EWN - Other Light Phenomena

Other Light Phenomena

Single Slit Diffraction 

Single slits also produce a diffraction pattern of their own. The difference is that the light diffracted by one edge of the slit interferes with light diffracted across the rest of the slit. The result is a bright central maximum and higher order maxima with a severely reduced intensity.

In fact, any object with a sharp edge can create a diffraction patter. In most cases, however, the dim fringes are washed out by the brightness of the central maximum and are not noticed.

Diffraction Grating 

In chemistry you may have done experiments in spectroscopy. The purpose is to analyze the specific frequencies of light emitted by a source. To analyze the light it is passed through a diffraction grating. The diffraction grating works just like the double slit except it contains 100's or 1000's of slits in a row. The result is that the various wavelengths of light from the source are separated very clearly (much like our second double slit practice problem involving the white light). Most lab diffraction grating produce interference patterns that appear as crisp lines to either side of the source. Other gratings, depending on the arrangement of the slits, may produce interference patterns all around the source. This image shows what it the pattern would look like if you used the common "Rainbow Glasses" diffraction gratings to look at an incandescent bulb. For calculation purposes, you can still use our double slit diffraction equation and set-up. The only difference is that d is now the distance between the slits. Often you won't be given d directly. Instead the grating will list lines per meter or something similar. If N equals the number of lines per meter:

So, for a grating with 10,000 lines/cm:

Polarization

Have you ever seen or worn a pair of sunglasses designed to reduce glare? These glasses have polarizing lenses. Another of light's wave properties is its ability to be polarized. Imagine that you are shaking a rope up and down to produce a continuous transverse wave. If you place a barrier with a vertical slit in the path of the wave, the only way it will pass through is if the orientation of the oscillation aligns with the slit. All other orientations of waves will be reflected off the barrier. Remember that light waves are transverse (a transverse electric field wave perpendicular to a transverse magnetic field wave). Many sources of light are unpolarized, meaning that the light will have many different orientations of oscillations. If your sunglasses have a set of slits that will only allow light oscillating in one direction to pass, they will deflect most of the light trying to enter your eye. The light that passes through your sunglasses lenses is now polarized, meaning it only oscillates in one direction. Light is also polarized when it reflects off of water. For a fisherman to be able to see fish just below the surface on a sunny day, he'll need to wear polarizing glasses designed to block the polarized light reflected off the water surface. The following video breaks down the concept of polarization and provides an excellent visual.

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