Ray Optics and ...

When Astronauts fly at higher altitude, the sky appears dark because ;

Two light rays $$P$$ and $$Q$$ are incident an optical device $$'X'$$ which finally goes along $$'R'$$ and $$'S'$$. Identify optical device $$'X'$$.

The focal length of a lens of refractive index $$\mu $$ is $$f$$. If the lens is immersed in a liquid of refractive index $$\mu $$ then the focal length of lens is 

A convex glass lens is immersed in water. Compared to the power in air, its power in water will :

A glass slab is placed in the path of beam of convergent light. The point of convergence of light 

What are the colours of the Sun observed most during sunrise/sunset and noon? 

Red light is used as danger signal because it is:

The line joining the pole and the centre of curvature of a mirror is called the :

The relation between u, v and r is:

An achromatic combination of lenses produce :