Why Does Interference Still Occur with Red Light Passing Through Slits?

[primarygun]

Why interference still occurs when a beam of red light passes through a hole created by the slits? The screen captures different brightness of red light.
If there are no interferences took place, the brightness should be uniform.

 

[HallsofIvy]

The effect of the slits (or holes) is to "start" the light over- the light spreads out in spheres from the two holes (with slits, think about each different plane and use circles). At each point between the two holes, the distance from each hole to that point is different- since light travels at the same speed from each hole, and starts at the same "phase" (that of the incoming light) but travels different distances, the waves will be at different phases- different places in max or min intensity. That's what causes the interference.

 

[primarygun]

But , there is only one hole.

 

[KingOfTwilight]

The one hole is a center of an elementary wave (I'm not sure about this word) (according to Huygens). Elementary wave sends waves to many directions and these waves interference with each other causing the maxs and mins.

 

[primarygun]

Thank you.
Only light ray has this characterisitic?
Elementary wave sends waves to many directions and these waves interference with each other causing the maxs and mins."

 

[KingOfTwilight]

Yes. Here's a (terrible) picture I've made.
1 = the center of the elementary wave
The screen or wall or whatever is on right side
If you've seen this same experiment with water, you'll see the similarity.

And here's some pictures I found on the net:

http://www.phy.davidson.edu/StuHome/cabell_f/diffractionfinal/images/huygens.gif

http://www.lems.brown.edu/~leymarie/WaveRender/Figures/FigHuygens2.jpg

 

[primarygun]

The brightness is proportional to the magnitude of amplitude?