Reflection of LightCambridge IGCSE Physics: Revision notes
Section 1
What are the key terms used in describing reflection?
When light reflects off a surface, we use specific terminology to describe the geometry of the reflection.
The normal is an imaginary line drawn perpendicular to the reflecting surface at the point where light strikes it. It acts as the reference line for measuring angles.
The angle of incidence is the angle between the incoming ray and the normal. It is always measured from the normal, not from the surface itself.
The angle of reflection is the angle between the reflected ray and the normal. Like the angle of incidence, it is measured from the normal.
The incident ray is the light ray travelling towards the mirror, and the reflected ray is the light ray bouncing away from it.
| Term | Definition |
|---|---|
| Normal | Perpendicular line to the surface at the point of incidence |
| Angle of incidence (i) | Angle between incident ray and the normal |
| Angle of reflection (r) | Angle between reflected ray and the normal |
| Incident ray | Light ray approaching the mirror |
| Reflected ray | Light ray leaving the mirror after reflection |
Students often measure angles from the mirror surface instead of from the normal. Always measure from the normal line (perpendicular to surface), not from the surface itself.
Think of the normal as a vertical measuring rod placed on the mirror. Both angles are measured from this rod, not from the mirror's edge.
Section 2
What is the law of reflection?
The law of reflection is the fundamental rule governing how light bounces off smooth, shiny surfaces:
The angle of incidence equals the angle of reflection: i = r
This relationship always holds true for reflection at a plane (flat) mirror, regardless of the angle at which light hits the surface.
Key points:
- Both angles are measured from the normal, not from the mirror surface
- The incident ray, reflected ray, and normal all lie in the same plane (2D plane of the paper)
- This law applies to all smooth reflecting surfaces: mirrors, still water, polished metal
- The law is independent of the wavelength of light
If light hits at 30° to the normal, it reflects at 30° to the normal. If it hits at 0° (straight on, perpendicular to the surface), it reflects straight back at 0°.
Examiners expect you to state the law of reflection as 'angle of incidence equals angle of reflection' and to use the notation i = r. Show both angles clearly measured from the normal in diagrams.
If a light ray hits a mirror at an angle of incidence of 35°, the angle of reflection will be 35°. The reflected ray will travel away from the surface at the same angle that the incident ray approached it.
Section 3
How do we construct and measure reflection using the law of reflection?
Construction method using a plane mirror:
- Draw the mirror as a straight line
- Mark the point where the light ray strikes the mirror (point of incidence)
- Draw the normal perpendicular to the mirror at this point
- Draw the incident ray approaching the mirror, and measure the angle between the incident ray and the normal (this is the angle of incidence, i)
- On the opposite side of the normal, draw the reflected ray at the same angle to the normal (angle of reflection = i)
- Both rays should be on opposite sides of the normal and make equal angles with it
Measurement procedure:
- Use a protractor to measure angles accurately
- Always measure from the normal (the 90° line), not from the mirror surface
- The incident and reflected rays should make equal angles with the normal
- Check your construction: if i ≠ r, your diagram is incorrect
Calculations: When given one angle:
- If angle of incidence = 25°, then angle of reflection = 25°
- If angle of reflection = 48°, then angle of incidence = 48°
Exam papers often ask you to 'construct the reflected ray' or 'draw the path of the reflected light'. Always draw a clear normal line first, measure the angle of incidence with a protractor, then construct the reflected ray at an equal angle on the other side.
Students frequently draw the normal in the wrong direction or forget to draw it perpendicular to the mirror. The normal must always be at 90° to the mirror surface, regardless of the mirror's orientation.
Section 4
What image is formed by a plane mirror?
When you look into a plane (flat) mirror, an optical image is formed behind the mirror. This image has very specific properties:
Properties of the image formed by a plane mirror:
| Property | Characteristic |
|---|---|
| Size | Same size as the object |
| Distance | Same distance behind the mirror as the object is in front |
| Type | Virtual (cannot be projected onto a screen) |
| Orientation | Upright (not inverted) |
| Laterally reversed | Left and right are swapped |
Why is the image virtual? The image appears to be behind the mirror, but light rays do not actually meet there. The reflected rays, when traced backwards, appear to come from a point behind the mirror. This is why you cannot catch the image on a screen placed behind the mirror.
How the image forms:
- Light from the object reflects off the mirror following the law of reflection
- Your eye traces the reflected rays backwards in straight lines
- These backwards rays appear to meet at a point behind the mirror
- Your brain perceives this as an image at that location
The specification requires you to describe the image as 'same size, same distance from mirror, virtual'. Use these exact descriptors in exam answers when asked about plane mirror images.
If you stand 40 cm in front of a plane mirror and are 20 cm tall, your image appears to be 40 cm behind the mirror, is 20 cm tall, and is virtual (you cannot project it onto a screen behind the mirror).
Section 5
How can we determine image position using geometry and the law of reflection?
Geometric method to find image position:
For a plane mirror, you can locate the exact position of the image using these principles:
- Draw perpendiculars from the object to the mirror surface – this creates the shortest distance from object to mirror
- Measure this perpendicular distance (distance from object to mirror surface)
- Extend an equal distance behind the mirror – the image is located here
- The image distance equals the object distance – if the object is 5 cm from the mirror, the image is 5 cm behind it
Using the law of reflection to confirm image position:
- Draw two light rays from the object to the mirror at different points
- Apply the law of reflection (i = r) to each ray
- Trace the reflected rays backwards behind the mirror
- All reflected rays, when traced backwards, meet at the same point – this is the image location
- This confirms the image is equidistant from the mirror as the object
Calculation example: If an object is placed 12 cm in front of a plane mirror:
- Perpendicular distance to mirror = 12 cm
- Image distance behind mirror = 12 cm
- Total distance from object to image = 24 cm (12 cm + 12 cm)
When answering questions about image position, show your working clearly. Draw the perpendicular from the object to the mirror, measure it, and extend the same distance behind the mirror. State: 'image distance = object distance'.
An object 6 cm high stands 15 cm in front of a plane mirror. Find the image height and distance behind the mirror. Answer: Image height = 6 cm (same size); Image distance = 15 cm behind the mirror (same distance as object).
Must Know
- The normal is a line perpendicular to the reflecting surface at the point of incidence; angles are always measured from the normal, not from the surface
- The law of reflection: angle of incidence = angle of reflection (i = r), with both angles measured from the normal
- Images formed by plane mirrors have three key properties: same size as the object, same distance behind the mirror as the object is in front, and virtual (cannot be projected onto a screen)
- To construct a reflection diagram: draw the mirror, mark the point of incidence, draw the normal perpendicular to the mirror, measure the angle of incidence with a protractor, then draw the reflected ray at an equal angle on the opposite side of the normal
- Image position is determined by the perpendicular distance from object to mirror: the image appears at an equal distance behind the mirror (object distance = image distance)
- Virtual images form because reflected light rays, when traced backwards, appear to come from a point behind the mirror where they don't actually meet
That's the notes covered.
Carry on to the next subtopic.