Light wave enters from medium 1 to medium 2. Its velocity in $2^{\text {nd }}$ medium is double from $1^{\text {st }}$. For total internal reflection the angle of incidence must be greater than

• Correct answer
  A

  $30^{\circ}$
• B

  $60^{\circ}$
• C

  $45^{\circ}$
• D

  $90^{\circ}$

Solution

Total internal reflection occurs when the angle of incidence is greater than critical angle (C) in the denser medium.
${ }^1 \mu_2=\frac{\text { Velocity of light in medium '1' }}{\text { Velocity of light in medium ' } 2}$ ' ${ }^1 \mu_2=\frac{v}{2 v}=\frac{1}{2}$. Medium ' 1 ' is denser. or $\sin C=\frac{1}{2}=\sin 30^{\circ}$ or $C=30^{\circ}$.

Hence, the correct answer is option: 'A'.