Diffraction and interference

There is already diffraction taken place in a double slit arrangement. Should there be no diffraction, the light waves after passing through the slits would not spread out. In that case, light from the two slits would not be met and interference could not take place.

As you should have learnt that in a single slit diffraction, if the slit width is comparable to the wavelength of light, the central maximum occupies a large angle and the intensity of which falls moderately on the two sides. When these two slits are placed close to each other, the central maxima from these two slits thus overlap at some region, forming an interference pattern. Because the intensity of the overlapping region from the two slits are mor or less constant, the interference pattern produced is therefore of uniform intensity.

You can view the following web-page for an experience of a songle slit diffraction animation. Try to decrease the slit width to a value equal to the wavelength used, and choose the "intensity profile" to see how the pattern changes.


http://www.walter-fendt.de/ph14e/singleslit.htm

The following is an animation of double slit interference:

http://www.walter-fendt.de/ph14e/doubleslit.htm

for interference pattern having the SAME intensity
it's ideal interference
for practical interference, diffraction effect is taken in to consideration
see

http://www.physics.upenn.edu/courses/gladney/phys151/lectures/lecture_apr_23_2003.shtml

you should search for some REAL/PRACTICAL experiment photos


http://en.wikipedia.org/wiki/Double_slit_experiment

JAVA applets are not perfect
the designer may not insert all real restrictions into the software
say, for the single slit experiment
you can see that light can pass through a gap shorter than its wavelength.....