question_answer

A ray of light is incident at \[\text{5}{{\text{0}}^{\text{o}}}\] on the middle of one of the two mirrors arranged at an angle of \[\text{6}{{\text{0}}^{\text{o}}}\] between them. The ray then touches the second mirror, gets reflected back to the first mirror, making an angle of incidence .....

A) \[\text{5}{{\text{0}}^{\text{o}}}\]                                           

B)  \[\text{6}{{\text{0}}^{\text{o}}}\]

C)  \[\text{7}{{\text{0}}^{\text{o}}}\]                                          

D)  \[\text{8}{{\text{0}}^{\text{o}}}\]

Correct Answer: C

Solution :

                 Let required angle be \[\theta .\] In \[\Delta ABC,\] \[{{90}^{o}}-\theta +{{140}^{o}}+{{20}^{o}}={{180}^{o}}\]                 \[\Rightarrow \]               \[\theta ={{70}^{o}}\]