1.

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 ?

Answer»

`f_R GE f_V`
`f_R GT f_V`
`f_R = f_V`
`f_R le f_V`

Solution :`1/f=(n-1)[(1)/(R_1)-(1)/(R_2)]`
`therefore(1)/(f_R)=(n_R-1)((1)/(R_1)-(1)/(R_2))` and
`(1)/(f_V)=(n_V-1)((1)/(R_1)-(1)/(R_2))`
`therefore" RATIO "(f_V)/(f_R)=(n_R-1)/(n_V-1)`
`n_R lt n_V`
`therefore n_R-1 lt n_V-1`
`therefore (n_R-1)/(n_V-1) lt 1`
`therefore (f_V)/(f_R) lt 1`
`therefore f_V lt f_R`
`therefore f_R gt f_V`


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