For a given pair of transparent media, the critical angle for which colour is maximum?

An equiconvex lens made of glass of refractive index $$\frac{3}{2}$$ has focal length $$f$$ in air. It is completely immersed in water of refractive index $$\frac{4}{3}$$. The percentage change in the focal length is

A point object is moving at a constant speed of 1 ms$$^{-1}$$ along the principal axis of a convex lens of focal length 10 cm. The speed of the image is also 1 ms$$^{-1}$$, when the object is at ............. cm from the optical centre of the lens.

The power of a equi-concave lens is $$-4.5 \mathrm{D}$$ and is made of a material of refractive index 1.6, the radii of curvature of the lens is