A blue light beam forms an image of an object using a convex lens. If the blue light is replaced by red light, how will the image change?

1. Image will shift closer to the lens and become smaller.
2. Image will shift farther from the lens and become larger.
3. The position of the image will not change, only its brightness changes.
4. No change will be seen

Image will shift farther from the lens and become larger.

Reason — The refractive index of the lens material varies with the wavelength of light. Since blue light has a shorter wavelength, the refractive index of the lens is greater for blue light. On the other hand, red light has a longer wavelength, so the refractive index of the lens is less for red light.

Therefore, a convex lens refracts red light less than blue light. As a result, when blue light is replaced by red light, the power of the lens decreases and hence its focal length increases.

For the same object position, if the focal length increases, the image is formed farther from the lens. Also, the image distance increases, so the magnification increases, making the image larger.

Hence, when blue light is replaced by red light, the image shifts farther from the lens and becomes larger.