Diffraction

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.