Why does red light bend less than violet? “BUT” , sound diffracts more than light

Question

We can hear the sound that coming from the other room, but we can't see the light coming as well.

And in this case, sound diffracts more than light because of a wavelength of sound is bigger than light.

Now we can say that in general, waves with lower wavelength can curve around a corner less than a wave with high wavelength like sound.

So when a beam of light passes through a glass, the Violet (a lower wavelength ) diffracts more than the Red color ( a large wavelength)

And here I couldn't understand it because the two concepts should be the same and in real they're not.

Thank you

Answer 1

You are confusing diffraction and refraction. These are different things.

In the case of light through a prism it's refraction because the prism material has a refractive index dependent on the wavelength of the light.

Sound going around corners or through doorways is diffraction. The wave spreads out but there's no change in the properties of the medium (air).

Answer 2

In the example of a prism splitting light into various paths for different wavelengths, the phenomenon is a combination of refraction and dispersion.

Refraction refers to the bending of the path of a light ray due to a change in the speed change from one medium to another. But this by itself is not enough to split light into colors. There is also dispersion, which is the effect that different wavelengths have differing speeds, and shorter wavelengths slow down more than longer ones in most materials, especially glasses.

The effect of hearing low frequencies around corners better than high frequencies is called diffraction, and that's a totally different phenomenon, but it does apply to light as well as sound. Basically, diffraction tells us how good or bad an object will be in making a shadow for a wave hitting the object. If the wavelength is small compared to the object, the shadowing will be good; if the wavelength is large compared to the object, the shadowing will be poor.

Consider sound with a speed in air of about 345 m/s. A 100 Hz bass note will have a wavelength of 3.45 m, about the size of a small car, and much larger than the window openings. So bass notes from a subwoofer easily bend around common objects. On the other hand, a 600 Hz trumpet or 1000 Hz guitar riff will have a wavelength of less than a meter, and is easily shadowed by objects.