Have a look at what is happening to the right of the slits. The experiment is named after the guy who first carried it out – Young’s double slit experiment. What happens if there are two or more slits? We’ll end up with two or more diffracting waves, which we might expect to interfere with one another.īelow is a simulation of diffraction through two slits. So far we’ve only considered the case of a single slit or gap for the wave to pass through. The video below shows how you can use this method to work out how wavefronts are altered by a slit.ĭiffraction Through Two Slits Young’s Experiment For example – if you dropped a number of pebbles in a straight line, all in one go at exactly the same time, a straight (in science-speak plane) wavefront would be created. These wavelets superimpose and interfere to form more complicated wavefronts. A wavelet can be described as a circular wave much like the ripple you would get from dropping a small pebble into a pond. Huygens argued that a wavefront could be modelled as a series of wavelets. One way to explain diffraction is to use a mathematical method invented by 17th century physicist Christiaan Huygens. ![]() When the gap size is smaller than the wavelength (top movie), more diffraction occurs and the waves spread out greatly – the wavefronts are almost semicircular. When the gap width is larger than the wavelength (bottom movie), the wave passes through the gap and does not spread out much on the other side. slit is narrower than the wavelength Gap width = two wavelengths i.e. When the size of the gap changes, how does this change the diffraction of the wave? When does maximum diffraction occur? (Think about your previous findings on the diffraction of sound around an obstacle). The difference between the movies is the size of the gap. This is shown in the two animations below. The swish of the tyre and wind-noise contains a lot of high frequency energy, and you should find that this does not diffract around the corner as effectively as the rumble of engine.Diffraction also occurs when a wave passes through a gap (or slit) in a barrier. You can experiment with this by listening to traffic noise from a busy road from around the corner of a building (not in a direct line-of-sight to the traffic), and then moving to a location a similar distance from the road but in direct view of the passing cars. However with a short barrier (the same length as the wavelength) diffraction is very effective and there is almost no zone of silence behind it.įrom this, we can reach the conclusion that with sound waves, it is the low frequencies (which have long wavelengths) which diffract around corners. Our simulation shows that with a ‘long’ barrier, there’s a lot of reflection of incident energy back towards the source, but although there is some diffraction or bending of the wave around the barrier, this still leaves a zone of silence behind it. The obstacle in the right animation has the same width as the wavelength of the sound.īy examining the three animations, decide which of these statements is correct in the following quiz. Ripple tanks with large, medium and small objects (left to right) obstructing a wave. The key to understanding diffraction is understanding how the relative size of the object and the wavelength influence what goes on. Have a look at this a simulation of three ripple tanks, each containing an object of different width, which obstructs the propagation of a wave. ![]() Diffraction can be clearly demonstrated using water waves in a ripple tank. ![]() The amount of diffraction (spreading or bending of the wave) depends on the wavelength and the size of the object. Waves can spread in a rather unusual way when they reach the edge of an object – this is called diffraction. What is the reason for this? Do light and sound share any properties that might cause this effect? Diffraction Around An Object Have you ever wondered why you can hear someone who is round the corner of a building, long before you see them? It appears that sound can travel round corners and light cannot.
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