what is rarer medium and denser medium??????????????????

What is Refractive Index?

Explain....

 What is astigmation?How it is cured and what is its cause?

 How to prove that

Refractive Index=Real Depth / Apparent Depth

if a glass slab is placed over paper on which letters VIBGYOR has been written  with its corresponding colour,than which letter wil appear to b raised most and why??????

what is an equiconvex lense?

plz explain what is convex and concave lens plz.. i dont have the basic concepts of them.

what is diff between :-1)Magnified and diminished image
2)Real and virtual image
3)inverted and errect image

what is difference b/w refraction and rarefaction?

 please explain with diagram principal section of a prism at mark the principal section in your diagram

1. bubbles rising up in a fish tank appear silvery
2. air bubbles trapped in a glass paper weight appear silvery
3. a crack in a window pane appears silvery
4. top surface of water contained in a beaker and held above the eye level appears silvery
5. how is totally reflecting prism is used as an erecting prism
6. what do you understand by the term angle of minimum deviation ? in this position how is the angle of incidence related to angle of emergence

Explain rainbow formation with a diagram.

a glass prism is dipped in water  will it dispersive power changes ?&why?

what is meant by the statement 'the critical angle for diamond is 24 degree'?

 Explain mirage in desertand looming in polar regions.