Problem about double-slit interference

[ckj1026]

Hi. What is the intensity pattern of double-slit interference? I googled it, there are two different types of graphs coming out. One has the same maximum intensity, the other one has different maximum intensity. I am really confused about this. Can anyone help me? 

Thanks. 

[kw0573]

The first one assumes y (consecutive peak distance) is insignificant to x (distance to screen), or x >> y and/or the angle is small. The second one assumes y is a decent portion or multiple of x. See also here: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html. You'll find equations for both the extreme and the general case in the data booklet.

[Guest SNJERIN]

On 4/6/2016 at 9:04 PM, kw0573 said:

The first one assumes y (consecutive peak distance) is insignificant to x (distance to screen), or x >> y and/or the angle is small. The second one assumes y is a decent portion or multiple of x. See also here: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/slits.html. You'll find equations for both the extreme and the general case in the data booklet.

Isn't that the first intensity pattern assumes that the width of each slit is negligibly small compared to the separation between the two slits, and in the second graph we had a non-zero slit width and thus the double slit intensity pattern is being modulated by the single slit diffraction pattern ? 

[kw0573]

@Haitham Wahid
I did make a mistake, but we both are just a bit off. According to the link I shared above, the main assumption is that the D >> a, or distance to screen is much larger than slit width, which gives the approximation that all peaks equally bright. The small angle approximation is only useful for calculations for large n, and does not actually affect behaviour near the centreline.

[Guest SNJERIN]

9 hours ago, kw0573 said:

@Haitham Wahid
I did make a mistake, but we both are just a bit off. According to the link I shared above, the main assumption is that the D >> a, or distance to screen is much larger than slit width, which gives the approximation that all peaks equally bright. The small angle approximation is only useful for calculations for large n, and does not actually affect behaviour near the centreline.

Now I understand! Thanks a lot!