And so even the light waves from the very top and very bottom of the slit are essentially right on top of each other, as well as all the waves inbetween.

Now, remember that the slit width, a, is only a few hundred nanometers in size. This means that in this diagram the two light rays have a path difference of about 2/3 x 2 p or 4 p / 3. We can see that along the parallel wave rays, the bottom wave has already completed about two-thirds of its cycle when the top wave begins its cycle. The quantity a sin T is called the path difference between the two light rays. This is to help us compare the phase of the waves. In the diagrams below the waves have been drawn from a side view, rather than a top view of wavefronts. Recall that we are considering points within the aperature as point sources from which new waves spread out. The equation (*) is the result of analysis of the PATH DIFFERENCE between light rays coming from the top and the bottom of the slit, and how this path difference relates to our discussion on INTERFERENCE.

And if we make the slit width smaller, the angle T increases, giving a wider central band.Īnd how can we derive the equation (*) for the location of the central diffraction minimum? If we increase the width size, a, the angle T at which the intensity first becomes zero decreases, resulting in a narrower central band. Note that the width of the central diffraction maximum is inversely proportional to the width of the slit. I will mention now that the intensity of light is proportional to the square of its amplitude. The first DIFFRACTION MINIMUM occurs at the angles given by sin T = l / a There are minor seconday bands on either side of the central maximum. Most of the light is concentrated in the broad CENTRAL DIFFRACTION MAXIMUM. The top part of the figure to the left is an imitation of a single slit diffraction pattern which may be observed on the screen (there would really be more blending between the bright and dark bands, see a real diffraction pattern at the top of this page).īelow the pattern is an intensity bar graph showing the intensity of the light in the diffraction pattern as a function of sin T. Let T represent the angle between the wave ray to a point on the screenĪnd the normal line between the slit and the screen. Let L represent the distance between the slit and the screen. When the light encounters the slit, the pattern of the resulting wave can be calculated by treating each point in the aperature as a point source from which new waves spread out. This is possible because the slit is narrow.Ĭonsider a slit of width a, light of wavelength l, and a smaller than l. The intensity at any point on the screen is independent of the angle made between the ray to the screen and the normal line between the slit and the screen (this angle is called T below).Cylindrical waves can be represented in 2D diagrams as cicular waves.The slit size is small, relative to the wavelength of light.Some assumptions must be made for this description of the single slit diffraction pattern: Light bends around obstacles like waves do, and it is this bending which causes the single slit diffraction pattern. Interference and Diffraction are the phenomena that distinguish waves from particles: waves interfere and diffract, particles do not. The key to understanding why light behaves like waves is in INTERFERENCE and DIFFRACTION. It wasn't until the 19th century that convincing evidence was found showing that light behaves like waves.īefore reading on, you may wish to review some wave terminology. Light is interesting and mysterious because it consists of both a beam of particles, and of waves in motion.Īll carriers of energy and momentum, such as light and electrons, propagate like a wave and exchange energy like a particle. Left: picture of a single slit diffraction pattern. The intention of this web page is to explain this pattern at the academic level mentioned above. The diffraction pattern observed with light and a small slit comes up in about every high school and first year university general physics class.

SINGLE SLIT DIFFRACTION PATTERN OF LIGHT SINGLE SLIT DIFFRACTION PATTERN OF LIGHT

Intensity equation light mcat