Stand around the corner from someone talking, out of their direct line of sight, and you can still hear them clearly, even though the sound has to bend around the corner to reach you. Stand in the same spot relative to a light source, and you're in complete darkness — light doesn't bend around the same corner in any noticeable way. Both sound and light are waves, and both are, in principle, subject to the exact same bending phenomenon: diffraction. The reason they behave so differently comes down to one number.
Diffraction: waves bending around obstacles and through gaps
Diffraction is the bending of waves around obstacles or through openings, and it was explained by Christiaan Huygens using a principle now bearing his name: every point on a wavefront can be treated as a source of new, smaller secondary wavelets, which spread out in all directions and recombine to form the wave's next position. Where a wave encounters an obstacle or passes through a gap, those secondary wavelets spread into the "shadow" region behind it, letting the wave curve into space it wouldn't reach by travelling in a straight line — which is the mechanism behind sound reaching you around a corner.
Wavelength versus obstacle size is what decides how much bending you'll actually notice
Diffraction's effect is most noticeable when a wave's wavelength is comparable to, or larger than, the size of the obstacle or gap it encounters. Audible sound has wavelengths ranging from roughly a couple of centimetres up to several metres — genuinely comparable to the size of doorways, corners, and furniture in an ordinary room — so sound diffracts around everyday obstacles enough to be clearly noticeable. Visible light has wavelengths of a few hundred nanometres, many thousands of times smaller than a doorway or a street corner, so while light technically does diffract around such large obstacles too, the effect is so minuscule relative to the obstacle's size that it's completely imperceptible in ordinary experience — for all practical purposes, light appears to travel in perfectly straight lines around large everyday objects, even though the same underlying wave physics governs both.
What we're still unsure about
Huygens' principle and the physics of diffraction are thoroughly settled and precisely predictable — this isn't a matter of live scientific dispute. Diffraction does become readily observable for light too, but only under conditions engineered specifically to make it visible: passing light through gaps or past obstacles small enough to be comparable to its wavelength, as in laboratory diffraction gratings, which is exactly how the wave nature of light was first experimentally confirmed. There's no genuine open question about the physics here; the only thing that occasionally surprises people is how dramatically different wavelength scales — audible sound versus visible light — make an identical physical phenomenon look like two completely unrelated experiences in ordinary daily life.
This sits inside Diffraction & Huygens' Principle, one of eight topics in Waves & Optics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.