(a) Under what conditions is the phenomenon of total internal reflection between the critical angle of incidence and the refractive index of the medium.
(b) Three lenses of focal lengths+10 cm, -10 cm and + 30 cm are arranged coaxially as in the figure given below. Find the position of the final image formed by the combination.

Explanation

`"For Lens "L_(1)`
`1/f=1/v-1/u,`
`1/v=(30-10)/300=20/300=1/15`
`For Lens"L_(2)`
`u=(+15-5)=10cm,`
`1/v=0,`
`"For Lens"L_(3)`
`u=oo`
`f=+30cm,`
`1/v=1/30,`
Final Image will be +30 cm from `L_(3)`.
`u=-30xm`
`110=1/v-1/(-30),`
`v=15 cm`
`f=-10cm,`
`v=oo`
`1/f=1/v-1/u,`
`v=+30 cm`
f=+10cm
`1/10-1/30-1/v`
1/f=1/v-1/10`
`1/(+30)=1/v-1/oo`