What is reflection?
Reflection happens when a wave hits a barrier and bounces back into the same medium. Water waves, sound and light all reflect. To study reflection we draw the normal, a line perpendicular to the reflecting surface at the point where the wave strikes.
The law of reflection
Two angles are measured from the normal: the angle of incidence (i) between the incident ray and the normal, and the angle of reflection (r) between the reflected ray and the normal.
Key formula / Key formula
Law of reflection: angle of incidence = angle of reflection, i.e. i = r.
The incident ray, reflected ray and normal all lie in the same plane.
What stays the same?
During reflection the wave stays in the same medium, so its speed, frequency and wavelength do not change. Only the direction of travel changes. In a ripple tank, straight (plane) water waves reflected off a straight barrier come back as straight waves; waves reflected from a curved barrier can be focused to a point.
Worked example
A plane water wave strikes a straight barrier so that the incident ray makes an angle of 35° with the barrier surface. Find the angle of reflection.
The angle of incidence is measured from the normal, not the surface: i = 90° − 35° = 55°.
By the law of reflection, r = i = 55°.
Everyday uses
- An echo is reflected sound.
- Sonar and ultrasound use reflected sound to measure distances.
- Mirrors reflect light so that i = r, forming clear images.
Remember
- Always measure i and r from the normal, not the surface.
- Reflection keeps speed, frequency and wavelength the same.
- Only the direction of the wave changes.