Coherence and Path Difference - A Level Physics

This video introduces coherence and its relationship with path difference for A Level Physics.
Coherence, path difference and phase difference are really important subjects to consider when we look at the interference of waves and how this leads onto double slit and diffraction grating patterns (to see how these work please look at these videos: bit.ly/1GQ5yEi and bit.ly/1Fmf2Wc). Coherence is a special case of interference (coherent waves interference).
DEFINITION OF COHERENT WAVES IS THAT THEY HAVE A CONSTANT PHASE DIFFERENCE.
Thanks for watching,
Lewis
This video is recommended for anyone studying A Level Physics in the following exam boards:
AQA
CIE
Edexcel
Edexcel IAL
Eduqas
IB
OCR A
OCR B
WJEC
_____________________________________
MY PHYSICS WEBSITES
Find even more videos organised by exam board and topic at:
GCSE Physics Online
► www.gcsephysic...
A Level Physics Online
► www.alevelphys...
MY KZbin CHANNEL
Your support in watching this video has been invaluable! To contribute towards the free videos on KZbin, make a small donation at:
► www.paypal.me/...
FOLLOW ME
► www.youtube.co...
► / physicsonline
► / gcseandalevelphysicson...
#waves #alevelphysics #physicsonline

[@mangotangochick]

Just a tip for anyone doing A Level/AS physics: - The path difference means the 2 waves will not necessarily have the same intensity at a point - You do get constructive/destructive interference but that does not necessarily mean it is completely constructive/destructive - Because their amplitudes are not necessarily equal - This is because I ∝ 1/r² and amplitude depends on the intensity of a wave, since it is dependent on the energy carried by that wave, with one wave carrying less energy because it has travelled further than the other - So the minimum between 2 maxima is not necessarily going to be 0 This was on my AS last year, and I was so confused about it for ages - it's just because I was so used to thinking all waves had the same amplitude because that's the way we learn about it. Hope this is helpful :)

[@oneinabillion654]

I thought intensity is proportional to the square of amplitude? Im taking AS this Oct Nov :DD

[@coolkid9770]

i dont get your 2nd point, can u elaborate pls :(

[@asimpleman6671]

@@oneinabillion654 A bit late to this sorry, the equation given is the inverse square law (which we get from I=P/A) so what I think she’s saying is that amplitude decreases as waves propagate through the medium according to the distance travelled from the source. You are right though, so as amplitude decreases so does intensity.

[@oneinabillion654]

@@asimpleman6671 Ahh, I forgot all these already. It's okay I secured A for physics. Thanks for the response tho

[@stressedoutkid2415]

@@oneinabillion654 hey i've my alevel exam 2 days from now.any tips that you wanna give?

[@harehnkaundun1406]

i think u should stress the point that interference happens irrespective of whether the waves are coherent or not. but that Coherence makes the interference observable.

[@harehnkaundun1406]

oh and the part about destructive interference, the equation u wrote in the video would be correct if n is an odd integer. The equation in the annotation would be then valid for all integer values

[@BiggusBenjamin]

You're incorrect, superposition happens regardless of whether the waves are coherent or not, but interference only happens when they are coherent. Also the destructive interference equation works for both even and odd integers

[@zhiwaskalimba4096]

great ..u sorta explained everything my teacher tried to explain for 45 min in 3min..👍

[@snowycrystalz101]

You should do a series where for each topic you work through some of the really tough exam questions. You are very good at explaining things coherently so I think many of us would benefit!

[@PhysicsOnline]

All part of my long term plan

[@snowycrystalz101]

@@PhysicsOnline Thanks!

[@PopaDom212]

your video is really good, I've been looking for alevel videos for this year and your videos are incredibly simple and comprehensible. Keep up the great work, you'll get more popular in no time with the quality of your videos!

[@usmanalamgir6971]

These are the best online lectures for physics i have ever found Thank You very much :)

[@helloitisiv992]

Ohh!your video are so useful! All the knowledge about physical are so easy to understand,thank you !

[@RamithHettiarachchi]

Destructive Interference could be (n/2)*λ - where n= {1,3,5,7...}These videos are great!

[@stefanmoszynski978]

Ramith Hettiarachchi or (n+1)/2 by the wavelength for all integer values of n

[@willk9847]

Constructive interference occurs if each wave is 'in phase', destructive interference occurs when each wave is 'out of phase'. What interference occurs when two waves meet at a point with a phase difference of 90 degrees? the resultant wave has a greater amplitude but yet the wave is not 'in phase'? So I'd say it ought to be constructive interference but the textbooks specify that this only occurs when waves are exactly in phase.

[@JohnSmith-qv3ew]

It is still constructive interference, as the amplitude is increasing. It is just not increasing as much as it would if the two waves were exactly in phase. If they had a phase difference of 180degrees, the amplitude would double. If they have a phase difference of 90degrees, I believe that the phase difference would increase by another half, so multiple by 1.5x.

[@snowycrystalz101]

This video is so so so helpful! Thank you so much!

[@bajranglalkhati1298]

l understood very well...... thxx.. for making short but sweet vedios..😊

[@r.b7979]

Could someone explain how you know the nx value? Is it given?

[@akshilahuja3580]

Thank you Helped me out a lot! ^_^

[@usmanchau4428]

Legendary Lecture!

[@PaulEdwardTonkinson]

Shouldn't the condition for destructive interference be (n +1/2) time the wavelength?

[@eliasnouaime4081]

yes. infact its (n minussss 0.5) lambda. if u put n+0.5 u wont be able to get the value 0.5lambda

[@saurabhraj201]

Wohooo! Got it..

[@oneinabillion654]

Hi, AS CIE theory paper today OMG.

[@danielverrier7438]

(n/2)y wouldn't work since if n = 2 you would get 1*y you need to use (n+0.5)y. Sorry but I had to use y instead of lander

[@armuhammad2148]

* Lambda ( λ)

[@thebeast1383]

you are a legend

[@AyaanKhan-ei3gk]

For path difference is it 2n+1 or n+1 lamda/2?

[@daujisingh4806]

Sir how to calculate coherence length when we are given two values of wavelengths (x.angstrn to y angstrn).

[@El650Jefe]

So can waves of different wave lengths not interfere?

[@PhysicsOnline]

They do, but the effects are less significant. Although, this is how a carrier wave is used for the radio in your car or at home.

[@NarendraSingh-kl2do]

Mistake in the path difference of destructive interference..there is 2n+1 at place of n..if n is whole number

[@ARCSTREAMS]

cant you get coherent light if you pass a regular light source through a pin hole aperture? the waves on the other side will have spatial coherence ? im actually a bit confused about spatial and temporal coherence,,is spatial have to do with the photos all traveling in the same direction and parallel to each other?

[@thegreatestgundy4214]

Thank you sooooo much!

[@seditt5146]

Can light source be made Coherent if two frequencies which are Harmonics of one another be used if they contain the same phase difference? For instance, Frequency A is 100 hz and Frequency B is 400 hz such that B is the Quarterwave of frequency A so every 4 peaks of B we get a peak of A such that the wave form while being complex is still in perfect phase difference. B would constructively interfer with A once every 4 cycles of B while increasing the overall Energy of the wave to Bf^2 or something along those lines.

[@saadahmed1999]

Hi!One question what do you mean by saying "It has got the same color" at 1:16 ??

[@PhysicsOnline]

+saad ahmed The colour of light depends on the wavelength (strictly speaking it's the frequency that defines colour). For example violet light is 450nm, red light is 700nm. If two light sources have exactly the same colour then they also have the same wavelength.

[@saadahmed1999]

+A Level Physics Online Thanks Got it! :D

[@maidaanwar1543]

What does a comstant phase difference mean?

[@DXPAlien]

u are the best

[@najahfathma1182]

in 2:05 is there a mistake ???

[@aqibkareem2278]

thank you

[@a.meforyou]

What is n?

[@lakshmidevianumalasetty608]

Superb

[@PhysicsOnline]

Thanks 🤗

[@waylag9144]

these vidoes are amazing its sad that I found it out so late....

[@lukewalsh9175]

+Waylag Plays Minecraft i found even later :/

[@PhysicsOnline]

+luke wsal there is still plenty of time left before your exams!

[@Gelenvader3]

+A Level Physics Online my exams today :(

[@anjali7480]

Thumbs up

[@fishwishpishpish9165]

I don’t understand why waves need a constant phase difference in order to interfere, could anyone help please?

[@El650Jefe]

So I commented the same question as you but in a different way. In order for waves to have a constant phase difference, they need to have the same frequency/wave length. So I asked do waves need to have the same frequency in order to interfere? When I googled it the answer was no. I guess interference of waves with the same frequency (or equivalently constant phase difference) is taught first because it’s easier to understand. And the math that arises is easier. Not sure if this helps.

[@hammadahmad7599]

Its (n + 1/2 )y And Not (n/2)y......

[@armuhammad2148]

both can be correct lol

[@naheemahakinwale8403]

At 0:48 you said they have the constant phase difference but there is no phase difference, they are in phase.

[@hammadmehmood2411]

For the waves to be coherent their phase difference should be constant not zero

[@berniewang536]

Could you tell me the brand of your blue pen? I really like it

[@funytech15]

pilot v7 signature series

[@readwithabid2907]

on the annotation where its written (n 'x' lambda) what does the 'x' stand for?

[@PhysicsOnline]

+Shamit Ofree That is just n multiplied by lambda.

[@readwithabid2907]

oooh yes yes, silly me

[@jesusisking4501]

Epic

[@rajessensoobrayen4108]

good ! ;)

[@ajayparkhe2577]

The

[@undisputeddespicable]

Sorry to say but it could be little better

[@rhysbaker7456]

This packs in as much terminology as possible without explaining ANY geometry, and quite frankly had no clear direction. 1 dislike.

[@harrynix98]

What is n ???

[@PhysicsOnline]

harry nixon It just stands for any integer, e.g. 1, 2, 3

[@harrynix98]

A Level Physics Online so for a calculation i could choose to put any integer in the equation?

[@harrynix98]

A Level Physics Online im doing a question now and it is asking when there is a dark fringe ( i assume it means a point of complete destructive inference) to find the path difference between the two slits. The mark scheme says i must do (2n +1) x lamda

[@superduck76]

harry nixon 2n must be even, as anything multiplied by 2 is even, +1 will make it odd, so 2n+1 is basically saying any odd integer (i think)

[@harrynix98]

superduck76 i think n is the point a which the two waves interfer so like first intersection the second thros and so on