Two lenses of power $$-15 \mathrm{D}$$ and $$+5 \mathrm{D}$$ are in contact with each other. The focal length of the combination is

Which of the following is NOT involved in the formation of secondary rainbow?

A double convex air bubble in water behaves as

If a ray of light in denser medium strikes a rarer medium at angle of incidence $$i$$, the angles of reflection and refraction are $$r$$ and $$r^{\prime}$$ respectively. If the reflected and refracted rays are at right angles to each other, the critical angle for the given pair of media is