3.2.1 Reflection of light
- Syllabus
- 0625–2026–2027
- Topic
- 3.2.1
- Level
- —
Learning objectives
3.2.1.1Terms normal, angle of incidence and• Define and use the terms normal, angle of incidence and angle of reflection3.2.1.2Formation of an optical image by a• Describe the formation of an optical image by a plane mirror and give its characteristics, i.e. same size, same distance from mirror, virtual3.2.1.3For reflection, the angle of incidence• State: for reflection, the angle of incidence is equal to the angle of reflection; recall and use this relationship3.2.1.4Simple constructions, measurements and• Use simple constructions, measurements and calculations for reflection by plane mirrors
Name and measure the angles of reflection
At the point where a light ray meets a reflecting surface, the normal is an imaginary line drawn at right angles to the surface.
| Term | Meaning |
|---|---|
| incident ray | ray travelling towards the mirror |
| reflected ray | ray travelling away from the mirror after reflection |
| angle of incidence, i | angle between the incident ray and the normal |
| angle of reflection, r | angle between the reflected ray and the normal |
Draw the normal through the exact point of incidence. Place the centre of a protractor there and measure each angle from the normal, on the appropriate side.
If the angle between an incident ray and the mirror surface is 35∘, the angle of incidence is 90∘−35∘=55∘ because the normal is perpendicular to the mirror.
Angles of incidence and reflection are not measured from the mirror surface. A labelled angle touching the surface is the complement of the corresponding angle to the normal.
Explain the image formed by a plane mirror
A plane mirror forms a virtual image behind the mirror where the reflected rays appear to come from.
Light from each point on the object reflects from the mirror and enters the observer's eye. The reflected rays do not actually meet behind the mirror, but the eye traces their straight-line paths backwards. The backward extensions meet at the corresponding image point.
| Property | Plane-mirror image |
|---|---|
| size | same size as the object |
| position | same perpendicular distance behind the mirror as the object is in front |
| type | virtual: the light rays do not pass through the image position |
If an object is 30cm in front of the mirror, its image is 30cm behind it, so object and image are 60cm apart.
The image is not located on the mirror surface and cannot be projected onto a screen at its apparent position. It is seen because reflected light reaches the eye, not because light travels out from behind the mirror.
Use the law of reflection
For reflection at a plane mirror, the angle of incidence equals the angle of reflection when both are measured from the normal.
i=r
First convert any angle stated from the mirror surface into an angle from the normal. Then set r=i. Keep the incident and reflected rays on opposite sides of the normal, with arrowheads showing travel towards and away from the mirror.
A ray makes 35∘ with the mirror surface. Its angle of incidence is i=90∘−35∘=55∘, so its angle of reflection is also r=55∘.
The law does not say that a ray leaves at the same angle to the mirror surface unless those surface angles are first recognised as complements of i and r.
Construct reflected rays and plane-mirror images
Plane-mirror constructions use straight rays, a perpendicular normal and i=r to locate a reflected path or virtual image accurately.
| Construction job | Ordered method |
|---|---|
| complete a reflected ray | 1. draw the normal at the point of incidence; 2. measure i; 3. mark the same angle r on the other side; 4. draw the reflected ray with an arrow away from the mirror |
| locate a point image | 1. draw two rays from the object to different mirror points; 2. reflect each using i=r; 3. extend the reflected rays backwards with dashed lines; 4. mark their intersection behind the mirror |
| find a ray seen by an eye | 1. place the image the same perpendicular distance behind the mirror; 2. join image to eye; 3. where this line crosses the mirror is the reflection point; 4. join object to that point |
| orient a mirror for two known ray directions | from the reflection point, use the branch back towards the source and the branch towards the destination; bisect the angle between these two outward branches to obtain the normal, then draw the mirror perpendicular to it |
Use a ruler for every ray and backward extension, a sharp pencil, and a protractor centred on the point of incidence. Preserve the arrow direction: object to mirror to observer.
A completed plane-mirror image must be the same perpendicular distance behind the mirror as the object is in front. Backward extensions are construction lines, not real light paths.