Reflection by a plane mirror
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 3 worked examples, 5 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Look in a mirror
- Rays and the normal
- The law of reflection
- The law
Read the transcript of the teaching part
Last time
Before we start: last time, in The electromagnetic spectrum, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Look in a mirror
Stand in front of a mirror, and you see yourself, apparently standing behind the glass. Where is that person? Nobody is behind the mirror at all. The picture is made by light bouncing off the mirror, and it follows one simple rule that you must be able to use.
Rays and the normal
We draw light as a ray: a straight line with an arrow that shows the way the light travels. The ray that arrives at the mirror is the incident ray. The ray that leaves is the reflected ray. The normal is a dashed line drawn at ninety degrees to the mirror, at the point where the ray hits. Every angle in this topic is measured from the normal, and not from the mirror.
The law of reflection
Here is a ray hitting a plane mirror. The angle between the incident ray and the normal is the angle of incidence. The angle between the reflected ray and the normal is the angle of reflection. The two angles are equal, and the incident ray, normal and reflected ray all lie in one flat plane.
The law
The law of reflection says: the angle of incidence is equal to the angle of reflection. We write it as i equals r. Both angles are on opposite sides of the normal. The law is true for every wave that reflects: light, sound and water.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Reflection and plane mirrors · Reflection by a plane mirror.
Chapter 3.2: Light
- Reflection by a plane mirror 7 min
- Refraction and refractive index 5 min
- Critical angle and total internal reflection 5 min
- Thin lenses, ray diagrams and magnification 6 min
- Uses of lenses and correcting sight 5 min
Also for O Level Physics 5054
Which topics come up most in O Level Physics 5054 past papers, counted from every paper since 2004.