# Student Projects/Diffraction and wave nature

We encounter a lot of waves in our daily life. Waves made by dropping a small stone in a pond are one of the best example. So basically waves are the disturbances in a medium like water, air, string etc. There are various waves and the phenomenon that we don't notice so carefully. For example sound waves which are nothing but disturbances in the air medium created by some vibrations like that of the membrane of a drum, strings of guitar or vocal chords in our throat.

Did we ever notice how we can listen to sound from one room to the other? The answer lies in diffraction, which is nothing but bending of the sound waves from the obstacles like the edges of door or windows of the room in this case. The only condition is that the size of obstacles must be comparable to the wavelength of wave. It is now obvious that the wavelengths of sound waves are comparable to the sizes of obstacles that we see in daily life (usually the human audible range from a few millimeters to meters).

One of the interpretation about the nature of light is that they behave as waves and are called electromagnetic waves. We usually see that light travels in a straight line and we never see them diffracting from obstacles around us. If we take the analogy from sound wave we should ask why don't we see the light waves diffract from the edges of the door and windows of the room and reach other parts of our house? Reading the above explanation one can easily say that its the wavelengths of light that are very small when compared to the obstacles that we see in daily life (the wavelengths of visible light ranges from 400 to 800 nano meters). The Young's double slit experiment with light was one of the important experiments that showed the diffraction of light and proved them to be waves.

To see the diffraction of light we need to have obstacles (or slits for that reason) of the sizes comparable to the wavelengths of light (visible or invisible like X-rays for that reason). The best example of such a obstacle is diffraction gratings for visible light. For X-rays we use the inter-atomic spaces as slits to demonstrate the diffraction of X-rays. We actually use the X-ray diffraction for finding the crystal structures using the laws governing diffraction. The reason behind the use of X-rays for that purpose is that the wavelengths of X-rays are comparable to inter-atomic separations.

Lets now talk about the least noticed waves the waves associated with particles. Yes, you read it correct particles do have waves or we can say particles can also behave like waves. Louis de Broglie in his work hypothesized the wave nature of particles (the wavelength associated with particles depends on the momentum of particles actually they are inversely proportional to each other) which was later on proved experimentally using electrons as waves. To prove that particle can behave as waves we need to demonstrate the similar diffraction of electrons by the slits or obstacles. It was demonstrated by Davisson and Germer using diffraction of electrons by the nickel crystal. One can now argue that every object that has a non zero momentum should diffract from obstacles if they are waves but then we again need to get back to our condition for diffraction. The obvious answer will then be that the momentum (product of mass and velocity of the particle) is too high and that is the reason the wavelengths are too small to diffract from the obstacles available. The reason behind high momentum is the masses of day today objects.

The diffraction and hence the wave nature of particles can only be observed if the mass of the particles is too small to make the wavelengths bigger enough to be compared with the obstacle sizes. This is the reason we cant apply Quantum Mechanics to daily life objects.