# B Interference vs Diffraction

1. Mar 1, 2016

### Santosh Acharya

I have a confusion regarding interference and diffraction phenomenon, I will be glad if you can post a video or something about interference and diffraction basics. I don't have meaningful concept over their differences.

[Mentor's Note: Thread moved to General Physics and thread title edited]

Last edited by a moderator: Mar 1, 2016
2. Mar 1, 2016

### blue_leaf77

Interference is a special case of superposition between two or more waves witht the same frequency, but sometimes there are also authors who use this term for waves with different frequencies.
Diffraction is the modification of the angular spectrum content of a wave due to the presence of an obstacle during its propagation. Note that, another wave effect, the refraction, also leads to a change in the angular spectrum but it's just a shift in frequency. On the other hand, diffraction usually leads a modification of the entire shape of the angular spectrum, e.g. to become broader like that in the diffraction from a slit or narrower like that in a focused light beam.
As for the videos, why not try to type "interference and diffraction" in youtube?

3. Mar 1, 2016

### Charles Link

I can add to blue_leaf's explanation above. Diffraction patterns are also the result of interference between the Huygen's sources that originate from the different locations on the wavefront. The diffraction patterns often originate from a continuum of (Huygen's) sources, like across the finite width of a small slit, but diffraction is also a form of interference.

4. Mar 1, 2016

### sophiecentaur

. . . . and I sympathise, because there is such a lot of fuzzy talk about them. The easy answer is that everything to do with wave propagation is Diffraction, in one form or another and Interference is a sub-set. The simplest form of Diffraction is what you get when there are a finite number of (very) small sources (e.g. holes or slits or multiple radio antennae or loudspeakers etc. etc.). These sources can be treated as points (looking on one plane) and they will produce a diffraction pattern that's referred to as an Interference pattern. Interference is the simplest form of diffraction to calculate and describe. Young's Slits is the most familiar and the positions of the maxes and mins can be calculated easily by calculating the directions in which the path difference between the two sources is either a whole number of wavelengths or an odd number of half wavelengths. For multiple points or slits, the calculation is a simple Summation of contributions from an integer number of sources. ("Σ" is the operation).
To work out the diffraction pattern when the wave is formed by a large object / aperture getting in the way of the wave, you look at every point on the wave front and calculate the way all those points contribute to the result in any direction. That involves Integration ("∫" is the operation). Diffraction occurs whenever waves encounter anything on their journey; a hole, a solid object, a lens, a mirror etc. etc. will all produce a diffraction pattern. If they are large, you get an almost perfect optical image / shadow etc.. Charles Link has referred to Huygen's principle and his (Huygens') was the first approach to calculating the progress of any wave, based on treating it as a set of point sources all across the wave front, which interfere with each other and produce the next step in the wave's progress. It's clever in that it actually predicts that the wave will only travel forwards, when undisturbed, and the interference 'backwards' or to the side, produces nothing.

5. Mar 1, 2016

### Charles Link

Perhaps one of the most useful results in interference/diffraction theory is that for two sinusoidal (in time) point sources of wavelength $\lambda$ placed a distance "d" apart, the phase difference $\phi$ between them in the far field at angle $\theta$ is given by $\phi=(2\pi/\lambda)*d*\sin(\theta)$. Meanwhile for the Huygen's sources across a finite size slit of width $b$, the phase $\phi$ from the Huygen's source at location $x$ where $0<x<b$ at angle $\theta$ in the far field is given by $\phi=(2\pi/\lambda)*x*\sin(\theta)$

6. Mar 1, 2016

### Santosh Acharya

If interference and diffraction are similar topics why not to study interference under diffraction?
Do anyone have animated video or like that to make me much more clear... I would be pleased.

7. Mar 2, 2016

### blue_leaf77

Usually, the topic of diffraction follows immediately that of interference, probably this is because the formula involved to describe diffraction requires a higher level of math.
Have you tried googling it? But even if you do find videos discussing interference and diffraction in one topic, I don't think it will give you much insight into the matter - these two phenomena are visually (for light) and aurally (for sound) hardly distinguishable. The difference between them is more subtle than what you may think, or what you may have realized given the fact that you asked this question. To get a deeper understanding on this, a literature study is highly recommended, it's a lot of work though but will be worth it if you do want to know,

Last edited: Mar 2, 2016
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