system
Given the following post, respond as if you're commenting on Reddit:
human
Why can I block most of the light when I hold my hand between me and a light source, but I can't block most of the sound when I hold my hand between me and a sound source?
gpt
Diffraction. Light and sound are both waves, and all waves exhibit five common behaviours:

1) Reflection, where a wave hitting a surface is reflected back or at an angle identical to its angle of incidence,

2) Refraction, where a wave moves through an interface between two different media, and in doing so changes its direction,

3) Diffraction, where a wave moving past an object is bent around the object to some extent,

4) Interference, where coincident waves will sum together to produce a disturbance which is either of greater amplitude (constructive interference) or lesser amplitude (destructive interference) than the constituent waves, and

5) Polarization, where the wave is filtered to permit excitation of the medium or transverse amplitude of the wave in only specific directions.

Diffraction, as outlined above, is a wavelength dependent phenomena. Sound waves typically travel through air at about three hundred forty meters per second, and have frequencies between 20 Hz and 20 kHz. This corresponds to wavelengths between about 17 m and 17 mm. Light, on the other hand, travels at 300,000,000 meters per second, and has wavelengths between 450 and 750 nanometres.  The longer sound waves diffract much more easily around objects which are comparable to the wavelength or larger. That said, you can see diffraction if you look at a shadow cast by your hand. When it is close to the surface, the edge of the shadow is well defined. As you move your hand away from the surface and closer to the light source, the edge becomes fuzzy and less distinct. Some of that is parallax, because the light is not a true point source, and some is diffraction, which changes the direction of some of the incoming light to scatter a bit.

Sound waves also reflect off of nearby surfaces and objects, taking alternate paths to reach your ears. Light does the same thing, which is why if you hold up your hand to block out a light source in an otherwise dark room, you can still see the back of your hand.