Measure reflection angles from the normal
Locate the point of incidence and measure each ray angle from a perpendicular reference.
Exam scope and assessment notes
Mapped to the named 0973 section using teaching from the corresponding single-science course. A 0973-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade. This qualification reports two grades on the 9–1 scale; confirm the entry route for your zone and exam series.
Before you begin
- Recognise perpendicular lines.
- Distinguish an approaching ray from a departing ray.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
A normal meets a plane mirror at what angle?
What you will learn
- Define normal, angle of incidence and angle of reflection.
- Convert a supplied surface angle to a normal angle.
Build the reference at the contact point
A ray represents the direction in which light travels. Its arrow points toward the mirror for an incident ray and away for a reflected ray. The point where the incident ray meets the surface is the point of incidence. Draw the normal through that point at 90° to the mirror; it is a reference line, not another light ray.
The angle of incidence lies between the incident ray and the normal on the incident side. The angle of reflection lies between the reflected ray and the same normal. A protractor reading from the surface instead is a different angle. Mark the surface and normal explicitly before measuring either direction.
Convert rather than relabel an angle
The surface and normal form a right angle. If an approaching ray makes 25° with the surface, its angle of incidence is 90° − 25° = 65°. The diagram's orientation on the page does not change this rule. A vertical mirror requires a horizontal normal at the contact point.
For normal incidence the incident ray lies along the normal, giving an incidence angle of 0°, although it makes 90° with the mirror. Conversely a ray that almost skims the surface has an incidence angle close to 90°. The labels always identify the two lines used for measurement.
Sources for this lesson
Pause and explain
A ray approaches at 20° to the surface. What is its incidence angle?
Worked example
An incident ray makes 25° with a plane mirror. State its angle of incidence.
Show the worked solution
- Draw the normal perpendicular to the mirror.
- The surface and normal angles add to 90°.
- i = 90° − 25° = 65°.
Answer Angle of incidence 65°.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find i for a ray making 30° with a mirror surface.
Give me a hint
Use the right angle between surface and normal.
Compare my reasoning
- Draw the normal at the incidence point.
- Subtract the surface angle from 90°.
- i = 90° − 30° = 60°.
60° to the normal.
Look for these in your work
- Identify the reference.
- State the angle unit.
An incidence angle is 42°. Find the incident ray's angle to the mirror.
Give me a hint
The two acute reference angles are complementary.
Compare my reasoning
- The normal is 90° to the surface.
- Surface angle = 90° − 42°.
- The result is 48°.
48° to the surface.
Look for these in your work
- Retain which angle is given.
- Use the complement.
A mirror is vertical and a ray approaches horizontally. Locate the normal and find i.
Give me a hint
A vertical surface has a horizontal perpendicular.
Compare my reasoning
- Draw the horizontal normal through the contact point.
- The incident ray lies on that line.
- Their angle is 0°, not 90°.
Horizontal normal; i = 0°.
Look for these in your work
- Rotate the reference with the mirror.
- Do not use page vertical as the definition.
Common mistakes
- Measure from the normal, not the surface.
- The normal is a reference, not a travelling ray.
- Arrow direction distinguishes incident and reflected light.
What can you explain now?
What is the angle of incidence for a ray along the normal?
Compare with the explanation
0°.
The incident direction and normal coincide.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Define both reflection angles using the normal and explain a surface-angle conversion.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.