Physics · Grades 7–10
Reflection and the normal
Measure angles from a perpendicular reference line, not from the mirror.
Learn the idea
Normal
The normal is perpendicular to the reflecting surface at the point where the ray meets it.
Law
The angle of incidence equals the angle of reflection, measured from the normal.
Plane mirror
A plane mirror forms a virtual image the same perpendicular distance behind it as the object is in front.
Visible versus virtual
A virtual image can be seen because rays reach the eye, but those rays do not actually converge behind the mirror.
Worked examples
A ray meets a plane mirror at 35 degrees to the normal. Find its reflection angle to the normal.
The angles measured from the normal are equal.
35 degrees
A ray is 20 degrees to the mirror surface. Find its angle to the normal.
The normal is at 90 degrees to the surface: 90 − 20 = 70.
70 degrees
Check your understanding
What is the normal line?
- A line always parallel to the ray
- A line along the mirror
- A line perpendicular to the surface
An object is 0.8 m in front of a plane mirror. Find image distance behind the mirror.
Why can measuring from the mirror surface produce the wrong reported reflection angle?
Answers and marking notes
Answers and marking notes
- A line perpendicular to the surface
The normal is defined at the point of incidence.
- 0.8 m
Plane-mirror image and object distances are equal in magnitude.
- The law's angles are defined from the normal, giving complementary angles to the surface.
Credit the reference-line distinction.
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Independent educational review required. Grade bands are approximate; these are not officially aligned curricula.