The focal length of a convex lens refers to the distance between the lens and its focal point. In the given question, option 1 states that the focal length is the same for all colors. This is incorrect because the focal length of a lens is dependent on the refractive index of the material it is made of, which in turn varies for different colors. Therefore, the focal length is not the same for all colors.

Option 2 states that the focal length is shorter for blue light than for red. This is the correct answer. This is because blue light has a shorter wavelength compared to red light. When light passes through a convex lens, it gets refracted or bent. The refractive index of a material depends on the wavelength of light. The shorter the wavelength, the higher the refractive index. As blue light has a higher refractive index than red light, it bends more when passing through the lens, resulting in a shorter focal length.

Option 3 suggests the opposite, that the focal length is shorter for red light than for blue. This is incorrect because red light has a longer wavelength and a lower refractive index, resulting in a longer focal length.

Option 4 states that the maximum focal length is for yellow light. This is an incorrect statement.