LinusX's questions

[LinusX]

Do we need to know about the quantitative measurement of fringe spacing?
If you look at the worked solutions for Q5a of Heinemann chapter 11 review you can see that they have done some weird calculation which I havent seen before.
Also, in terms of the history and comparisons with the classical wave model etc, what do we need to know?

[Lasercookie]

That equation is given on Page 414. The Nelson book gives it directly, along with a better explanation.
It's really just the stuff you should already know about interference explained using maths.
I think it must be on the study design - I made good use of it when doing trial exams.


is the fringe spacing.
is the wavelength of the light (m).
is the distance from the screen (m).
is the separation between the slits (m).

You can play around with inputting numbers and see how it will affect the fringe spacing.

Also, in terms of the history and comparisons with the classical wave model etc, what do we need to know?

Basically, take a look at each of the 'properties' (diffraction, interference, photoelectric, de broglie) and learn how the correct model explains it and also learn why the other model cannot explain it. For the photoelectric effect, VCAA 2010 had a great question on it. Pretty sure VCAA 2009 had similar question as well.

Read the study design, it should clear stuff up.

[LinusX]

Thanks for the reply laserdd,I did check the study design:
• explain the results of Young’s double slit experiment in terms of
– evidence for the wave-like nature of light
– constructive and destructive interference of coherent waves in terms of path differences,
pd = n λ, pd = (n – ˝) λ respectively
– qualitative effect of wavelength, distance of screen and slit separation on interference
patterns;
• explain the effects of varying the width of gap or diameter of an obstacle on the diffraction pattern
produced by light of appropriate wavelength in terms of the ratio λ/w (qualitative);

I can't find anything at all on that equation, all I see is a qualitative effects of wavelength, etc on interference patterns. I want to look at the current study design VCAA papers but I don't want to spoil it for trials later (I remember things for a loooong time).
Anyone done them?

[Lasercookie]

Thanks for the reply laserdd,I did check the study design:
• explain the results of Young’s double slit experiment in terms of
– evidence for the wave-like nature of light
– constructive and destructive interference of coherent waves in terms of path differences,
pd = n λ, pd = (n – ˝) λ respectively
– qualitative effect of wavelength, distance of screen and slit separation on interference
patterns;
• explain the effects of varying the width of gap or diameter of an obstacle on the diffraction pattern
produced by light of appropriate wavelength in terms of the ratio λ/w (qualitative);

I can't find anything at all on that equation, all I see is a qualitative effects of wavelength, etc on interference patterns. I want to look at the current study design VCAA papers but I don't want to spoil it for trials later (I remember things for a loooong time).
Anyone done them?

I did utilise the equation to figure out the qualitative effect. It works for me, it gives me a clear way of figuring out what will happen (e.g. if that number is increased, the other numbers will go down to maintain that fringe spacing etc.). Saves space on the cheat sheet as well. I'd say it's definitely worth understanding.

The textbooks seem to like asking for quantitative answers though.

This is what I had on my cheatsheet for the Light and Matter sac. I tried to upload the PDF file, but it kept giving an error. I had the particle model and wave model explanations for interference and photoelectric effect. I don't guarantee that the wording is 100% correct, you might want to double check. You could probably go for a bit more detail, this was just notes to jog my memory.

Interference (Wave Model is correct)
Particle model: Incorrect. Predicted that two bands would appear.
Wave model: Correct. Antinodes occur when waves are in phase (constructive). Nodes occur when waves are out of phase (destructive)

Photoelectric Effect (Particle Model is correct)
Observation: Photocurrent depends on the intensity of the incident light.
Particle Model: Greater intensity means more photons arriving per second.
Wave Model: Greater intensity means greater energy, but this energy has the same frequency so the photocurrent shouldn’t change.

Observation: Energy of emitted electrons is independent of intensity. It depends only on the frequency.
Particle Model: E=hf. Photons give their energy to the single electron they interact with. Changing intensity varies number of photons not the energy.
Wave Model: Energy = amplitude. Intensity is related to the amplitude.

Observation: Cut off potential is constant for each metal. Ek = hf -W
Particle Model: Same amount of excess energy to each photoelectron. Increasing photocurrent does not increase Ek of a released photoelectron.
Wave Model: Different intensities means different energies. The energy of the emitted electrons depends on the metal surface involved. Work function is the energy needed to remove one electron.

Observation: No time delay.
Particle model: Photons are emitted as packets of energy in random succession. A photon may be ready immediately after a source is exposed, or it may take a random time later before it is emitted
Wave model: Energy delivered in timed frequencies. There should be a time delay.

[LinusX]

Thanks for clearing that up laserdd 

I had another question though:
In my textbook and many places it says that the 'extent' of diffraction is maximised by increasing the ration of (lambda)/slit width above one. I thought this meant that the extent of diffraction is maximised by increasing that ratio, so a huge wavelength with a tiny slit would cause a lot of diffraction. After this, I'm being told that the wavelength and the slit width should be approx  equal (same order of magnitude). Which is correct to maximise diffraction? Also, what do they mean by the 'extent', is it the fringe spacing width?

[Lasercookie]

In my textbook and many places it says that the 'extent' of diffraction is maximised by increasing the ration of (lambda)/slit width above one. I thought this meant that the extent of diffraction is maximised by increasing that ratio, so a huge wavelength with a tiny slit would cause a lot of diffraction. After this, I'm being told that the wavelength and the slit width should be approx  equal (same order of magnitude). Which is correct to maximise diffraction? Also, what do they mean by the 'extent', is it the fringe spacing width?

With my original answer for this I went off into a major tangent (because I looked for a bit more depth, which just confused me lol). I've rewritten it in dot points and hopefully it more directly answers your question:

- Heinemann sucks in their explanations for this. Take a look at Nelson and Jacaranda (Jacaranda had a nice page on it (page 304).
- Diffraction is when a wave spreads out after passing through some obstacle (e.g. a slit). The relationship between interference and diffraction seems to be a bit ambiguous.
- Extent of diffraction is how much a wave will spread out.
- That is referred to in the study design, but we only need to understand it qualitatively.
- Diffraction is insignificant when the ratio is less than 1. It is significant when it is greater or equal to one than 1.
- If the ratio is greater than 1, the light will simply spread out more. Nelson had some nice pictures that explained this with water waves.

And now for the tangent:
- Read Wikipedia and Feynman (Volume 1, Chapter 30) for more depth. Clearly my understanding of this isn't as thorough as I would like (but should be good enough for VCAA lol).
http://en.wikipedia.org/wiki/Diffraction#Single-slit_diffraction
http://en.wikipedia.org/wiki/Diffraction_formalism#A_simple_quantitative_description
- So, what does 'extent of diffraction' actually refer to, well it's derived from this , where is the angle at which minimum intensity occurs (Feynman explains why it's minimum). We don't need to learn about the angle stuff.

Edit: I'll PM you with links for Jacaranda and Feynman's Lectures if I can feynd (pun intended) them again.
Edit: Actually, Jacaranda has a damn good explanation for this, without going into too much of the stuff we don't need to know. I can't find the link for Jacaranda (it's on AN somewhere), but I'll PM you page 304.

[LinusX]

Okay thanks !