If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

## Physics library

### Course: Physics library>Unit 14

Lesson 2: Interference of electromagnetic waves

# Young's double slit problem solving

Continue to explore Young's double slit equation through a sample problem involving a double slit experiment. See how light waves of specific wavelengths interact when they pass through two slits, creating a pattern of bright spots on a screen. Get insights into the math behind calculating these distances and understand the impact of changing slit spacing. Created by David SantoPietro.

## Want to join the conversation?

• Why wasn't d (the distance between the two slits) calculated as the distance from the center of one opening to the center of the other opening, 1500nm instead of 1300nm? This seems more intuitive, to me. • The question is stating that the 200 nm slits are spaced 1300 nm apart, and the video demonstrates that the measurement is from the center of one slit to the center of the other. This is not the same as saying the object between the slits is 1300 nm, and it would in fact be 1100 nm.
• How can we find distance between 1st and 2nd bright dot ??plzzz help ..
(1 vote) • Hello, I'm kind of a nube, forgive me. I've been doing the experiment at home and can't seem to figure out what kind of measuring device i can use to collapse the wave and make it act like a particle. I would like to preform this for my friends and family to inspire some curiosity. Is this part of the experiment beyond my DIY capabilities? • OK. So I have thought of a way to DO it. But a) you can not Prove its happening and b) the experiment / demonstration is a bit 'unexciting'.
However; here goes

First, if you want to show wavefunction collapse, then you need to start with a wave-like thing and then change into something with a specific momentum, position, energy... etc. (ie NOT delocalised).

OK, so how do we 'prove' that we have a wave-like - thing'? Well, if we can diffract it, then it shows us that we have a wave. (Diffraction is one of the 'tests' of wave nature of a thing)
Then if we want the wave function to collapse, we need to 'observe' the thing. ie measure its position, energy momentum etc.

OK, so how to do it at home?? Well, I did say it was not exciting....

the answer is to squint !!

I mean if you look at a light source; for example a street lamp outside at night or a star or any bright-ish light. and then squint. I mean like almost close your eyes. You will see the light 'spreading out'. It makes a star-like shape in your eye. Try it n see. This is due to the light 'diffracting' as it goes past your eyelid.

So, now we have showed that the light going into your eye is a wave. And therefore has a wavefunction.
THEN, the light passes through your eye (jelly bits and lens) and then lands on your retina at the back of your eye. How do we know it lands there? Because you see it.
So now you know the exact position of the light. Exactly on your retina. You also know that the energy required to stimulate your rods and cones in your retina will come from the photon that 'formed' on your retina at the time of observation. Now, no longer a wave but 'collapsed' to a measurable thing.

I did say it was not an exciting experiment and there is no proof of wavefunction collapse but it is a demonstration :) and worthy of discussion.

Great question. Let me know what you think

Well done
• at , since d=distance from the centres of the two slits, wouldnt we add 1300 to 0.5*slit length to find the distance between slits 'd'? So the correct answer would be 1300+(0.5*200*2)=1300+200=1500 nm. • What happens when the distance from the screen increases? Do the distance between the dots increase? • Why did you say that sin=tan? this rule is only applicable for small angles (less than 10)
I used the rule Δy= Lλ/d
and I got 1.62 • To calculate the distance between the slits, do we take the distance from the center of the slits to the other center or simply from one end to another? Also, if I use theta = wavelength/ b, then i get theta to be 3.5. Why is this wrong?
(1 vote) • Then what is this .....Xn= n(Lamda)(D)/d? • My guess is Xn is the distance from the central maxima to the nth maxima (the central maxima being n=0).
From the diagram:
tan(θ)=Xn/D where D is the distance to the screen
and
sin(θ)=n(lambda)/d
For very small angles, we can say that tan(θ)=sin(θ)
So: Xn/D=n(lambda)/d
Xn =n(lambda)(D)/d
However, in this question, it is not a "very small angle"
The angle is bigger than 30 degrees! So we first had to find θ from sin(θ) before finding tan(θ) instead of simply equating sin(θ) and tan(θ).  