# Two slits diffraction vs interference

1. Mar 4, 2013

### bgq

Hi,

What is the difference in the setups of the Young's double slit experiment and the two slits diffraction?

I found textbooks discuss each differently, yet I found no difference in the setup.

Thanks to any help.

2. Mar 4, 2013

### Staff: Mentor

Where is the difference in those discussions?

3. Mar 4, 2013

### bgq

They say the diffraction in the case of the two slits produces interference enveloped by diffraction pattern. They also pointed out that for small angles, in interference we obtain bright fringes with approximately same intensity; while in diffraction the central bright fringe is of larger intensity even with small angles.

4. Mar 4, 2013

### Staff: Mentor

That looks like a comparison of a single-slit pattern with a double-slit pattern.

5. Mar 4, 2013

### bgq

Here is the interference pattern with two slits:

And here is the diffraction pattern with two slits:

As you see, they are not the same; however, the setup of each experiment (according to my understanding) is the same.

6. Mar 4, 2013

### Born2bwire

The presence of two slits gives rise to interference when you take the superposition of the light that comes out of both slits. That interference is due to the difference in the path length that the light has to travel to the observation point with regard to the two slits. The interference resulting from this path length difference is periodic and is shown in your first picture. However, the light emanating from a slit is diffracted. This means that the brightness of the light emanating from a single slit decreases as you increase the angle. This is the diffraction envelope. So the actual pattern of the incident light from two slits is going to be the interference pattern scaled by the diffraction envelope as you see in your second picture.

7. Mar 5, 2013

### BeBattey

You'd have to give a clearer description of the texts you're talking about, but the significance of Young's experiment was that he couldn't produce light that was coherent, as in light that was all in the same phase, with one light source that could go through both slits. Plug a lamp into a double slit experiment and see what happens, you get no interference pattern. The only reason lasers are used is because they already emit coherent light.

However, by using any light source and a single slit, you get around that problem. As the light hits the single slit, it is (of course) in phase with itself at that point. Set any two slits equidistant from that point and the light that arrives there must also be in phase, therefore produce an interference pattern.

Young's Experiment requires a single slit to be in front of the double slit, and any light source can be used. The double slit experiment does not require a single slit, as we've found other ways to produce coherent light.

Of course, correct me if I'm wrong, physicsforums denizens!

8. Mar 5, 2013

### bgq

This is a good explanation, thank you, so what is really obtained on the screen is like the second picture. Am I right?

9. Mar 5, 2013

### Staff: Mentor

The first picture needs slits with a width similar to the wavelength of light (or a very large double-slit separation and a microscope). The second picture is more realistic for real double slits.

10. Mar 5, 2013

### Staff: Mentor

Here's the two-slit "diffraction" pattern from a previous post:

If you close or block one slit, you would see (only) the pattern indicated by the dashed line, but with lower overall intensity because only half as much light enters one slit versus two slits. The small-scale variations (solid line) disappear.

If you gradually reduce the widths of the two slits, while keeping them the same distance apart (center-to-center), the pattern indicated by the dashed line (envelope) spreads out while the small-scale variations maintain the same width. Eventually the central maximum of the envelope "fills the field of view" so to speak, and the amplitude of the small-scale variations is nearly constant. The overall amplitude of the pattern decreases because there is less light going through the slits.

If you move the slits further apart, while keeping them the same width, the small-scale variations (solid line) become narrower, but the dashed-line envelope remains the same. If you move the slits closer together, the small-scale variations become wider, but again the envelope remains the same. When the two slits get close enough to merge, you end up with a single-slit pattern.

Last edited: Mar 5, 2013
11. Mar 5, 2013

### bgq

Thank you very much.

12. Mar 5, 2013

### sophiecentaur

You need to interpret this pair of pictures carefully in order not to get the wrong idea - they have different horizontal scales. It is unfortunate that the coarse variations appear about the same in the two pictures. The variations on the first picture is for the two (infinitely thin) slits but these correspond to the fine variations in the second picture. In the second picture, rapid variations correspond to wide (slit) spacing, slower variations correspond to a narrow aperture (slit width).

13. Feb 19, 2015

### abhaysingh650

You just did a great job,that's all correct.It cleared my very confusing doubt.You are a living legend.

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14. Feb 20, 2015