![]() The narrower the slit, the more the light spreads out. This different amount of bending gives the blobs their colored edges: blue on the inside, red on the outside. ![]() Red light, for instance, has a longer wavelength than blue light, so it bends more than blue light does. The angle at which the light bends is proportional to the wavelength of the light. Where the trough of one wave overlaps with the crest of another wave, the waves cancel each other out, and you see a dark band. Where the crest of one wave overlaps with the crest of another wave, the two waves combine to make a bigger wave, and you see a bright blob of light. The light waves that go through the slit spread out, overlap, and add together, producing the diffraction pattern you see. ![]() The black bands between the blobs of light show that a wave is associated with the light. Rotate each object while you look through it. Look at the light through a piece of cloth, a feather, a diffraction grating, or a piece of metal screen. Rotate the hair and watch the line of blobs rotate. Move the hair until it is between your eye and the light source, and notice that the light is spread into a line of blobs by the hair, just as it was by the slit. Stretch a hair tight and hold it about 1 inch (2.5 cm) from your eye. Notice that the blobs have blue and red edges and that the blue edges are closer to the light source. As you squeeze the slit together, the blobs of light grow larger and spread apart, moving away from the central light source and becoming easier to see. If you look closely you may see that the line is composed of tiny blobs of light. While looking through the slit, rotate the pencils until they are horizontal, and notice that the line of light becomes vertical. Notice that there is a line of light perpendicular to the slit. Squeeze the pencils together, making the slit smaller. Hold both pencils close to one eye (about 1 inch away) and look at the light source through the slit between the pencils. The tape wrapped around one pencil should keep the pencils slightly apart, forming a thin slit between them, just below the tape. Hold up the two pencils, side by side, with the erasers at the top. 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.Place the light on a stable surface at least one arm’s length away from you. 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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