A light ray strikes a flat mirror at an angle of 25° measured from the mirror surface. What is the angle of reflection, measured from the normal?
A25°, because the angle of reflection equals the angle of incidence and that angle is 25°
B65°, because the angle of incidence from the normal is 90° − 25° = 65°
C50°, because the reflection doubles the surface angle
D90°, because light always reflects perpendicular to the surface
The law of reflection requires both angles to be measured from the **normal**, not from the surface. A ray arriving at 25° from the surface is arriving at 90° − 25° = 65° from the normal. It therefore reflects at 65° from the normal. The most common error is using the surface angle (25°) as the angle of incidence — option A is exactly this mistake. Always draw the normal first and measure from it.
Question 2 Multiple Choice
A sheet of white paper reflects light diffusely — you can see it from any direction, but you don't see your reflection in it. What explains this diffuse scattering?
APaper violates the law of reflection, scattering light randomly at each surface point
BPaper absorbs most of the light and re-emits it in all directions
CThe law of reflection holds at each microscopic surface point, but the surface normals point in many different directions, so reflected rays scatter outward
DPaper is transparent enough that light passes through and scatters inside the material
The law of reflection is not violated by diffuse surfaces — it holds perfectly at every microscopic facet. The scatter comes from the surface geometry: a rough surface has countless tiny facets each facing a slightly different direction. Parallel incoming rays obey the law at each facet, but because neighboring facets face different directions, the reflected rays spread in all directions. This is why you can see paper from any angle (scattered light reaches your eye) but it doesn't form an image (the reflected rays are not parallel).
Question 3 True / False
The law of reflection applies mainly to light waves, not to sound or water waves.
TTrue
FFalse
Answer: False
The law of reflection follows from the general physics of wave behavior at boundaries — not from anything special about light. Water waves bounce off the edge of a tank, sound echoes off walls, and radar pulses reflect off aircraft — all obeying the same rule: angle of incidence equals angle of reflection, measured from the normal. This universality is one reason the law is considered fundamental.
Question 4 True / False
A ray hitting a mirror at 40° from the normal reflects at 40° from the normal, on the opposite side of the normal.
TTrue
FFalse
Answer: True
This is the law of reflection stated correctly: both angles — incidence and reflection — are measured from the normal, and they are equal. The reflected ray lies on the opposite side of the normal from the incident ray, in the same plane. Note that 40° from the normal corresponds to 50° from the mirror surface; if the question had said '40° from the surface,' the angles of incidence and reflection would each be 50° from the normal.
Question 5 Short Answer
Why is the normal — rather than the surface itself — used as the reference line for measuring angles in the law of reflection?
Think about your answer, then reveal below.
Model answer: The normal is perpendicular to the surface at the point of contact and provides a symmetric reference: the incident and reflected rays make equal angles on either side of it. Using the surface as reference gives the complementary angle and is inconsistent across different surface orientations. The normal also generalizes naturally to curved surfaces, where the tangent plane and its perpendicular normal can be defined at each point — making it the only reference that works universally.
In practice, using the surface instead of the normal gives the complementary angle. A ray hitting at 30° from the surface has an angle of incidence of 60° from the normal — and reflects at 60°, not 30°. The normal-based convention is both mathematically consistent and physically meaningful: it correctly describes wave behavior at curved surfaces, where the local normal varies from point to point.