Question

Find the angle of incidence for which angle of deviation from a liquid drop is minimum in a primary rainbow.

• A. 30⁰
• B. 40⁰
• C. 50⁰
• D. 60⁰

Answer 1

Answer: (D)

60⁰

Answer 2

sin i = µ sin r … (1)

differentiating eq. (1) cos i di = µ cos r dr

or = $\frac { \cos i } { \mu \cos r }$ = $\frac { 1 } { 2 }$

or 2 cos i = µ cos r

or 4 cos² i = µ² cos² r = µ² (1 – sin² r)

or 4 cos² i = µ²= µ² – (1 – cos² i) or

3 cos² i = µ² –1

or cos i =$\sqrt { \frac { \mu ^ { 2 } - 1 } { 3 } }$= $\sqrt { \frac { \left( \frac { 4 } { 3 } \right) ^ { 2 } - 1 } { 3 } }$ = $\sqrt { \frac { 7 } { 27 } }$

or cos i =$\sqrt { .26 }$= .5

or i = 60^0.