What is reflection?
Reflection happens when light bounces off a surface. To describe it we use some terms. The incident ray is the ray hitting the surface. The reflected ray is the ray bouncing away. The normal is an imaginary line drawn at 90° to the surface at the point where the light hits. Angles are measured from the normal, not the surface.
Key idea
Law of reflection: angle of incidence (i) = angle of reflection (r). Both angles are measured from the normal.
Smooth and rough surfaces
A smooth, shiny surface such as a mirror gives regular reflection: parallel rays stay parallel and form a clear image. A rough surface such as paper or a wall gives diffuse reflection: rays scatter in many directions, so no clear image forms but we can still see the object.
Example
If a ray hits a mirror at 30° to the normal, the reflected ray also leaves at 30° to the normal.
Image in a plane mirror
A plane mirror forms an image that is upright, the same size as the object, laterally inverted, and virtual (it appears to be behind the mirror and cannot be caught on a screen). The image is as far behind the mirror as the object is in front.
Remember
- Angle of incidence = angle of reflection.
- Angles are measured from the normal.
- A plane-mirror image is virtual and laterally inverted.