Young's double slit equation | Light waves | Physics | Khan Academy

Let's derive a formula that relates all the variables in Young's double slit experiment. Created by David SantoPietro.

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[kenpachizaraki]

I had such a difficult time learning this in class for some reason. I really appreciate this. I don't know why but watching this made perfect sense to me. Maybe I'm just more of a visual learner. My instructor teaches concepts sometimes without illustrating. It makes it hard for me to visualize.

[Hunar]

I'm a physics student and i'm planning to get BIG marks in order to become a teacher, when i do that i will use the best method to teach my students, these and alot of other videos made it very clear how my method would be .. thanks i hope i will be as helpful as you guys :)

[muzzammilsajid]

1:58 I have a problem. How are those path lengths equal? I mean that's a right angled triangle in which hypotenuse is always greater, isn't it?

[ДенисМ-йж]

2:04 why the paths will be the same length? Aren't they cathetus and hypotenuse of right triangle, and can't be same length?

[adishijha]

please explain ...why both of the angles are same

[aaronge]

I hate to be that guy, but just in case any one else is confused, I gotta make a small correction. At 2:20, he says that the two sides highlighted in turquoise would be the same length, but that's impossible since one of the turquoise sides is the hypotenuse of a right triangle while the other side is not the hypotenuse.

[nurul]

thank u very much sir...i cant focused in my online class and my test is in three days and thankfully you save my lifeee <3

[yorkregiontutoring]

The most important part of the video (just before and after 2:30) was quite poorly explained. Please consider making another video for students that demonstrates parallelism of lines, why the path lengths would be considered equal and why you placed the second theta where you had.

[adityachandeliya]

awesome explaination :) !!
Every faculty must teach like this one !!

[SA-sqsf]

hats of to u guys
u guys r just amazin'
u make every thing so easy for us .
love de khan academy

[maizpalomeropuebla]

Hello. I just watch this video, is very good! thanks. But I want you check at 2:16. I have a question on this. If you take the right angle on where it is marked the two light blue paths marked are not equal. you most take the right angle on the white line so the both light blue paths have the same lenght...  am I right?

[danpovey]

Arouynd 2:15, I notice a lot of people are pausing. I think he draws the right-angle symbol in the wrong place and it should be at the center, white line. Otherwise the argument doesn't seem to work. But I'm not a physicist and have not checked the argument all the way to the end.

[ShashiKumar-chk]

A lot of people are struggling to understand why both lengths are equal at around 2:15, here's why,
The distance between both the slits (d) is very small and distance between the slits and screen is very very big, then by pythogorous theorem in that big triangle (X1)² = (X2)² + h²(h is the hypotenuse of the smaller triangle containing d and perpendicular in the bigger triangle), since h²<<<<(X2)², we can neglect h to get X1=X2.
Hope it helps!

[JG]

I worked out a formula for Δx without using the sine approximation...



I used the law of cosines a few times. If "d" is the distance between the centers of the slits, "L" is the perpendicular distance between the slit system and the back wall, and "y" is the height to the first order point, then:
Δx = ( 2dy + d^2 ) / ( 2 * sqrt((y + d/2)^2 + L^2) )

The sine approximation (Δx = dsinθ) is good when "d" is very small compared to "y" (and to a lesser extent "L").

[JimbobFaz]

Im confused at 2:10. Since if as he claims the 2 blue lengths are equal, then this forms an iscoles triangle and so the base angles are also equal. However, since the lower base angle is 90 degrees, this would also make the upper angle 90 degrees and you would not have a triangle but just a line. Consequently, the only thing that makes sense here is if this is just acttually an approximation. If this is the case then it would explain why he said the distance to the screen most be a lot further than the slit difference, as the approximation would get better then. If this is what he means here, why did he not clarify that for large comparable distance this is a good approximation.

[noname-sgqx]

Isn't the equation for this a=D(lambda)/d, where a is the fringe spacing and D is the distance from the screen to slit and d the distance between the 2 slits?

[gilbertfeliu]

Can someone please explain why the angle opposite to the Δx side in the right triangle is equal to theta? Theta originally being the angle between the line cutting through the central maximum and the line going to the int. fringe.

[islampeace]

how are the angles the same at 2:34, don't say that the distance between the screen and the source is much larger than the distance between the slits... I know that but how does that allow you to assume that the angles are the same

[syedsaadqainhaider]

what is the effect of light intensity ion the fringe spacing??

[AVirk]

How did he equate the Q angles in the two triangles