Reflection of Light and Plane Mirrors
Light bounces off a surface according to one simple law. The marks are for correct labelling, for measuring from the normal, and for the four properties of a mirror image.
1. The law of reflection
The angle of incidence = the angle of reflection i = r
Both angles are measured from the NORMAL — the line drawn at 90° to the mirror at the point where the ray hits.
The vocabulary, all of which is examinable:
| Term | Meaning |
|---|---|
| Incident ray | the ray travelling towards the mirror |
| Reflected ray | the ray travelling away from the mirror |
| Normal | the line at 90° to the surface at the point of incidence |
| Angle of incidence (i) | between the incident ray and the normal |
| Angle of reflection (r) | between the reflected ray and the normal |
Learn which ray is which. Confusion over the definitions of the incident and reflected rays was a recorded error — incident = incoming, and it shares a root with “incoming”.
Measure from the NORMAL, never from the mirror surface. The angle to the surface is the complement (90° − i), and using it is the most common error in the topic.
Reflection is bouncing, not bending. Defining reflection as “the bending of light” was recorded — that is refraction. Reflection is light bouncing off a surface.
The equality is between the ANGLES, not the rays. A recorded confusion treated the rays themselves as equal; it is i = r, the two angles.
2. Drawing a reflection diagram
- Draw the mirror, with hatching on the back
- Mark the point where the ray hits
- Draw the normal as a dashed line at 90° to the mirror there
- Draw the incident ray with an arrow towards the mirror
- Draw the reflected ray with an arrow away, at the same angle on the other side of the normal
- Label i and r
The normal must be dashed so it isn’t mistaken for a ray, and it must be at 90° to the mirror surface — not vertical on the page, unless the mirror is horizontal.
Put arrows on both rays to show the direction of travel.
Special cases:
- A ray hitting along the normal (i = 0°) reflects straight back on itself
- Two mirrors at an angle: the ray reflects off each in turn — apply i = r separately at each mirror
Don’t assume the same angle at both mirrors. A recorded error assumed the angle of reflection was identical at two mirrors without allowing for their inclination to each other. Work through them one at a time.
3. The image in a plane mirror
Four properties — learn all four, because questions ask for them together:
1. VIRTUAL — it cannot be projected onto a screen 2. UPRIGHT — the same way up as the object 3. SAME SIZE as the object 4. LATERALLY INVERTED — left and right appear swapped
And:
The image is the SAME DISTANCE behind the mirror as the object is in front, on a line perpendicular to the mirror.
Locating the image by construction: draw two rays from the object to the mirror, reflect each using i = r, then extend the reflected rays backwards as dashed lines behind the mirror. They meet at the image.
Draw the rays behind the mirror as DASHED lines — the light does not actually go there, which is exactly why the image is virtual.
Lateral inversion is why AMBULANCE is written reversed on the front of the vehicle: seen in a driver’s rear-view mirror it reads correctly.
Only left and right swap — not top and bottom. Writing a mirror image wrongly was a recorded error. Letters that are symmetrical about a vertical axis (A, H, I, M, O, T, U, V, W, X, Y) look unchanged; others reverse.
4. Types of reflection
Specular (regular) reflection — from a smooth surface like a mirror; parallel rays stay parallel and a clear image forms. Diffuse reflection — from a rough surface; the normals point in different directions, so parallel rays scatter and no image forms.
The law of reflection still holds at every point in diffuse reflection — it is the surface that is irregular, not the law. This is why you can see a matt wall from any angle but see no reflection in it.
5. Mistakes that cost marks
Measuring angles from the mirror instead of the normal.
Defining reflection as bending.
Confusing incident with reflected ray.
Drawing the normal as a solid line, or not at 90°.
Omitting arrows on rays.
Solid lines for virtual rays behind the mirror.
Giving fewer than four image properties.
Saying the image is real.
Saying top and bottom are inverted rather than left and right.
Assuming equal angles at two differently angled mirrors.
Frequently asked questions
What is the law of reflection? The angle of incidence equals the angle of reflection, both measured from the normal.
What is the normal? A line drawn at 90° to the surface at the point where the ray hits.
Where do I measure the angles from? Always from the normal.
What is the difference between reflection and refraction? Reflection is light bouncing off a surface; refraction is light changing direction because its speed changes.
What are the properties of an image in a plane mirror? Virtual, upright, same size, and laterally inverted, formed the same distance behind the mirror.
What does laterally inverted mean? Left and right appear swapped — top and bottom are unchanged.
Why is AMBULANCE written backwards? So it reads correctly in a driver’s rear-view mirror.
Why is the image virtual? The rays only appear to come from behind the mirror; they don’t actually meet there.
What is diffuse reflection? Reflection from a rough surface, scattering light in many directions so no image forms.
Does the law still apply to rough surfaces? Yes — at every point. The surface irregularity is what scatters the light.
Quick revision checklist
- I can state the law of reflection
- I know what the normal is and always measure from it
- I can define incident and reflected rays
- I know reflection is bouncing, not bending
- I can draw a labelled reflection diagram with a dashed normal and arrows
- I can handle a ray along the normal
- I apply i = r separately at each of two mirrors
- I know all four image properties
- I know the image is the same distance behind the mirror
- I can locate an image with dashed construction lines
- I understand lateral inversion
- I can distinguish specular from diffuse reflection
These notes cover reflection of light and plane mirrors in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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