Mini Physics
Part of Mini EducationSwitch subject
Search

Drawing Ray Diagrams For Plane Mirrors

Key idea: How to draw ray diagrams for plane mirrors in O Level Physics: normal, i = r, and locating virtual images step-by-step.

  • SEC G3 Physics 2027
On this page

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. Ray diagram (plane mirror)

A ray diagram is a drawing that shows the paths of light rays. For a plane mirror, it helps you apply the law of reflection (i = r) to predict what an observer sees.

2. Key Ideas

  • Draw rays as straight lines with arrowheads.
  • At the point where a ray hits the mirror, draw the normal.
  • Use the law of reflection: i = r (angles measured from the normal).
  • To locate the image in a plane mirror, extend reflected rays behind the mirror with dashed lines (virtual rays).
Prerequisite

Review the key terms and the laws first: Reflection of light.

3. Detailed Explanations

A. Step-by-step method (plane mirror)

  1. Draw the mirror and object to scale where dimensions are given.
  2. Choose two rays from the top of the object to different points on the mirror.
  3. At each point of incidence, draw a normal and reflect the ray so that i = r.
  4. Add arrowheads towards the observer on the real reflected paths.
  5. Extend the reflected rays backwards behind the mirror using dashed lines. Their apparent intersection locates the top of the virtual image.
  6. Draw the image upright, the same size as the object, and the same perpendicular distance behind the mirror.
Constructing a plane-mirror imageA vertical plane mirror separates an upright object from its virtual image. Two solid incident and reflected rays obey equal angles at the mirror, while dashed extensions behind the mirror meet at the top of the image.Solid rays are real paths; dashed lines are backward extensionsplane mirrorobjectvirtual imagenormali = robserverdistance dsame distance d
Scroll diagram horizontally to read all labels.
Two rays leave the top of the object, reflect with i = r, and reach the eye. Dashed backward extensions meet behind the mirror at the virtual image. The image is the same perpendicular distance behind the mirror as the object is in front.

4. Common Mistakes

  • Measuring angles from the mirror surface instead of from the normal.
  • Drawing only one ray (you need at least two to locate an image position).
  • Forgetting arrowheads (direction of travel matters).
  • Drawing dashed lines in front of the mirror (extensions go behind the mirror).

5. Exam Tips

  • Use a ruler and keep rays straight.
  • Keep the diagram to scale if distances are given.
  • Use dashed lines only for extensions (virtual rays).
  • Label i and r clearly if angles are part of the question.

6. Worked Examples

Modelled example 1

Spotting a law-of-reflection error

Core

Problem

A ray diagram labels i = 25° and r = 40° at a plane mirror. Diagnose and correct it.
Study the worked solution

  1. Recall the governing check

    Method

    Compare the two angles measured from the normal.

    Reason

    The law of reflection requires i = r.

    Working

    25° ≠ 40°, so the diagram is inconsistent.

  2. Correct the reflected ray

    Method

    Keep i = 25° and set r = 25°.

    Reason

    The reflected ray must make the equal angle on the other side of the normal.

    Working

    r = 25°.

Guided practice 2

Locating an image (method question)

About 6 min

Problem

Describe how to locate the image of an object in a plane mirror using a ray diagram.

Try this before viewing the solution

Hints

Hint 1: begin with two incident rays
Choose two different points of incidence from the same object point.
Hint 2: separate real paths and extensions
Use solid reflected rays in front and dashed backward extensions behind the mirror.
View solution step by step

  1. Construct the reflected rays

    Method

    Draw two rays, normals and reflected paths satisfying i = r.

    Reason

    Two independent paths are needed to locate one apparent source point.

    Working

    Add arrowheads on the real light paths.

  2. Locate the virtual image

    Method

    Extend both reflected rays backwards with dashed lines.

    Reason

    The observer traces the rays back in straight lines.

    Working

    The dashed-line intersection marks the image position.

Common misconception 3

Interpreting dashed lines

Find and correct the mistake

Learner claim

A learner says the dashed lines behind a plane mirror show light passing through the mirror. Locate and correct the error.

Try this before viewing the solution

Correct meaning

View solution step by step

  1. Identify the line convention

    Method

    Read dashed lines as backward extensions.

    Reason

    The reflected rays appear to originate behind the mirror.

    Working

    They are construction lines, not transmitted rays.

  2. State the image consequence

    Method

    Their apparent intersection locates a virtual image.

    Reason

    No real rays converge at that point.

    Working

    Virtual image behind the mirror.

Examiner practice 4

Angle from the normal (common trap)

3 marks

Examination question

A ray strikes a plane mirror at 30° to the mirror surface. Find i and r. [3 marks]

Try this before viewing the solution

View solution step by step

  1. Identify the reference

    1 mark

    Method

    Use the normal as the angle reference.

    Reason

    The normal is perpendicular to the mirror.

    Working

    i = 90°-30°.

  2. Find incidence

    1 mark

    Method

    Calculate i = 60°.

    Reason

    The surface angle and normal angle are complementary.

    Working

    i = 60°.

  3. Apply reflection

    1 mark

    Method

    Set r = i.

    Reason

    The law of reflection applies.

    Working

    r = 60°.

Challenge 5

Image distance in a plane mirror

Minimal support

Construction-to-distance transfer

An object is 8.0 cm in front of a plane mirror. Use the plane-mirror construction property to locate the image.

Try this before viewing the solution

Unit: cm

Hints

Hint 1: use perpendicular symmetry
The image lies as far behind the mirror as the object lies in front.
View solution step by step

  1. Recall the image property

    Method

    Set image distance equal to object distance.

    Reason

    A plane-mirror image is symmetric about the mirror plane.

    Working

    dᵢ = dₒ.

  2. Locate the image

    Method

    Place it 8.0 cm behind the mirror.

    Reason

    The given object distance is perpendicular to the mirror.

    Working

    dᵢ = 8.0 cm behind.

7. Mind Stretchers

Mind stretcher 1: Why two rays are enoughExtension

Why do you only need two rays to locate the image in a plane mirror ray diagram?

Show Answer

Two straight lines intersect at a single point. The image position is where the backward extensions of the reflected rays meet, so two rays are sufficient to determine that point.

Mind stretcher 2: Image position vs observer positionExtension

If you move your eye to a different position, does the image position behind a plane mirror change? Explain using ray diagrams.

Show Answer

The image position does not change. Different rays are reflected to different eye positions, but their backward extensions still meet at the same point behind the mirror (the virtual image position).

8. Practice and next step

Construct a two-ray image without tracing an existing diagram, then verify equal object and image distances perpendicular to the mirror. Continue to refraction of light.

Continue with the next resource in this course.

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