Question

How does the focal length of a convex lens change if monochromatic red light is used instead of violet light?

Answer 1

Answer:

Focal length of the convex lens increases with the increase in wavelength. We know wavelength of red is more than the wavelength of violet. So, as the wavelength decreases the focal length also decreases.

Answer 2

How does the focal length of convex lens change if monochromatic red light is used instead of violet light.

We know, focal length of a lens is given as

$\frac{1}{f}=(\mu-1)\left(\frac{1}{R_1}-\frac{1}{R_2}\right)$

where μ is refractive index of lens with respect to the given medium.

refractive index of a material is inversely proportional to wavelength of light passing through it.

wavelength is minimum for violet and maximum for red light.

hence, refractive index of glass must be maximum for violet and minimum for red light.

now if monochromatic red light is used instead of violet light.

(μ - 1) will be lesser than it initial value.

hence, 1/f decreases

and hence, f increases

Therefore the focal length of a convex lens increase when we use red light instead of violet light.