Question

A girl focused monochromatic orange light instead of monochromatic green light through a convex lens What will happen to focal length? (increase decrease does not change changes by a factor of

Answer 1

When a monochromatic light passes through a lens, its focal length is affected by the refractive index of the lens material for that specific wavelength of light. The phenomenon that describes the dependency of the refractive index on the wavelength of light is called dispersion.
In general, the refractive index of most materials decreases as the wavelength of light increases. This means that materials tend to have a higher refractive index for blue/violet light (shorter wavelength) and a lower refractive index for red/orange light (longer wavelength). 
Given: - Orange light has a longer wavelength than green light. - The focal length (f) of a lens is inversely proportional to the refractive index (n) for a specific wavelength, provided the lens shape remains constant. 
Thus, when the girl switches from focusing monochromatic green light (shorter wavelength, higher refractive index) to monochromatic orange light (longer wavelength, lower refractive index), the focal length for the orange light will be slightly longer compared to that for the green light. 
In simpler terms, the focal length will **increase** when she switches from green to orange light.