Question

When one light ray is reflected from a plane mirror with 30° angle of reflection, the angle of deviation of the ray after reflection is :

• A. 140°
• B. 110°
• C. 120°
• D. 130°

Answer 1

The Correct Option is C

Given:

• Angle of reflection (\( r \)) = \( 30^\circ \)

Step 1: Law of Reflection

According to the law of reflection:

\[ i = r, \]

where \( i \) is the angle of incidence. Therefore:

\[ i = 30^\circ. \]

Step 2: Determine the Angle of Deviation

The angle of deviation (\( \delta \)) is the angle between the incident ray and the reflected ray. It is given by:

\[ \delta = 180^\circ - 2i. \]

Since \( i = 30^\circ \), substitute the value into the formula:

\[ \delta = 180^\circ - 2(30^\circ). \]

Step 3: Calculate the Angle of Deviation

Simplify the expression:

\[ \delta = 180^\circ - 60^\circ = 120^\circ. \]

Final Answer:

The angle of deviation is \( 120^\circ \).