# Thread: Diffraction and interference of wave. Confused!

1. Interference pattern is observed on screen even when wave passes through single slit.How can it be? For interference it must be to have two wave's source but interference pattern can be seen in single source.Can you clearify me? Thanks!

2.

3. It's a result of Huygen's Principle - if you have a slit which is larger than about the wavelength of the incoming wave itself, every point within the slit acts a source of an outgoing wave, which will then interfere at the screen. Like so :

File:Huygens Fresnel Principle.gif - Wikipedia, the free encyclopedia

4. I've seen your attachment.Every point within the slit act like a source hence,interference pattern accurs.But,as we know,every point of wavefront act like a source which produces waves and its combined effect form new secondary wavefront.Here,there is nothing like interference phenomenon is happening.My statement contradict your statement.Explain!!..

5. Originally Posted by sagarkaran
I've seen your attachment.Every point within the slit act like a source hence,interference pattern occurs
Yes, indeed.

But,as we know,every point of wavefront act like a source which produces waves and its combined effect form new secondary wavefront.
Also true.

Here,there is nothing like interference phenomenon is happening
Yes, there is, you just can't see them. The difference is that far from the slit, every point acts as a source of a spherical wave propagating in all directions; due to the superposition principle, every point in space is thus reached by an equal amount of wave crests and wave troughs, so the net interference effect from points outside the slit is exactly zero. All that remains is the original wave front. Not so at the slit itself - it cuts all outgoing wave fronts in half, so there is only forward propagation; it also cuts the incoming wave fronts in a direction perpendicular to the direction of propagation. Every point inside the slit is therefore a net source of wave fronts, whereas all other points behind the screen are not.

All of this is made mathematically rigorous in Kirchhoff's diffraction formula :

Kirchhoff's diffraction formula - Wikipedia, the free encyclopedia

6. every point acts as a source of a spherical wave propagating in all directions; due to the superposition principle, every point in space is thus reached by an equal amount of wave crests and wave troughs, so the net interference effect from points outside the slit is exactly zero.

Ok

[QUOTE]All that remains is the original wave front.

How?Since the net interference is zero all that remain should be nothing.May I understood wrong,so please explain same in different way!

[QUOTE]Not so at the slit itself - it cuts all outgoing wave fronts in half, so there is only forward propagation; it also cuts the incoming wave fronts in a direction perpendicular to the direction of propagation.

Also not understood well."It cuts all outgoing wave fronts in half"-means,as a point produce small circular wave it's backward semicircle is cancelled out and is remained it's forward semicircle.

7. Originally Posted by sagarkaran
How?Since the net interference is zero all that remain should be nothing.
The net interference from all points outside the slit adds to zero, but the wave originating from the slit itself is superimposed on that.

"It cuts all outgoing wave fronts in half"-means,as a point produce small circular wave it's backward semicircle is cancelled out and is remained it's forward semicircle.
Yes, since there is a solid wall on either side of the slit, so it can't go back ( and we shall assume that the wave doesn't get reflected, or else things become complicated ).

8. Sorry!Still,I am unable to understand.Why a wave shows interference pattern after passing through single slit?Does wave not shows interference pattern without slit? Simple and best answer is more better.

9. Originally Posted by sagarkaran
Why a wave shows interference pattern after passing through single slit?
It only shows such a pattern if the width of the slit is more than one wavelength; otherwise there is no interference, but there will still be diffraction.

Does wave not shows interference pattern without slit?
No. You need slits or a diffraction grating to produce interference.

10. We are making sure to be aware of the difference between a spherical wave passing through a slit and a plane wave passing through a slit, right?

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