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

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

Text Solution

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From lens maker's formula `(1)/(f) = (mu - 1)((1)/(R_1) - (1)/(R_2))`
AS `mu_b gt mu_r :. f_b lt f_r` or `f_r gt f_b`
i.e., focal length of a lens for red light will be more than the focal length of lens for blue light.

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If the focal length of a double convex lens for red light is f_(R) its focal length for the violet light is

`f_(R)`

Greater than `f_(R)`

less than `f_(R)`

`2f_(R)`

f_(B) and f_(R) are focal lengths of a convex lens for blue and red light respectively and F_(B_ and Fr are the focal lengths of the concave lens for blue and red light respectively. We must then have

`f_(B)gtf_(R)` and `f_(B)ltf_(R)`

`f_(B)ltf_(R)` and `f_(B)gtf_(R)`

`f_(B)gtf_(R)` and `f_(B)gtf_(R)`

`f_(B)ltf_(R)` and `f_(B)f_(R)`

Assertion : The focal length of the lens does not change when red light is replaced by blue light. Reason : The focal length of lens does not depends on colour of light used.

If both assertion and reason are true and reason is the correct explanation of assertion.

If both assertion and reason are true but reason is not the correct explanation of assertion.

If assertion is true but reason is false.

If assertion and reason both are false.