Laser light passes through two small slits and strikes a screen, as seen in the photograph of the apparatus at the left below. A double slit interference pattern, like that shown at the right below, is produced.
Now the LEFT slit (as you look along the optic axis in the direction the beam is moving) is closed by sliding a small piece of metal in front of it. What will the pattern look like after the left slit is blocked off?
Here are five possible results:
a) 
b) 
c) 
d) 
e) 
It looks like the first choice is the same as the original pattern (the pattern does not change); the second looks the same except that its intensity is less (only one slit is open); the third looks similar, but without all of the little ridges; the fourth and the fifth appear to be images of only one of the slits, where one is inverted and the other is not.
Which of the selections is the correct pattern after one of the slits is blocked off?
After April 25th, 2014, click Read More for the answer.
The answer is (c): the pattern is the characteristic diffraction pattern of a single slit, as seen in the photograph at the left below.
When laser light passes through a single slit, a single slit diffraction pattern is created, as per the upper photograph above. Either slit alone will create an identical pattern. When two identical slits are illuminated, as in the lower photograph above, interference between the two slits occurs, creating the fine minima and maxima, the double slit interference pattern. No light can strike the screen at any location excluded by the single slit diffraction pattern of either slit. Therefore the pattern from a double slit is a combination: the fine interference pattern due to interference between the two slits modulated by the overall single slit diffraction pattern of either individual slit. If the individual slits were very narrow (about the wavelength of the laser light), no diffraction pattern would occur, and the fine minima and maxima of the two-slit interference pattern would extend to infinity.