IGCSE Physics / Light waves
Reflection of light and plane mirrors
Edexcel International GCSE Physics (4PH1), chapter 12: Light waves, specification point 3.14, 3.15, 3.16. Free lesson.
Read the revision notes · IGCSE Physics past papers
Light is a transverse wave that can reflect from a surface. The law of reflection helps you draw rays and describe the virtual image formed by a plane mirror.
What happens when light reflects
Light is a transverse wave: its oscillations are at right angles to the direction it travels. Light can be reflected or refracted. Reflection is when light bounces off a surface. Refraction is when it changes direction as it crosses into a different material. A plane mirror is a flat mirror.
How to measure angles of reflection
The incident ray travels towards the mirror. At the point where it hits, draw the normal, a line at 90° to the mirror. The angle of incidence, i, is measured between the incident ray and the normal. The reflected ray leaves on the other side of the normal. The law of reflection is i = r: the angle of reflection equals the angle of incidence. Measure both angles from the normal, not from the mirror surface. An angle measured from the mirror and the angle measured from the normal add up to 90°.
How to draw a reflection ray diagram
Neat, measured lines are needed for a clear ray diagram. Use a ruler for the mirror and rays, and a protractor to measure the angle from the normal. Use a pencil, ruler and protractor. Freehand lines do not score. Draw the normal as a dashed line at 90° to the mirror. Add arrows to show the direction light travels.
How a plane mirror forms an image
Two rays from the top of an object can show where its image appears. Each ray reflects according to the law of reflection. Extend the reflected rays backwards behind the mirror with dashed lines. Where those extensions meet is the top of the image. The image is virtual, so no light really reaches it and it cannot be caught on a screen. It is upright and the same size as the object. It is the same distance behind the mirror as the object is in front.
Finding the angle from the mirror surface
If an angle is given between a ray and the mirror surface, first convert it to an angle from the normal. Then use the law of reflection.