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Reflection Of Light

Key idea: O Level reflection: define reflection, state the laws (same plane, i = r), draw ray diagrams using the normal, and compare regular vs diffuse reflection.

  • SEC G3 Physics 2027
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Learning objectives

  • Describe wave generation by vibrating sources, ropes and springs
  • Describe ripple-tank waves using wavefronts
  • Explain that waves transfer energy
  • Explain that wave energy transfer does not transfer matter
  • Use amplitude, frequency and wavelength to describe wave motion
  • Define and use wave speed and period and interpret wave graphs
  • Recall and apply wave speed = frequency × wavelength
  • Compare transverse and longitudinal waves and give examples
  • Explain sound production by vibration and the need for a medium
  • Describe sound using compressions and rarefactions
  • Relate sound loudness to amplitude and pitch to frequency
  • Explain reflected-sound echoes and use them to measure distance
  • Explain ultrasound use in sonar and soft-tissue scanning
  • Use the normal, angle of incidence and angle of reflection
  • Apply the law of reflection in constructions, measurements and calculations
  • Use the normal, angle of incidence and angle of refraction
  • Apply sin i divided by sin r as a constant for a fixed pair of media
  • Define refractive index as vacuum light speed divided by medium light speed
  • Explain the critical angle
  • Explain the conditions for total internal reflection
  • Apply total internal reflection to optical fibres and state advantages
  • Describe how a thin converging lens acts on a light beam
  • Define the focal length of a converging lens
  • Construct real and virtual image ray diagrams for a thin converging lens
  • Describe lens images as real or virtual, magnified or diminished, and upright or inverted

1. Definition

A. Reflection of light

Reflection of light is the change in direction of a light ray at a surface so that it bounces back into the original medium.

2. Key Ideas

A. Key terms (must know)

  • Incident ray: the incoming ray that strikes the surface.
  • Reflected ray: the ray that leaves the surface after reflection.
  • Normal: a line drawn perpendicular (90°) to the surface at the point of incidence.
  • Angle of incidence, i: the angle between the incident ray and the normal.
  • Angle of reflection, r: the angle between the reflected ray and the normal.

B. Laws of reflection

  1. The incident ray, the reflected ray and the normal all lie in the same plane.
  2. The angle of incidence equals the angle of reflection: i = r.
An incident ray and reflected ray meet a plane mirror at a point where a dashed normal is perpendicular to the mirror. The angle of incidence i and angle of reflection r are measured from the normal and are equal.
Both angles are measured from the normal—not from the mirror surface—and the law of reflection gives i = r.

3. Detailed Explanations

A. How to apply the law of reflection (ray construction)

To draw a reflected ray:

  1. Mark the point where the incident ray hits the surface.
  2. Draw the normal at that point (90° to the surface).
  3. Measure the angle of incidence i from the normal.
  4. Draw the reflected ray on the other side of the normal with the same angle r = i.

B. Regular vs diffuse reflection

Both types obey i = r at every point, but the surface shapes the direction of the reflected rays.

Regular and diffuse reflectionParallel incident rays reflect in parallel from a smooth surface. The same parallel incident rays reflect in different directions from a rough surface because the local normals differ, although each ray obeys angle of incidence equals angle of reflection.Smooth surface: regular reflectionRough surface: diffuse reflectionparallel rays remain paralleldifferent local normals give different directions
Smooth surfaces give regular reflection; rough surfaces give diffuse reflection. Each individual ray still obeys i = r at its local normal.
  • Regular reflection (smooth surface): parallel incident rays remain parallel after reflection.
  • Diffuse reflection (rough surface): reflected rays go in many directions because the normals vary from point to point.

4. Common Mistakes

  • Measuring angles from the mirror surface instead of from the normal.
  • Forgetting to draw the normal at the point of incidence.
  • Mixing up i and r (they are equal, but on different sides of the normal).
  • Drawing the reflected ray on the wrong side of the normal (it must “bounce back” into the original medium).

5. Exam Tips

  • Always draw and label the normal first.
  • Use degrees (°) for angles.
  • If you are asked to “state the laws”, write:
    • “same plane”
    • “i = r”
  • For diagram questions, include arrowheads and label i and r clearly.

6. Worked Examples

Modelled example 1

Finding the angle of reflection

Core

Problem

A ray strikes a plane mirror at an angle of incidence i = 35°. Find the angle of reflection and state where both angles are measured from.
Study the worked solution

  1. Identify the reference line

    Method

    Measure the incident and reflected angles from the normal.

    Reason

    The normal is perpendicular to the mirror at the point of incidence and defines i and r.

    Working

    Angle to normal: i = 35°.

  2. Apply the law

    Method

    Set the reflection angle equal to the incidence angle.

    Reason

    The law of reflection gives i = r.

    Working

    r = i = 35°

Guided practice 2

Angle to the surface vs angle to the normal

About 4 min

Problem

A ray makes an angle of 20° with the surface of a plane mirror. Find the angles of incidence and reflection.

Convert to the required reference line

Unit: degrees

Hints

Hint 1: draw the normal
The normal makes 90° with the mirror surface.
Hint 2: use complementary angles
Subtract the surface angle from 90°.
View solution step by step

  1. Find the incidence angle

    Method

    Convert the angle to the surface into an angle to the normal.

    Reason

    Incidence is defined from the normal, not the mirror.

    Working

    i = 90°-20° = 70°

  2. Find the reflection angle

    Method

    Apply r = i.

    Reason

    Both angles use the same normal reference.

    Working

    r = 70°

Common misconception 3

Angle between incident and reflected rays

Find and correct the mistake

Learner response

A student says the angle between the incident and reflected rays is i because i = r. Diagnose the statement and derive the correct angle.

Add the two angles beside the normal

Angle between rays

View solution step by step

  1. Locate both parts of the separation

    Method

    Add the incident-to-normal angle and normal-to-reflected angle.

    Reason

    The two rays lie on opposite sides of the normal.

    Working

    Angle between rays = i + r.

  2. Apply the reflection law

    Method

    Replace r with i.

    Reason

    The law gives equal angles, but the required separation includes both.

    Working

    i + r = i + i = 2i

Examiner practice 4

Using a protractor in a ray diagram

3 marks

Examination construction

In a ray diagram, the incident ray makes i = 50° to the normal. Describe how to complete the reflected ray accurately. [3 marks]

State placement, measurement and direction

View solution step by step

  1. Construct the reflected direction

    2 marks

    Method

    Measure 50° from the normal on the other side and draw a straight ray from the point of incidence.

    Reason

    The law requires r = i = 50°, with both angles measured from the normal.

    Working

    Protractor centre at incidence point; baseline aligned with normal.

  2. Show ray travel

    1 mark

    Method

    Add an arrowhead pointing away from the mirror.

    Reason

    A ray diagram must show the direction of propagation.

    Working

    Reflected-ray arrow points into the original medium.

Challenge 5

Mirror rotation (angle change)

Minimal support

Rotated-mirror transfer

A plane mirror is rotated by 10° while the incident ray direction stays fixed. By how much does the reflected ray direction change?

Track the normal and both equal-angle changes

Hints

Hint 1: rotate the reference line
The normal rotates through the same 10° as the mirror.
Hint 2: use both sides
The incidence angle changes by 10°, and equality produces another 10° change on the reflected side.
View solution step by step

  1. Track the rotated normal

    Method

    Rotate the normal by 10° with the mirror.

    Reason

    The normal remains perpendicular to the mirror surface.

    Working

    Normal direction change = 10°.

  2. Track the reflected ray

    Method

    Apply the equal-angle change on the reflected side as well.

    Reason

    The incident ray stays fixed while the new configuration must still satisfy i = r.

    Working

    Reflected-ray change = 2(10°) = 20°.

7. Mind Stretchers

Mind stretcher 1: Why you can see paper from many directionsExtension

You can see a sheet of paper from many directions, but a mirror can give a glare only in one direction. Explain using reflection.

Show Answer

Paper gives diffuse reflection, scattering reflected rays in many directions, so light can reach your eyes from many viewing angles. A mirror gives regular reflection, so most rays reflect in one direction, producing glare only when your eye is in that direction.

Mind stretcher 2: Same law, different appearanceExtension

Both smooth and rough surfaces obey i = r. Why does a rough surface not form a clear image like a mirror?

Show Answer

On a rough surface the normal changes from point to point, so reflected rays from different parts of the surface go in different directions. The rays do not stay organised to form a clear image.

8. Practice and next step

Construct reflected rays for three incident angles, always drawing the normal first. Practise image location in plane-mirror ray diagrams, then continue to refraction.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027