Diffraction and interference

Question

Diffraction grating

interference 同diffraction 同時出現

Diffraction pattern due to a single slit is superimposed on interference patteren.

所以 fringe internsity varies.

但係 Young's double slit experiment

為甚麼 只有 interference pattern Fringe intensity 係一樣

當WAVE PASS through double slits Diffraction occurs.

但為什麼只看到interference pattern

而無diffraction pattern. 而且fringe intensity 不變?

當中有甚麼不同

如何令到只有interference pattern

而甚麼條件下可做到 diffraction grating 的效果

更新:

The intensity of the bright fringes in an interference is equally bright, equally spaced. In a diffraction pattern, the central maximum has the greatest brightness, with each successive bright fringe getting narrower and dimmer.

更新 2:

為甚麼 interference pattern is equally bright, equally spaced

但diffraction pattern getting narrower and dimmer.

更新 3:

For double slit,

interference pattern is equally bright, equally spaced

但為甚麼diffraction pattern getting narrower and dimmer.

都是用double slit 甚麼造成pattern 的intensity不同

Answer 1

最愛解答

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

Answer 2

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/phys1...

you should search for some REAL/PRACTICAL experiment photos

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

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.....