Q. Calculate the R.I. of the medium for which the critical angle is 42 $°$  . °
Q. Mention the conditions for total internal reflection of a light ray. 
Q. Mention any two applications Of total internal reflection.  Any two applications 
Q. Write any two advantages of optical fibres over metallic cable in  communication. Any two advantages 
Q. A tank contains a slab or glass 8 cm thick and of refractive index 1.6. Above this, is a depth of 4.5 cm Of an oil and upon this a layer of water 6 cm thick and of refractive index 4/3. To an observer, looking down from above, a mark at the bottom appears to be raised upto position 6 cm from the bottom of the slab. The refractive index of oil is 
a) 0.8 
b) 1.2 
c) 1.5 
d) 1.8