Shadows: particles and waves

picture of light and paint shadows

Light casts shadows. But shadows can be made by particles or waves. They do tell us about the nature of light, but we need to be quantitative. This page supports the multimedia tutorial The Nature of Light.

Shadows: light and particles (droplets)

In the familiar, macroscopic world, light casts a shadow, as it does here below my hand. A beam of particles, such as the droplets of paint in this film clip, also makes a shadow. Many great thinkers, including Newton, thought that light was made of particles.

Particles make a shadow. Light makes a shadow. But be careful what we conclude.

Before we jump to conclusions, let's look at shadows cast by sound.

Sound shadows: short vs long wavelengths

In this experiment, a small source of sound (the pipe, with diameter 8 mm) produces sinusoidal sound waves with known frequencies and wavelengths, in a room treated to reduce reflections and reverberation. Close above the the source, I hold a book above it. Above that, I hold a microphone which I scan sideways. The sound track for one scan is shown in each experiment.
Does sound cast a shadow? At left, the frequency is 10 kHz and the wavelength 34 mm. At right, the frequency is 340 Hz and the wavelength 1 m. When do we see shadows?

In the first experiment, the wavelength is six times smaller than the width of the book. We see a sound shadow above the book, but we see interesting effects at the edges (which we'll return to in the chapter on diffraction). In the second, the wavelength is several times longer than the width of the book and we see no shadow. Conclusion: to observe clear shadows, we need the wavelength to be rather smaller than the size of the objects investigated.


Water waves, sound waves and light waves

Living by the sea, I often notice this effect. Run the movie to compare the behaviour of light and sound.

Water waves provide a sound source with long and short wavelengths. How do sound and light compare?

So, light behaves more like the short wavelength sound than the long wavelength sound. Not surprising, given the very short wavelength of light. One critical test for both is whether they exhibit interference – a property of waves. Go to this page about Young's experiment. This link will return you to the multimedia tutorial The Nature of Light.

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