sir,how focallength of a lens changes when red light is replaced by blue light??????????

        According to Cauchy equation

Hence the refractive  index of a substance is inversely proptional to the wavelength.

Since according the Lens makers formula, the refractive index of a material is directly dependent on its refractive index. Hence in this way we can say that the focal length is dependent on the wavelength of light . Hence blue and red light will show a great variation in thier focal lengths.

In fact his is the cause beind the chromatic abberation produced by the glasses.