We know that the focus of a lens is given by:

where f is the focal length of the lens, n is the refractive index of the material of the lens, and R₁ and R₂ are the radius of the refracting surface on the side of the object and on the other side respectively.

Now, refractive index (n) is larger blue light than red light. Hence, reciprocal of focal length is larger for blue light. Thus, focal length (f) is smaller for blue light. Hence, the focal length is more for red light than for blue light.