Reflection
A short lesson with diagrams, a worked example, a problem to try, and a quick check. Then quiz yourself on 5 practice questions.
Light bounces off a mirror at the same angle it arrives: θreflection = θincidence, both measured from the normal (the line perpendicular to the surface). A flat mirror makes an image that is virtual, upright, the same size, and as far behind the mirror as the object is in front.
1. Where the image is
2. Flat-mirror facts
If a ray hits 35° from the surface, it’s 55° from the normal, so it reflects at 55°.
A flat mirror reverses front and back, which is why your image raises its “left” hand when you raise your right.
To see your whole body, a mirror needs to be only half your height, no matter how far away you stand. Light from your feet reflects off a point halfway between your eyes and your feet.
3. Example, then try it
You stand 3.0 m in front of a flat mirror and walk toward it at 1.0 m/s. How far are you from your image at first? How fast do you approach it?
Your image is 3.0 m behind the mirror, so you are 6.0 m apart.
As you get closer, so does your image, so the gap closes at 2.0 m/s.
A person 1.6 m tall has eyes 0.10 m below the top of their head. What is the shortest wall mirror that shows their whole body, and where should its top and bottom edges be?
Top edge: light from the top of the head reflects halfway between the head (1.6 m) and the eyes (1.5 m): 1.55 m above the floor.
Bottom edge: light from the feet reflects halfway between the feet (0 m) and the eyes (1.5 m): 0.75 m above the floor.
Height: 1.55 − 0.75 = 0.80 m, half the person’s height. Distance from the mirror doesn’t matter.
Answer: a 0.80 m mirror, from 0.75 m to 1.55 m above the floor.
4. Common mistakes
A ray hits a flat mirror 25° from the normal. What is the angle between the incoming and reflected rays?
The reflected ray is also 25° from the normal, on the other side, so the two rays are 25° + 25° = 50° apart.
Ready to practice?
5 questions on Reflection, each with a hint and an explanation for every wrong answer.