Reflection of Waves

This lesson covers the reflection of waves, including the law of reflection, how to draw ray diagrams for plane mirrors, and how reflection applies to different types of wave, as required by the Edexcel GCSE Combined Science specification (1SC0).

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What Is Reflection?

Reflection occurs when a wave bounces off a surface. The wave changes direction but stays in the same medium.

Key points about reflection:

• The speed, frequency and wavelength of the wave do not change during reflection.
• Only the direction of travel changes.
• Reflection can happen with all types of wave — light, sound, water waves, etc.

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The Law of Reflection

The law of reflection states:

The angle of incidence is equal to the angle of reflection.

$\text{angle of incidence} = \text{angle of reflection}$angle of incidence=angle of reflection

$\theta_i = \theta_r$θi​=θr​

Important Definitions

Term	Definition
Normal	An imaginary line drawn at 90° to the reflecting surface at the point where the ray hits
Angle of incidence (θᵢ)	The angle between the incident ray and the normal
Angle of reflection (θᵣ)	The angle between the reflected ray and the normal
Incident ray	The incoming ray of light approaching the surface
Reflected ray	The ray of light bouncing off the surface

Exam Tip: Angles are ALWAYS measured from the normal, not from the surface. This is a very common mistake that costs marks.

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Ray Diagram for a Plane Mirror

graph TD
    A["Incident ray"] -->|"θᵢ"| B["Point of incidence"]
    B -->|"θᵣ"| C["Reflected ray"]
    D["Normal (dashed, 90° to surface)"] --- B
    E["Mirror surface (horizontal)"] --- B

How to Draw a Reflection Ray Diagram

1. Draw the mirror as a straight line (with hatching on the back).
2. Draw the normal — a dashed line at 90° to the mirror at the point of incidence.
3. Draw the incident ray arriving at the mirror.
4. Measure the angle of incidence (between incident ray and normal).
5. Draw the reflected ray on the other side of the normal at the same angle.
6. Label all angles, rays and the normal.

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Reflection in a Plane Mirror — Image Properties

When you look at an object in a plane (flat) mirror, the image has the following properties:

Property	Description
Same size	The image is the same size as the object
Same distance	The image is the same distance behind the mirror as the object is in front
Virtual	The image cannot be projected onto a screen; rays appear to come from behind the mirror
Laterally inverted	Left and right are swapped (mirror image)
Upright	The image is the same way up as the object

Exam Tip: The image in a plane mirror is described as virtual, upright, laterally inverted and the same size as the object. Learn this list — it often comes up as a 2–3 mark question.

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Specular and Diffuse Reflection

Specular Reflection

• Occurs on smooth, flat surfaces (e.g. a mirror, calm water).
• All reflected rays are parallel — you see a clear image.

Diffuse Reflection

• Occurs on rough, uneven surfaces (e.g. paper, a wall, rough water).
• Reflected rays are scattered in many directions — no clear image is formed.

Type	Surface	Reflected rays	Image?
Specular	Smooth	Parallel	Clear image
Diffuse	Rough	Scattered	No clear image

The law of reflection still applies at each individual point on a rough surface, but because the normals point in different directions at different points, the overall reflected rays go in many directions.

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Reflection of Sound — Echoes

Sound waves also reflect off hard surfaces. A reflected sound wave is called an echo.

Conditions for Hearing an Echo

• The reflecting surface must be at least about 17 m away from the listener (so there is a detectable time delay between the original sound and the echo).
• Hard, flat surfaces (cliffs, buildings, walls) produce the clearest echoes.

Using Echoes to Measure Distance

You can use the equation:

$\text{distance} = \frac{\text{speed} \times \text{time}}{2}$distance=2speed×time​

The division by 2 is because the sound travels there and back.

Worked Example

A student claps near a wall and hears the echo 0.6 s later. The speed of sound in air is 340 m/s. Calculate the distance to the wall.

$\text{distance} = \frac{v \times t}{2} = \frac{340 \times 0.6}{2} = \frac{204}{2} = 102 \text{ m}$distance=2v×t​=2340×0.6​=2204​=102 m

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Reflection of Water Waves

You can observe the reflection of water waves using a ripple tank.

• Straight waves reflect off a flat barrier following the law of reflection.
• The angle of incidence equals the angle of reflection (measured from the normal to the barrier).

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Worked Example — Calculating Angles

A ray of light hits a plane mirror. The angle between the incident ray and the surface of the mirror is 35°. What is the angle of reflection?

Step 1: The angle of incidence is measured from the normal, not the surface.

$\theta_i = 90° - 35° = 55°$θi​=90°−35°=55°

Step 2: By the law of reflection:

$\theta_r = \theta_i = 55°$θr​=θi​=55°

Exam Tip: If a question gives you the angle from the surface, you must subtract from 90° to find the angle from the normal before applying the law of reflection.

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Summary

• Reflection occurs when a wave bounces off a surface, changing direction but remaining in the same medium.
• The law of reflection: angle of incidence = angle of reflection (both measured from the normal).
• Images in a plane mirror are virtual, upright, laterally inverted, and the same size as the object.
• Specular reflection (smooth surfaces) gives a clear image; diffuse reflection (rough surfaces) scatters light.
• Sound reflection produces echoes — you can calculate distance using distance = (speed × time) / 2.
• The law of reflection applies to all wave types: light, sound and water waves.

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Why Does Reflection Happen?