Diffraction of light: if light is a wave, why does it cast clear shadows? Why does it form beams? How does its behaviour compare with that of sound waves, water waves, electron waves? This page compares diffraction of various waves and uses Huygens' construction to explain diffraction and beam formation. It supports the multimedia tutorial Diffraction.
The pictures at right show shadows formed by my hand from light and from a beam of paint particles. Macroscopic objects cast sharp shadows in light: how is this poossible if light is a wave?
Why does sound go around corners, but not light?
It's a fair question: I put my hand in front of my mouth and you can still hear me, even if you can't see my lips. But does sound always 'go around corners'?
Sound waves sometimes go around corners
You can try this wherever you have a broad band source of sound, such as waves, the wind in the trees, or a busy road. Obviously, light doesn't (noticeably) bend around the corner. For sound, low frequencies diffract more than high. With a friend, you can probably notice the effect when with your voice. Roughly speaking, vowels have mainly low frequency and consonants high frequency.
The ripple tank
The ripple tank, viewed from above.
This clip is just to show the ripple tank. An electromechanical oscillator (like a loudspeaker but with a shaft instead of a cone) drives the horizontal bar (220 mm long) up and down, making a set of parallel wavefronts. A rubber (50 mm long) casts a shadow. This arrangement is unsuitable for filming because we can only see the waves clearly from some angles.
Projecting the waves on a screen
The geometry for projecting images of water waves on a screen
The ripple tank is suspended over an overhead project. The refraction of light at the air-water surface produces dark and bright bands on the screen.
Water wave shadows and beams
Shadow, short wavelength
Shadow, longer wavelength
Beam, short wavelength
Beam, longer wavelength
Two different geometries times two different wavelengths. In the top row, the parallel wavefronts from the source strike an object. When the wavelength is several times smaller than the object (at left), we see a reasonably clear shadow. When the wavelength is longer (roughly half the length of the object), the shadows are rather less clear.
In the bottom row, an aperture in a barrier creates a beam of waves. For the shorter wavelength (left) the beam is reasonably clearly defined, and its edges are reasonably distinct. With the larger wavelength, we see stronger effects of diffraction at the edges of the beam.
'Photo 51', an X-ray image by Raymond Gosling and Rosalind Franklin in 1952; A neutron diffraction image from the ISIS neutron source; Electron diffraction in the UNSW 2nd year lab; Water waves in a ripple tank from Diffraction
Four examples of the patterns formed by waves of different sorts: X-rays, like visible light, are electromagnetic waves, neutrons and electrons are matter waves. The wavelengths of the X-rays, neutrons and electrons here are all measured in picometres, which is why they can form interference patterns when interacting with matter. The water waves have a wavelength of about a centimetre.
So, what about the the diffraction of (visible) light? We'll see this most clearly when the objects or apertures have wavelengths comparable with the wavelength of visible light, i.e. sizes of microns. However, effects are also visible with larger objects. Go to Diffraction.