For a point object, which of the following always produces virtual image in air?

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.