Particle Accelerator

6:04 AM

Hi! Welcome to "Interferometry" Chat Room.
This room is primarily meant for discussions related to Physics.

Why was the name "Interferometry" chosen? : This is because I like the concept of interference and this chat room would be helpful to prevent interference from other conversations.

Note: Room name might be changed in future if I come across even interesting concepts in the subject.

Feeds

Guru Vishnu has added John Rennie to the list of this room's owners.

Guru Vishnu

@JohnRennie: Hi sir. Good Morning :-)

John Rennie

@GuruVishnu hi :-)

Guru Vishnu

6:25 AM

@JohnRennie: I have created this room to store conversations in more accessible form for future reference. This will also help to save screenshots easily for me, so that I do not need to cut out other intermediate conversations in the middle. Is that fine for you to discuss in this room, sir?

John Rennie

Yes. that's fine.

Guru Vishnu

@JohnRennie Thank you sir. Are you free now?

John Rennie

@GuruVishnu yes, for about half an hour.

Guru Vishnu

@JohnRennie Fine sir. It'll not take more time. I received the following comment for the question I asked yesterday regarding astronomical spectroscopy. Is this how they determine elements using the absorption spectra, sir?

How do they actually do this? +1. By visually comparing cellulose slides on top of each other? — Mazura 2 hours ago

For reference : This is the question - physics.stackexchange.com/q/526618/238167

John Rennie

I don't know how it's actually done. I'd guess it's computerised these days.

In the early days when Fraunhofer was working they didn't even have photographs so I guess they measured by hand straight from the screen.

Guru Vishnu

6:31 AM

Ok sir. Is that "cellulose" referring to paper?

John Rennie

I suspect it means a cel as used in the film industry.

Cel

A cel, short for celluloid, is a transparent sheet on which objects are drawn or painted for traditional, hand-drawn animation. Actual celluloid (consisting of cellulose nitrate and camphor) was used during the first half of the 20th century, but since it was flammable and dimensionally unstable it was largely replaced by cellulose acetate. With the advent of computer-assisted animation production, the use of cels has been all but abandoned in major productions. Disney studios stopped using cels in 1990 when Computer Animation Production System (CAPS) replaced this element in their animation process...

Your question has attracted a lot of attention (and upvotes :-)

Guru Vishnu

Ok sir. Thank you. Now I somehow understood how they could have done this long before computers were invented.

@JohnRennie Yes sir. The credit goes to you. It's because it entered HNQ :-)

Thank you for your time and support sir.

John Rennie

:-)

Guru Vishnu

room topic changed to Interferometry: Intentionally left blank [john-rennie]

room topic changed to Interferometry: Intentionally left blank [guru-vishnu] [john-rennie]

Guru Vishnu

10:33 AM

@JohnRennie: Hi sir. Are you free now?

John Rennie

@GuruVishnu hi, yes I'm free

Guru Vishnu

Fine sir. I asked the following question on the main site about an hour ago when you were busy in another room. If possible, could you help me regarding that?

0

Q: Is the equation for dispersive power applicable for all dispersive elements?

The following text is from Concepts of Physics by Dr. H.C.Verma, from the chapter "Dispersion and Spectra", page 434, topic "Dispersive Power": The dispersive power of a material is defined as the ratio of angular dispersion to the to the average deviation when a light beam is transmitted thr...

I have received a useful comment there.

John Rennie

There are various ways of quantifying dispersion. I would have guessed the most common one was the Abbe number:

Abbe number

In optics and lens design, the Abbe number, also known as the V-number or constringence of a transparent material, is an approximate measure of the material's dispersion (change of refractive index versus wavelength), with high values of V indicating low dispersion. It is named after Ernst Abbe (1840–1905), the German physicist who defined it. The Abbe number, VD, of a material is defined as V D = n D...

Guru Vishnu

But now that user says, the formula is applicable for all cases. I couldn't believe how it's applicable for everything if it's derived for a particular case, sir.

John Rennie

Actually the Abbe number is just $1/\omega$

Guru Vishnu

10:38 AM

@JohnRennie Is $\omega$ dispersive power, sir? I think I need to learn about this from the linked article as I didn't come across it.

John Rennie

The Abbe number / $\omega$ is only a guideline because the refractive index is a complicated function of wavelength and you can't represent that complicated function by a single number.

It's value is as a guideline for whether a material is highly dispersive or weakly dispersive.

For most materials the refractive index doesn't change by very much across the visual spectrum so we can approximate it as a linear dependence i.e. something like $\mu = a\nu + b$.

Guru Vishnu

Thank you sir. So, "dispersive power" is an outdated measure, whereas "abbe number" is it's modern version. Am I right, sir?

John Rennie

I don't know to be honest. But both are only approximate measures.

Guru Vishnu

@JohnRennie Ok sir. No problem. I'm reading that article and if I have any doubts I'll ask them. So far it seems to be the reciprocal of dispersive power.

John Rennie

I wonder what happens if you take $\mu = a\nu + \mu_o$ and substitute it into the equation for $\omega$ ...

Guru Vishnu

10:48 AM

My book gives another different expression for dependence of index of refraction with wavelength: $$\mu=\mu_0+A/\lambda^2$$ This seems to be slightly different from the expression you gave sir.

John Rennie

OK, that's approximating it as a quadratic rather than a straight line.

Guru Vishnu

It's said that $A$ is Cauchy's constant in the above expression.

Sir, I think according to "Abbe numbers are used in the design of achromatic lenses, as their reciprocal is proportional to dispersion (slope of refractive index versus wavelength)…", both dispersion and dispersive powers mean the same mathematically. But till now I've understood dispersion as a phenomenon rather than a mathematical expression like "power".

John Rennie

Well it is a phenomenon, and the Abbe number and it's reciprocal $\omega$ are attempts to provide a simple way of quantifying the phenomenon.

Dispersion happens because the refractive index changes with wavelength, so using $d\mu/d\lambda$ to quantify it seems an obvious approach.

Guru Vishnu

@JohnRennie: Thank you sir. So, the expression is applicable for all geometries despite it's being derived for a particular case of thin prism. Do you agree with this sir?

John Rennie

$d\mu/d\lambda$ isn't constant across the visual spectrum, but we can treat it as approximately constant, and that gives the Abbe number.

@GuruVishnu yes

Guru Vishnu

11:01 AM

@JohnRennie Fine sir. Or in other words, dispersive power of any device is given by the above equation. Ok sir?

John Rennie

Any medium not any device. The Abbe number is a property of the medium from with the prism/lens/whatever is made.

Guru Vishnu

@JohnRennie Ok sir. So, there is no meaning of "dispersive power of a prism" but it exists for "dispersive power of glass".

John Rennie

Yes, exactly.

The dispersion in the prism will be proportional to the Abbe number but will also depend on the prism angles etc.

Guru Vishnu

@JohnRennie: Ok sir. Now understood. I thought "dispersive power" is a property of the dispersing device rather than the material, as we have for "optical power" which depends on the device (whether it's convex lens, concave lens, etc.)

Thank you sir.

John Rennie

:-)

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♦ Particle Accelerator