A school astronomy club clamps a laser pointer so that its beam strikes a small plane mirror mounted on a wall bracket. The beam hits the mirror at an angle of incidence of 35° to the normal. What is the angle between the incident ray and the reflected ray?

IGCSE Physics 0625: Angle Between Incident and Reflected Rays at a Mirror

Step 1: Recall the law of reflection

The law of reflection states that the angle of incidence equals the angle of reflection, both measured from the normal (the line perpendicular to the mirror at the point where the ray strikes it):

$i = r$

Here $i = 35°$ , so:

$r = 35°$

Step 2: Find the angle between the incident and reflected rays

The incident ray and the reflected ray each make an angle with the normal, but on opposite sides of it. The total angle between the two rays is the sum of these two angles:

$\text{angle between rays} = i + r = 35° + 35°$

$\text{angle between rays} = 70°$

Why the other options are wrong

Option	How it arises	Error
$35°$	Just $i$	Gives only the angle of incidence, not the angle between the two rays
$55°$	$90° - 35°$	Measures from the mirror surface instead of from the normal
$110°$	$2 \times 55°$	Doubles the (incorrect) surface angle instead of the normal angle

Final answers

Angle between the incident and reflected rays $= \boxed{70°}$ , option C