Light and reflection · Grade 4
When light hits a smooth, shiny surface like a mirror, it reflects in a predictable way: the angle of incidence equals the angle of reflection, both measured from the normal (a line at 90° to the mirror). Rough surfaces scatter light in all directions instead, which is why paper does not show your reflection.
Key facts
- Law of reflection: angle of incidence = angle of reflection.
- Angles are measured from the normal, not from the mirror surface.
- A plane mirror image is upright, the same size, laterally inverted and virtual – as far behind the mirror as the object is in front.
- Smooth surfaces give specular (regular) reflection; rough surfaces give diffuse reflection.
- Turning a mirror through θ turns the reflected ray through 2θ.
- Two mirrors at angle θ make 360 ÷ θ − 1 images.
Worked examples
Example 1
A ray strikes a mirror at 28° to its surface. Find the angle of reflection and the angle between the incident and reflected rays.
Answer: 62°; 124°
Example 2
Kai stands 1.5 m in front of a mirror and walks towards it at 0.5 m/s. How far is he from his image at the start, and how fast do they approach each other?
Answer: 3.0 m; 1.0 m/s