Reflection of Light

This lesson covers the reflection of light as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand the law of reflection, the difference between specular and diffuse reflection, how to draw accurate ray diagrams, and how images are formed in plane mirrors.

---

The Law of Reflection

When a ray of light hits a surface, it bounces off (reflects). The behaviour of the reflected ray follows a precise rule called the law of reflection.

The law of reflection states:

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

Both angles are measured from the normal — an imaginary line drawn perpendicular (at 90°) to the surface at the point where the light ray hits.

Key Definitions

Term	Definition
Incident ray	The incoming ray of light that strikes the surface
Reflected ray	The ray of light that bounces off the surface
Normal	An imaginary line drawn at 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

The Rule

$\text{Angle of incidence} = \text{Angle of reflection}$Angle of incidence=Angle of reflection $i = r$i=r

Exam Tip: A very common mistake is to measure the angle from the surface rather than from the normal. Always measure angles from the normal, not from the mirror surface. If the angle of incidence is measured from the surface, it will be wrong and you will lose marks.

---

Drawing Ray Diagrams for Reflection

You must be able to draw accurate ray diagrams in the exam. Follow these steps:

1. Draw the mirror surface as a straight line.
2. Mark the point where the light ray hits the mirror — this is the point of incidence.
3. Draw the normal as a dashed line at 90° to the mirror at the point of incidence.
4. Draw the incident ray arriving at the point of incidence and label the angle of incidence (i) between the incident ray and the normal.
5. Draw the reflected ray leaving the surface, ensuring the angle of reflection (r) equals the angle of incidence.
6. Add arrows on the rays to show the direction of light travel.

flowchart TD
    A["Draw the mirror surface"] --> B["Mark the point of incidence"]
    B --> C["Draw the normal at 90° to the surface<br/>(dashed line)"]
    C --> D["Draw the incident ray"]
    D --> E["Measure angle of incidence from normal"]
    E --> F["Draw reflected ray at<br/>angle of reflection = angle of incidence"]
    F --> G["Add arrows showing direction of light"]

    style A fill:#2c3e50,color:#fff
    style G fill:#27ae60,color:#fff

Exam Tip: Use a ruler and protractor when drawing ray diagrams. Freehand ray diagrams will lose marks. Make sure all angles are measured from the normal, and label i and r clearly.

---

Specular Reflection vs Diffuse Reflection

Not all surfaces reflect light in the same way. The type of reflection depends on the smoothness of the surface.

Specular Reflection

Specular reflection occurs when light reflects off a smooth, flat surface (such as a mirror, still water, or polished metal).

• All the reflected rays are parallel to each other.
• This produces a clear, sharp image — you can see your reflection.
• The normal at every point on the surface is in the same direction.

Diffuse Reflection

Diffuse reflection occurs when light reflects off a rough, uneven surface (such as paper, a brick wall, or frosted glass).

• The normals at different points on the surface point in different directions because the surface is uneven.
• The reflected rays are scattered in many directions.
• This means no clear image is formed, but the surface is visible because light still reaches your eyes from all parts of it.

Comparison Table

Feature	Specular Reflection	Diffuse Reflection
Surface	Smooth and flat	Rough and uneven
Reflected rays	Parallel	Scattered in many directions
Image	Clear reflection formed	No clear image
Examples	Mirror, still water, polished metal	Paper, wall, frosted glass

Exam Tip: Both specular and diffuse reflection obey the law of reflection at each individual point on the surface. The difference is that on a rough surface, the normals point in different directions at different points, so the reflected rays are scattered overall.

---

Plane Mirrors

A plane mirror is a flat mirror. When you look into a plane mirror, you see an image of yourself. This image has specific properties that you need to know.

Properties of the Image in a Plane Mirror

The image formed by a plane mirror is:

1. Virtual — it cannot be projected onto a screen; the light rays do not actually come from the image position.
2. Upright (erect) — the image is the same way up as the object.
3. Laterally inverted — left and right are swapped (your left hand appears to be your right hand in the mirror).
4. The same size as the object — the magnification is exactly 1.
5. The same distance behind the mirror as the object is in front of it.

Exam Tip: Remember the mnemonic "VULS" — Virtual, Upright, Laterally inverted, Same size. This covers the four key properties examiners expect you to state.

Locating the Image in a Plane Mirror

To find the position of the image:

• The image appears to be located behind the mirror.
• The distance from the mirror to the image equals the distance from the mirror to the object.
• If the object is 30 cm in front of the mirror, the image appears 30 cm behind the mirror.
• The total distance from object to image is therefore twice the object distance.

Real vs Virtual Images

Feature	Real Image	Virtual Image
Can be projected onto a screen?	Yes	No
Where rays meet	Rays actually converge at the image position	Rays only appear to come from the image position
Formed by	Converging lenses, concave mirrors	Plane mirrors, diverging lenses

---

Worked Example

A ray of light strikes a plane mirror at an angle of 35° to the mirror surface. What is the angle of reflection?

Step 1: The angle to the mirror surface is 35°. But angles must be measured from the normal.

Step 2: The normal is at 90° to the surface, so the angle of incidence = 90° − 35° = 55°.

Step 3: By the law of reflection, the angle of reflection = the angle of incidence = 55°.

Exam Tip: This is a very common exam trap. If the question gives you the angle to the surface (not the normal), you must subtract it from 90° to find the angle of incidence. Read the question carefully.

---

Worked Example 2

An object is placed 25 cm in front of a plane mirror. How far is the image from the object?

Step 1: The image is the same distance behind the mirror as the object is in front. So the image is 25 cm behind the mirror.

Step 2: Distance from object to image = 25 cm + 25 cm = 50 cm.

---

Applications of Reflection

• Periscopes use two plane mirrors at 45° to allow you to see over obstacles.
• Rear-view mirrors in cars use plane mirrors to show what is behind the vehicle.
• Kaleidoscopes use multiple mirrors to create symmetrical patterns.
• Safety mirrors in shops and on roads (these are curved, but the principle of reflection still applies).

---

Summary

• The law of reflection states that the angle of incidence equals the angle of reflection (measured from the normal).
• Specular reflection (smooth surfaces) gives clear images; diffuse reflection (rough surfaces) scatters light.
• A plane mirror produces an image that is virtual, upright, laterally inverted, and the same size as the object.
• The image in a plane mirror appears to be the same distance behind the mirror as the object is in front.
• Always measure angles from the normal, not from the surface.
• Use a ruler and protractor for accurate ray diagrams in the exam.

---

Extended Worked Examples and Deeper Understanding

Worked Example 3 — Reflection from Two Mirrors

Two plane mirrors are placed at right angles to each other. A light ray hits the first mirror at an angle of incidence of 30°. The reflected ray then strikes the second mirror. Calculate the angle of reflection from the second mirror, and show that the final ray is parallel to the original incident ray but travelling in the opposite direction.

Step 1: The first reflection obeys the law of reflection, so the angle of reflection from mirror 1 is 30°.

Step 2: Using simple geometry (the two mirrors meet at 90°), the angle at which the ray hits mirror 2 can be worked out. The angle between the reflected ray from mirror 1 and the normal of mirror 2 is (90° - 30°) = 60°. So the angle of incidence at mirror 2 is 60°, and by the law of reflection the angle of reflection from mirror 2 is also 60°.

Step 3: The total turning of the ray is (180° - 2 x 30°) + (180° - 2 x 60°) = 120° + 60° = 180°, so the final ray travels parallel to the original but in the opposite direction. This is the principle behind a corner cube reflector (retroreflector), which is used on bicycles, cars and even on the Moon (lunar retroreflectors reflect laser pulses back to Earth).

Common mistake callout: Students often add the two angles of incidence together. The correct approach is to follow the geometry step by step, measuring each angle from the normal at each mirror.

Worked Example 4 — Finding the Image Position

An object 2 m tall stands 3 m in front of a plane mirror. A student stands 4 m in front of the same mirror. How far apart are the student and the image of the object?

Step 1: The image of the object forms 3 m behind the mirror.

Step 2: The student is 4 m in front of the mirror.

Step 3: Total separation = 4 m + 3 m = 7 m.

Exam Tip: Image distance behind the mirror always equals object distance in front. Draw a clear diagram: mirror in the middle, object on one side, image on the other.

Diagram Description — Periscope Using Two Mirrors

A periscope is a vertical tube containing two plane mirrors, each mounted at 45° to the tube's axis. Light from outside enters the top opening horizontally, strikes the upper mirror at 45° (angle of incidence = 45°, angle of reflection = 45°), and is reflected vertically downwards. It travels down the tube to the lower mirror (also at 45°) and is reflected horizontally into the observer's eye. The observer therefore sees an image from above obstructions.

flowchart TD
    A["Light enters top of periscope"] --> B["Upper mirror at 45°<br/>i = 45°, r = 45°"]
    B --> C["Light travels vertically<br/>down the tube"]
    C --> D["Lower mirror at 45°<br/>i = 45°, r = 45°"]
    D --> E["Light enters observer’s eye<br/>horizontally"]

    style A fill:#3498db,color:#fff
    style E fill:#27ae60,color:#fff

Comparison Table — Reflection Scenarios

Surface	Angle i	Angle r	Image formed?	Example application
Plane mirror	0° (along normal)	0°	Yes, virtual	Bathroom mirror
Plane mirror	45°	45°	Yes, virtual	Periscope element
White paper	25° (at one point)	25° at that point, but rays scattered	No clear image	Reading page
Polished steel	60°	60°	Yes, slight distortion	Reflective kitchenware
Matt black surface	30°	No reflected ray — absorbed	No image	Optical lab lining

Common Mistakes Callout

• Mistake 1: Measuring from the surface instead of the normal. Always from the normal.
• Mistake 2: Writing "light reflects at the same angle" — be specific: "the angle of reflection equals the angle of incidence, both measured from the normal at the point of incidence".
• Mistake 3: Describing a plane mirror image as "real". It is virtual — the rays do not actually pass through the image position.
• Mistake 4: Forgetting that diffuse reflection still obeys the law of reflection at each point. The normals simply point in different directions on a rough surface.
• Mistake 5: Drawing the reflected ray as a continuation of the incident ray (passing through the mirror). The ray must bounce back into the same medium.

Exam-Style Extended Response

"Explain why you can see a printed page clearly but cannot see your own face in it, even though both use reflected light." (4 marks)

A model answer would state: the printed page has a rough surface (1). Light striking it undergoes diffuse reflection — the normals at different points on the surface are in different directions, so the reflected rays are scattered in many directions (1). You can therefore see the page from any angle because some reflected light reaches your eye. However, because the rays are scattered rather than parallel, no image of your face is formed (1). A mirror has a smooth surface, producing specular reflection, where parallel rays form a sharp image (1).

Answering at different grade levels

This table shows how the depth of answer should change as the target grade rises. The precise vocabulary — angle of incidence, angle of reflection, normal, specular, diffuse, virtual — must be used accurately at all levels.

Target grade	Expected answer to: "Describe what happens when light hits a plane mirror"
Grade 3-4	"The light bounces off. The angle it comes in at equals the angle it goes out at." Uses everyday language. No mention of the normal.
Grade 5-6	"The angle of incidence equals the angle of reflection, both measured from the normal. The image formed is virtual and the same size as the object." Correct vocabulary but limited detail.
Grade 7-8	"Light undergoes specular reflection at the smooth surface of the plane mirror. The angle of incidence equals the angle of reflection, with both measured from the normal (a line at 90° to the surface at the point of incidence). The image is virtual, upright, laterally inverted and the same size as the object, appearing the same distance behind the mirror as the object is in front."
Grade 9	As grade 7-8, plus: explicit comparison with diffuse reflection on a rough surface (where the normal points in different directions at each point, scattering reflected rays); reasoning that the image is virtual because the reflected rays only appear to originate from behind the mirror — they do not actually pass through that point; and linking to applications such as periscopes (two mirrors at 45°) or retroreflectors using corner geometry rather than lenses.

Edexcel alignment: This content is aligned with Edexcel GCSE Physics (1PH0) specification Topic 5 Light and the electromagnetic spectrum — specifically 5.1 Light and refraction (reflection at plane surfaces, law of reflection, specular vs diffuse reflection, virtual image formation by plane mirrors). Assessed on Paper 2.