Will the focal length of a lens for red ...

Will the focal length of a lens for red light be more, sam or less than that for blue light

Text Solution

Verified by Experts

If focal length of a given lens is f in air the accoding to lens maker formula,
`1/f = (mu -1 ) [ (1)/(R_1) - (1)/(R_2)]`
`therefore 1/f prop (mu -1)`
`( because` remaining terms remain constans
`rArr` We know that `mu_b gt u_r` and so `f_b lt f_r`
Thus, focal length of a given lens is smaller for blue light as compared to red light.

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Focal length of thin lens ......

decreases with increase in refractive index.

increases with increase in refractive index.

doesn.t change with refractive index.

none of these

A: When white light passes through lens then refraction of violet light is more than that of red light. R: For given lens, focal length of red light is more than that of violet light.

Both assertion and reason are true and the reason is correct explanation of the assertion.

Both assertion and reason are true but reason is not correct explanation of the assertion

Assertion is true but the reason is false.

Both assertion and reason are false.

If the focal length of a lens for red and violet light rays respectively are f_R and f_V then which of the following is true relationship ?

`f_R ge f_V`

`f_R gt f_V`

`f_R = f_V`

`f_R le f_V`