Diffraction is when waves bend around the corner of an obstacle.

It is most easily seen when a wave spreads out after passing through a gap.

diffraction waves


Medium gap: some diffraction, but mostly straight



Wavelength-size gap: most diffraction

diffraction waves in harbor


The maximum effect is when the gap and wavelength are similar in size.

Here we see diffraction as waves enter a harbor:




Diffraction is very important for Radio Waves: it lets them get around hills and into valleys:


wave diffraction radio
Radio waves with wavelengths of kilometers diffract
over hills and through valleys so you can easily get reception.

wave diffraction microwave
But microwaves with wavelength of centimeters tend to go straight.