Diffraction grating | Light waves | Physics | Khan Academy

[@bboy14]

Khan academy needs to keep you on board, your explanations and commentary style is a treat!

[@Cooltaha]

such a clear and concise voice

[@fluffzxx9615]

I don’t get why in the normal double slit it would be smudgy in the first place, why?

[@albertvandrejer5003]

@@fluffzxx9615 because for a large numer of slits, the condition for constructive interference of all the superimposed waves is only given at very specific positions. For a double slit what you see is just the interference pattern of two waves, which will have areas of constructive interference, destructive interference and everything in between.

[@williamstephenjones3863]

Agreed

[@SAKA701]

Grate video, must say.

[@chrisbuckle6930]

Aly Azeemi great*

[@SAKA701]

Chris Buckle Ik dude, it was a pun :p diffraction grate-ing. YES IM THE LORD OF ALL BAD PUNS.

[@divyasingh9291]

Aly Azeemi ikr😂😂

[@SAKA701]

TOP COMMENT WOWOWOOW, FeelsGoodMan :D

[@the_golden_one4733]

Dont grate on my nerves

[@ponypony4695]

If only my teacher at university could explain this in the way you did... thanks, now it looks much more easier to understand ;)

[@LASTCHANCE-mi8zy]

says who

[@sarahholland5980]

you just explained a full lecture in 15 minutes, and beautifully and simply. bless you.

[@ufo717212]

I just tried to reorganize your points in my own words as the following. At a magical spot, each light travel one wavelength further(or less) than its adjacent light. Therefore all lights results in constructive interference at this spot which is bright. At a non-magical spot, each light has a slight phase difference with its adjacent light. Since there could be hundreds of different lights from hundreds of holes, each light can always be paired with another light with a phase difference of about half wavelength. Thus each pair results in destructive interference. Overall they result in darkness at the spot.

[@kissmyussr1993]

Who are you so wise in the ways of science?

[@vandnashukla3598]

This is exactly how I tried understanding it.i took approximately half of the total number of holes and then each point in between the initial and final hole would pair up destructively with another.

[@Ahmed-vs1ui]

How does that magical spot even exist mathematically

[@mr.j_krr_80]

10:55 "in between you'll get *darkness* " me: I know... *turns off light, sits at the corner and falls into depression*

[@yashgaikwad7516]

Mr. J_Krr_ :(

[@MJ-hq1ie]

A whole mood

[@splinter1817]

sed lyf

[@Integrelle]

2:12 "Why? Well, lets talk about why." LOL, reminded me of the GMM quote, "Lets talk about that".

"And in between these bright spots you will get darkness...which is grate."

[@fluffzxx9615]

I don’t get why in the normal double slit it would be smudgy in the first place, why?

[@epickirby101]

Fluffz Xx cause the waves won’t completely cancel out in between, partial destructive or constructive interference

[@kushalchakrabarti240]

4:03 .. how about a third hole? This is where it gets interesting... LMAO

[@expectopetronum3851]

😂😂😂😂😂

[@abdallababikir4473]

Brother,😂

[@AbdulHamid-uw2oe]

Mouth..vagina...&...... The thid....

[@zeenathunnisa3571]

Gosh!! Such an awesome and clear explanation. Jazakallah Khair.

[@chaosforever]

11:50 no this actually all made perfect sense, thank you!

[@Postermaestro]

This was very well explained. You guys at Khan are so good at making it easy to understand, which makes the learning process so much more fun and time-efficient!

[@Lyoko-wy3si]

I barely understood the idea of diffraction grating until I watched this! Thanks for the help

[@cortwill4085]

OH... ME... GOSH!! IS THIS... ARE YOU... KHAN ACADEMY KIDS?!?!?!?!?!?!?!?!? I LOVE Khan Academy Kids so much! l couldn't even help myself but to get a game of you guys on my iPad! I like all the characters: Peck, Ollo, Kodi, Raya, and Sandy! But I think... Sandy's my favorite! Khan Academy Kids is all l play! Like, I literally don't want to play anything else on my iPad! CONGRATS!🎉 You get 5 stars!⭐⭐⭐⭐⭐ I wanted to give you 5 stars in the game, but it required a PIN that only Mum and Dad knew. And they won't give me permission to give you 5 stars. Here you go!:⭐⭐⭐⭐⭐

[@4darekord]

crystal clear explanation man

[@luigiprz5319]

Hi, to keep angles, so green lines, the same, the purple lines should be parallel. We cannot approximate angles because the difference distance is much bigger than wave length. Correct me if I'm not right. It looks the same when draw is not precise but geometrically not. What if first hole will be perpendicular to bright spot and the x hole will be at the border of light point :) but you got good intentions it's the plus

[@simonhwang5624]

Right at 3:00, you say "draw a right angle", but if it's a right angle there's no way the two lines (minus the "extra part") are equal, because geometry: hypotenuse of right triangle must be longer than sides.

[@adityamathur5972]

Same bro, even I didn't get that

[@nononnomonohjghdgdshrsrhsjgd]

@@adityamathur5972 i was scrolling through the comments to find that question and some explanation

[@divyansh_kashyap2901]

​@@nononnomonohjghdgdshrsrhsjgdbecause the angle between both hypotenuse and one side is very very very small...approx 0⁰.. Cos0⁰=1=(base/hypotenuse)

[@Kolinnor]

I think he meant to draw an isoceles triangle.

[@oscarpalmowski9003]

best video on the internet about why diffraction gratings give discrete maxima. THANKYOU SO MUCH. I wish I cam across this video first

[@haristufail2121]

Beautifully explained Sir.You should get a Noble prize.

[@FDGod99]

-_-"

[@FDGod99]

+Onkar Apte -_-"

[@gracepancalstatela4422]

I agree👍.

[@LogicalCod]

Nobel

[@harrisabdullah2278]

If there was a nobel prize for teaching

[@annaszinyei]

You did such a great job explaining! So clear and easy!

[@4rthur4rmstrong]

All David’s videos on physics has been absolutely amazing!

[@RahulGupta-pf4yt]

Your style of teaching is remarkable.

[@trailertrashtactics]

This guy is like a young sassy Sal Haha. Love it thank you for the videos.

[@MrRak18]

It helped me understand x- ray diffraction.. thank you.

[@animeBHARATIYA]

The explanation is very clear, thank you

[@jinks6887]

really well explained thanks a lot ..love from india

[@BlueRangeTV]

Thanks, the Bulgarian student books don't explain this well at all! Finally understood it

[@alivira1558]

Never understood what the concept was before but this was like magic.. well done and thank you so very much.

[@RedBar3D]

Actually drawing them parallell seems to help understanding a whole lot

[@smtusharibnesalam8688]

Its Great. Mind blowing. Top class video. Thank you very much for teaching this

[@physestryshorts5214]

omg ..... no words for this lecture

[@Aditya-f8t5z]

Thank you for making this video. 🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏🙏

[@SohamChakraborty42069]

Beautiful explanation. One part I don't understand though. The angle theta is the angle between the perpendicular line joining the middle of the distance between the pair of slits and the screen and another line that joins the middle point and the point of interest on the screen. In diffraction grating, we are choosing different pairs of slits each time, so the middle point changes whereas the point on the screen remains fixed. How is the angle the same, then?

[@workingemail1016]

Very well explained. Thank you.

[@ChanakyaVivekananda]

Mannnnnnnn This is such a good explanation!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[@mariamnaeem6963]

Wow!! Thank you for posting such a wonderful explanation! I was so confused that how wavelength is determined and you have cleared this concept briliantly!

[@wooluke7144]

Wow. Very Well Done. All Capitals Letters.

[@miyu6720]

khan academy saving my grades I FREAKING LOVE YOUUUU

[@CalvinUlbricht]

very well explained video, thanks a lot, really helped me understand it better.

[@Cherrilady83]

God Blessed this man.. Tysm❤

[@sotirissarros3522]

many thanks man. it's clear that you really want to explain the subject and you do it perfectly!! well done!!

[@mastersanjay7642]

Amazing explanation and content hat tip

[@TimeGrowing]

Hey man you just killed it's been one month on school teaching me,but I didn't understand. And you, just in 14m you make it easier than drinking water. Thank you so much.

[@hokwanchan4142]

the video gives me a clear idea of diffraction.thanks

[@MohdSameer-rx9gj]

Very very nicely explained.....!!

[@yeonjin8]

First time watching an English explaination but still it's so good❤️👍

[@5vart5ol]

Dude! You really have to become a teacher! Cause the way you explain! You make it so it feels sooo interesting! Which it is! TEACH MORE!

[@GoogleUser-ee8ro]

so clearly explained

[@abuhshadid5938]

You explained it just perfect.... You make some great videos on physics.... You really are a great tutor... It turns out that my teacher doesn't have any time to explain everything like you do... So I'm very much grateful to you, David SantoPietro...

[@kushalchakrabarti240]

This guy is great!

[@georgiaantoniou8410]

ELITE TEACHING SKILLS

[@1Rise]

you explained the destructive concept perfectly and easy to understand.

[@adreekazafor2410]

i think i don't even need to go through the topic anymore thanks

[@lewiduressa]

This explanation is very clear. I read my physics book on this topic, and added with this video, it makes sense! Thanks

[@aashwinsharma1859]

Just understood the concept in 15 min which I couldn't understand in days. Great explanation

[@MariaShchurova]

That;s incredible. Thank you for your work

[@markren626]

This super amazing explanation made me think of the animation of 3b1b’s fourier transformation animation, the little dots moving on the sine wave and add up their values is just like the process of wrapping a metal wire around a circle and find its center of mass!

[@poiuwnwang7109]

That makes sense to me, Khan.

[@astroknight5]

Beautifully illustrated!

[@chrisjieyanglee5360]

thanks bro you've helped me alot!!

[@fahamidaani9597]

wow you explain it so easily

[@edit..x]

Super interesting ❤️

[@inku.1593]

You are amazing 👑👑. Even I don't speak English every time I see you explaining something I feel happy inside of me . Becouse you make studing physics fun .

[@lordasjad4896]

"It's great actually "

[@udays6170]

Beautiful explanation!

[@smallbowl7143]

Thanks, but I'm slightly confused If d (distance between 2 slits remains constant), then at any time no more than 2 waves can be in phase. Furthermore in the maths used to find the equation dsin(theta) = n lambda, you say that the wavelength is the change in x and make a right angled triangle. As it is a right angled triangle the angle opposite the right angle must be the hypotenuse and therefore the two lines must not be the same length and the waves are still out of phase. If I didn't explain clearly then sorry but that's why I'm still confused

[@derdudernan]

Small Bowl I think I understand your question, so let me take a shot.. d*sin(theta) represents the extra distance light has to travel from one opening compared to another. The reason light from several different openings can be in phase is because that distance for those openings will be an integer number of wavelengths. So if the light travels say 2 or 3 or 4 or... extra wavelengths in distance compared to another opening they will still hit the screen in phase. Also, yes in the right triangle you talk about the hypotenuse is a little bit longer then the longest leg, it is assumed the rays are parallel when when leaving the slits, so it is an approximation. But since the distance between the slits is so small compared to the distance to the screen the angle is really small and so assuming they have the same length gives a good approximation, even on the scale of a nanometer. Hope I understood the question, hope that helps. Still attempting to understand it fully myself...

[@smallbowl7143]

derdudernan Since asking the question I studied more physics and came across small angle approximations. In very small angles it is assumed that sin(x) = x = tan(x) This is the reason it works as the hypotenuse is effectively the same length as the adjacent.

[@gausiyakhan6478]

Because of two coherent sources .in huygens theory every point are source .

[@craetydonutkey1348]

For the deviated spot, i think if you just draw out all the possible waves on the same graph you could tell they are interfered destructively by their adjacent waves at each intersection. In other words, I think any space between the integer wavelength lambda will always be dark, resulting in discrete bright spots on the screen.

[@chaitanyavarma1747]

"Double slits are cool" Awesome 😂😂😂

[@sayan_debnath]

Great explanation.. Thank you

[@wildmindentertainment2685]

Subscribed, very good explanation. We see stuff so superficially at high school that sometimes I must understand more graphically thr whole phenomenon and not just admit results as they would want us to do it.

[@kitanomarumilitoni9293]

I understood that very well. Great job sir thank u very much!

[@bhaveshohal3390]

Excellent video.....

[@Rachel_PureBold]

Hi, one thing I found confusing about this was your drawing of the little triangles near the diffraction screen. Because the angles differ at which the rays leave the screen, these little triangles cannot be similar, or even right-angled, if the short sides are equal to one wavelength in every case. Elsewhere online I found an image of interlocking semicircles with their intersection points joined by straight lines projected to the screen. I found this more helpful (and not confusing).

[@Awai_quotes]

And i also have to admit you only make it understandable

[@tom_something]

This is super clear, thank you. One thing I was wondering as I looked at this. Shorter wavelengths should, then, encounter these positive interference peaks at smaller angles. And I was trying to reconcile that because I know "blue light diffracts more than red light". But I looked it up, and in a diffraction grating, blue dots would be _closer_ together. I had assumed a diffraction grating would work the same as a prism or the sky. But nope. So the observation of dot spacing with regard to wavelength through a diffraction grating is totally consistent with the model you've outlined, which is really satisfying.

[@banterbus4297]

thanks so much you helped me understand it so much better

[@manswind3417]

Quite a marvellous experiment and of great utility, as the instructor points out and I rightfully admit. However, as opposed to YDSE, wouldn't this experiment be a nightmare to actually execute? We are dealing in lambdas of the order of below microns here, so in the process of exacting our Ds across the wall, even if we miss the mark by a nanometer in making the hole we might mess up the whole pattern right? In YDSE we didn't have such a concern since there were only 2 holes so a slight error would only cause a slight shift in the interference pattern, but for diffraction grating wouldn't the error add up due to all holes and effectively yet a dark spot instead of a bright one all because of physical imperfections?

[@bithipaul6810]

Great video..sir..outstanding...

[@grizzers21]

easier than reading a textbook. Thanks!

[@intesar6341]

Thank you ... now its clear for me

[@supersonic174]

One thing that could be helpful to point out is that in areas of deconstructive, when ever the wave arrives at the screen there will always be a collection of smaller waves or troughs.. even as time passes, as time moves forward. As the trough hits the screen there will be waves hitting the same point causing deconstructive interference and vice versa.

[@shashanksadafule]

How is path difference constant for consecutive waves. The theta is different for both of them

[@brianummel5602]

I agree. His explanation is wrong.

[@jasonhng8135]

nice video about diffraction grating!

[@mrlisterresistor9695]

I got it the first time only thanks to your explanation

[@gracepancalstatela4422]

best explanation!!! Thanks a lot .

[@maximshekhovtsov4766]

Thank you very much from Ukraine!!!

[@angikmukherjee1226]

You nailed it man!

[@LanNguyen-iu3oe]

Thank you so much! your videos on diffraction and the slit experiment really helped me fully conceptually understand the process.

[@vardanhovakimyan7633]

good job, buddy! thanks

[@tylerlitch3992]

What screen recording software do you use? I would like to know a good one that allows me to pause recording.

[@vinaykumardaivajna2470]

Wow great thank you u guys are ....great...thank you so much.....

[@antoniadukmenic627]

Thank you sooooo much! 💞

[@AsmaaAlmurshedi]

2024 and this video still working great 😃👍 thx for the explanation you're doing amazing!

[@rishabhattri6055]

Very well explained

[@lovecatz101]

9:58 blew my mind :O

[@Cooltaha]

the cancelling out blew my mind :o

[@nomoreprozleft]

Cool taha watch the video above this one in their physics playlist. they explained it there

[@faizanrizwan786]

so comprehensive! thank you!

[@bondlets2342]

Thank you so much! You guys are awesome!

[@ilovestareon]

I'm confused 3:03 Why do you assume that drawing line perpendicular to the second line splits the difference in length between the two lines? It doesn't. To make those lines equal you have to create an isosceles triangle, a right-angle just makes a scalene triangle, and all the lines will be unequal.

[@gabrielragum]

I believe this is due to an approximation, which is only valid if the distance between the plane with the slits and the plane where the image is formed (D) is much greater than the distance between the slits (d), so D >> d. This is because the farther apart the two planes are, the more parallel the lines which represent the light rays become, and since they still must end at the same point, that perpendicular line you mentioned will split the second line in such a way that it gives us approximately the path length difference.

[@benitokestelman4545]

When you draw the right angle at 3:05 and snip off a wavelength from the bottom line, the remainder of the bottom line will be smaller than the top line (Pythagorean theorem), so the whole bottom line can't have been one wavelength longer than the top line. Can you explain this?

[@benitokestelman4545]

Are we just approximating and saying they're pretty much equal, since the length between the slits and the screen are so great? Is this ok?

[@adityamathur5972]

@@benitokestelman4545 That can't be because instead of bright spots we'd get fading ones that indicated the "appropriateness" of sinθ ~Δx/d. Don't you agree?

[@AndimatterUniverse]

You really should have emphasized that this only works if you assume the screen to be far away from the grating. Because in your animation at around 5:00 its clearly visible that the difference between the second and third ray is not 1 lambda. It only works if you assume the rays to be parallel, wich they are if the screen is far away.

[@ebtsoby]

this is what confuses me, we assume the light is parallel, yet we use triangles