/0's physics phread

[kamil9876]

ANd yes, i know what you mean now by "N to S is always positive is not enough". In my example what I chose is that Down is ALWAYS positive, as in, "The initial direction of N to S will be always positive". Just like the question in the vcaa exam says: "Take the direction from N to S in the figure as positive". In the figure being analogous to what I mean as "What N to S is initially".
I'm pretty sure that's true as it's the only interpretation that yields only one correct answer as far as our understanding of freedom to assign positive direction goes.

[/0]

thanks for all your help guys

[/0]

What is the formula for bright fringes in single-slit diffraction?

And also, the formula for dark fringes is

The pattern in single-slit diffraction is a lot b-r-o-a-d-e-r than the pattern in double-slit interference, so how can the same formula apply?

[kamil9876]

In single slit diffraction, the d ussually means half the width. So when you say broader, what are you keeping the same in both? ?

You might like this and the next post: http://vcenotes.com/forum/index.php/topic,9668.msg159096.html#msg159096
The second post explains your first question i hope.

[TrueTears]

What is the formula for bright fringes in single-slit diffraction?

And also, the formula for dark fringes is

The pattern in single-slit diffraction is a lot b-r-o-a-d-e-r than the pattern in double-slit interference, so how can the same formula apply?

just gives the extent of diffraction. where w is the width of the single slit.

[/0]

In single slit diffraction, the d ussually means half the width. So when you say broader, what are you keeping the same in both? ?

You might like this and the next post: http://vcenotes.com/forum/index.php/topic,9668.msg159096.html#msg159096
The second post explains your first question i hope.

Thanks kamil, I found that post useful

[TrueTears]

No it gives bright bands for single/double slits.

If you want dark bands it would be

[notice it could be n+0.5 due to what you take middle as, ie n = 1 or n = 0]

EDIT: oh you just deleted your last post but anyways I'll leave this up here.

[/0]

Thanks TT

Also I have another question

does this take into account relativistic effects?

I thought that electrons usually travel at around

[kamil9876]

Thanks TT

Also I have another question

[/tex]

Just out of curiosity, what is that?

[/0]

the de broglie wavelength

[TrueTears]

Thanks TT

Also I have another question

[/tex]

Just out of curiosity, what is that?

[/0]

1. Why is it that in double-slit experiments we don't consider the contributions from the top and bottom halves of each slit as we do in single-slit diffraction?


2. In single-slit diffraction, you know how they take parallel rays of light and use to find the path difference... well the common reasoning is that the screen is so far away and the slits are so close that the rays are virtually coincident.
But if we make approximations like that, then doesn't that mean that the path difference should also be 'insignificant'? I would think that saying two parallel rays converge at infinity is such an impractical estimate that it completely negates the accuracy of , which must be VERY accurate if we consider light waves.

[TrueTears]

2. Yes, all the formulas are all derived from assuming the point P is at infinity and the 2 lines are parallel, so it is in fact an approximation. But because the width of the slit are so small, the approximation is very very very very close to the path difference.

[/0]

Thanks TT, it is a tricky proof

Also, I'm having trouble with VCAA 2003 AOS4 Q5

"In a photoelectric effect experiment, light of various frequencies falls on a metal surface in a photocell. The photoelectrons are decelerated across a retarding voltage, and the stopping potential, , is measured for each frequency. Determine a value for the work function of this metal surface. Include the unit in your answer."

There is a graph which, using regression is approx where is in Volts and in Hz.

The intercept of this graph is .

From , when ,

Therefore,

But if I let then

And

Which method is wrong and Why? thanks

[kamil9876]

1. Why is it that in double-slit experiments we don't consider the contributions from the top and bottom halves of each slit as we do in single-slit diffraction?


2. In single-slit diffraction, you know how they take parallel rays of light and use to find the path difference... well the common reasoning is that the screen is so far away and the slits are so close that the rays are virtually coincident.
But if we make approximations like that, then doesn't that mean that the path difference should also be 'insignificant'? I would think that saying two parallel rays converge at infinity is such an impractical estimate that it completely negates the accuracy of , which must be VERY accurate if we consider light waves.

I think you should imagine a single slit not as really just one slit, but a continous domain of infinitely small slits the size of those found in double slit experiments. The reasoning is consistent then. As far as approximations try derive it without approximations, I think you have all the analytical geometry you need at your disposal