Question

A parallel beams is incident on a convex lens of focal length f it is. Then put in contact with a concave lens of focal length f/2 Then the nature and position of the image? ​

Answer 1

The position of the image is infinity.

The nature of the image is real and inverted.

Explanation:

Given that,

Focal length of convex lens= f

Focal length of concave lens $f' = \dfrac{f}{2}$

The focal length of convex lens

Using formula of lens

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$.....(I)

The focal length of concave lens

Using formula of lens

$\dfrac{1}{f'}=\dfrac{1}{v}-\dfrac{1}{u}$

Put the value into the formula

$-\dfrac{2}{f}=\dfrac{1}{v}-\dfrac{1}{u}$....(II)

We need to calculate the combination of focal length

Adding equation (I) and (II)  

$\dfrac{1}{f}-\dfrac{2}{f}=\dfrac{2}{v}-\dfrac{2}{u}$

$-\dfrac{1}{2f}=\dfrac{1}{v}-\dfrac{1}{u}$

Hence, The position of the image is infinity.

The nature of the image is real and inverted.

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Topic : Optics

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