When a light enters from medium 1 to 2, its speed increases by `20%` and when medium 1 to medium 3, its speed decreases by `20%`. Consider the situation in the following figure. Find r.

Explanation

`muprop1/v`
`mu_1/mu_2=v_2/v_1=1.2`
`mu_1/mu_3=v_3/v_1=0.8`
`musin30^@=mu_1sinr`
`sin30^@=(mu_1)/(mu_2)sinr`
`1/2=1.2sinr`
`sin r=(1)/(2.4)=(10)/(24)=(5)/(12)`
`r=sin^-1(5//12)`