Question

a convex lens of focal length x is put in contact with a concave lens of focal length y . the equivalent focal length of the combination is​

Answer 1

Answer:

f = xy/y-x

Step-by-step explanation:

Focal length for convex lens is positive and for concave lens it's negative.

Let,

focal length of convex lens = +X

focal length of concave lens = -y

A/q

1/f = 1/x + 1/-y

= 1/x - 1/y

= y-x/xy

therefore, f = xy/y-x .

Thats all !!

Answer 2

Answer:

The equivalent focal length of combination is $f=\frac{xy}{y} -x$

Step-by-step explanation:

The focal length of the lens is defined as the distance between the lens and the image when the subject is placed in focus.

We know that

Focal length for convex lens is positive.

For concave lens it is negative.

Assuming focal length of convex lens $=+x$

focal length of concave lens $=-y$

As per the question,

$\frac{1}{f} =\frac{1}{x} +\frac{1}{-y}$

$\frac{1}{f} =\frac{1}{x} -\frac{1}{y}$

$f=y-\frac{x}{xy}$

$f=\frac{xy}{y} -x$

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