1.

When monochromatic red light is used instead of blue light in a convex lens, its focal length will ......

Answer»

not DEPEND on COLOUR in light.
INCREASE
decrease
remain same

Solution :For red light, and blue light `n_r lt n_b`
Now `1/f=(n-1)((1)/(R_1)-(1)/(R_2))`
`1/f prop n`
`therefore (f_2)/(f_b)=(n_b)/(n_r)`
`therefore (f_r)/(f_b) gt 1 ""thereforef_r gt f_b`
`therefore` Focal length will increase.


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