Question

A light ray is incident on a flat surface, making an angle of ${20}^{\circ }$ with the surface. What is the angle of incidence and angle of reflection respectively?

• A. ${20}^{\circ },{20}^{\circ }$
• B. ${20}^{\circ },{70}^{\circ }$
• C. ${70}^{\circ },{70}^{\circ }$
• D. ${70}^{\circ },{20}^{\circ }$

Answer 1

The correct option is C ${70}^{\circ },{70}^{\circ }$

Let the angle of incidence and angle of reflection be i and r respectively
Since the angle between the surface and normal is ${90}^{\circ }$
∴ ∠ i + ${20}^{\circ }$ = ${90}^{\circ }$
⇒ ∠ i = ${90}^{\circ }$ - ${20}^{\circ }$ = ${70}^{\circ }$

We know that the law of reflection states that when a ray of light reflects off a surface, the angle of incidence is equal to the angle of reflection.

⇒ ∠ i = ∠ r = ${70}^{\circ }$