Reflection, Refraction and Diffraction of Waves
Simple Explanation
Reflection happens when a wave bounces off a boundary, obeying the law of reflection (angle of incidence equals angle of reflection). Refraction happens when a wave changes speed (and usually direction) crossing into a different medium. Diffraction happens when a wave spreads out after passing through a gap or around an obstacle — this spreading is most noticeable when the gap is similar in size to the wavelength.
Why Do We Need It?
These three behaviors apply to every kind of wave — water, sound, and light — which is why the same underlying physics explains echoes, mirages, and why you can hear around a corner but usually cannot see around one.
Worked Example
Explain why sound diffracts around a corner more than light
You can often hear someone talking around a corner, even though you cannot see them. Explain why, in terms of diffraction.
Why Does This Work?
Diffraction is a direct consequence of every point on a wavefront acting as a source of new, spreading wavelets (Huygens' principle) — when a gap is narrow compared to the wavelength, this spreading effect dominates and is easily visible; when the gap is huge compared to the wavelength, the spreading is there but too small to notice.
Real-Life Example
Radio waves reaching behind hills
Long-wavelength radio (AM) broadcasts can often be received behind hills or buildings, while short-wavelength FM or WiFi signals struggle with the same obstacle.
AM radio's much longer wavelength diffracts more strongly around large obstacles, letting the signal reach places a shorter-wavelength (less diffracting) signal cannot.
Practice
A wave enters a new medium where it travels more slowly. What effect is this an example of?
MediumCommon mistake
Confusing diffraction (spreading around obstacles/through gaps) with refraction (bending due to a speed change entering a new medium) — they are different effects with different causes, even though both can bend a wave's direction.
Quick Review
- Reflection: angle of incidence = angle of reflection.
- Refraction: change of speed (and usually direction) entering a new medium.
- Diffraction: spreading through gaps/around obstacles, strongest when the gap is comparable to the wavelength.