Question: We just learned in school that sound and light are both examples of something called “waves.” So if they’re both waves, then how come I can hear things around a corner, but not see things around a corner? — DB, Austin, TX
Answer: Light waves are much shorter than sound waves, and the wavelength affects their behavior. The sound waves you can hear have wavelengths from around 0.02 m to 0.2 m. By comparison, light waves you can see have wavelengths from around 0.0000004 m to 0.0000007 m. So light waves have wavelengths that are hundreds of thousands of times shorter than sound waves.
The top video shows what happens to water waves when they pass through an opening that’s around the same width as one wavelength. The width of the opening (W), and the wavelength (λ) are both around 3 cm. Note how the waves spread out after they get through the opening. The waves are essentially “going around the corner” in a process called diffraction.
If, instead going through a gap in a barrier, the waves just went past a corner where the barrier ends, you’d still observe the waves spreading the same amount around that corner. Unfortunately, I don’t have a video of that, but you could easily try it in a shallow pan of water as an experiment. In order to get the incoming parallel waves you see in the videos, tap horizontally on the rim of the pan with the handle of a screwdriver. For the barrier, use something heavy so it doesn’t float. Aluminum foil rolled into a tight cylinder works nicely.
So this is what happens with sound waves and why you can hear sounds around a corner quite easily. But watch what happens when we change λ and W. Now the wavelength is 2 cm, and the gap is 6 cm:
Notice how the amount of spreading has decreased. There’s still diffraction but not nearly as much. This is how we describe diffraction mathematically:
The amount of spreading (diffraction) is θ, which in this case is close to 20°. The gap width is W = 6 cm, and the wavelength is λ = 2 cm. The basic formula for wave diffraction is: θ = sin-1(λ/W). So as the wavelength λ increases, and/or the width of the gap W decreases, θ increases and you get more diffraction.
And as λ decreases, and/or W increases, the opposite occurs — less diffraction, as in this final video where the wavelength is 1 cm and the gap width is W = 20 cm:
There is still some spreading visible, but in this case the waves pretty much continue on without turning the corner. This is more like what happens with light waves and why you can’t see around a corner. For diffraction to happen you need a gap that is close to the size of the wavelength, i.e., λ = W. And for light waves that would have to be a very small gap.
If you’d like to see light waves going around corners, check out this video produced by the UCLA Physics Department:
And if you’d like to try the experiment yourself (without using dangerous lasers) here’s a lesson plan produced by the Exploratorium in San Fransisco:
The diffraction of waves of all types is based on the Huygens – Fresnel Principle. If you want to learn more about why waves diffract just follow that link. Thanks for a great question DB, and stay curious!
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