Talk:Optical depth

Physics Start‑class Mid‑importance

	Physics portal This article is within the scope of WikiProject Physics, a collaborative effort to improve the coverage of Physics on Wikipedia. If you would like to participate, please visit the project page, where you can join the discussion and see a list of open tasks.PhysicsWikipedia:WikiProject PhysicsTemplate:WikiProject Physicsphysics articles
Start	This article has been rated as Start-class on Wikipedia's content assessment scale.
Mid	This article has been rated as Mid-importance on the project's importance scale.

There is a disparity here... I think an optical depth is something like transmission=exp(-depth). But I'm not sure. William M. Connolley 19:54, 1 January 2006 (UTC).[reply]

corrected definition

The optical depth tau is related to the fraction of light scattered. The following equation expresses this relationship:

I/I_0= exp{-tau},

where I_0 is the incident light and I is the light that passes through the medium without being scattered. Equivalently, for a homogenous medium, tau is the ratio of the path length to the mean free path.

Corrected definition inserted in the article, some inexactitudes corrected, some concepts in atmospheric science added. I hope it is appreciated.Marenco 26 September 2006 (UTC)

Great! that's cleared up the definition finally. Deuar 20:13, 27 September 2006 (UTC)[reply]

Cognitive problem with explanation

I think there is a cognitive problem here. As I understand it, optical depth is a property of the material you are looking through, not of you the observer or the object you are observing. However, the article talks about it in analogy to taking an object and moving it backward, which makes it sound like the medium is not what is being described by optical depth.

It might be better to speak of optical depth increasing as the fog gets thicker, and the value of optical depth being the farthest thing you can see through that fog.

Raddick 20:10, 4 November 2007 (UTC)[reply]

I think the explanation can be reformulated to avoid misunderstandings, but is important to state that the optical depth change when there is more medium between the observer and the object (more light get scattered or absorved). So the optical depth is not just a property of the material, but it also depends on the distance between the object and the observer. The farther the object is, the less you see of it (because there is more medium on the way). Hsxavier 23:58, 5 November 2007 (UTC)[reply]

I'm not convinced that the first sentence is correct :

Optical depth is a measure of transparency, and is defined as the fraction of radiation (or light) that is scattered or absorbed on a path.

The equation is correct, but if tau is the fraction of the radiation that is removed then tau=0.5 should mean that half of the radiation has been absorbed or scattered. However e^(-0.5) ~ 0.6 and e^(-1) is certainly not equal to zero. --Maddoug (talk) 15:59, 22 January 2008 (UTC)[reply]

The current (Apr. 25, 2009) first sentence "Optical depth, or optical thickness is a measure of transparency, and is defined as the negative logarithm of the fraction of radiation (or light) that is scattered or absorbed on a path." is not correct. Given I/I0=e^-tau, tau=-ln(I/I0). Thus the first sentence needs to be "Optical depth, or optical thickness is a measure of transparency, and is defined as the negative logarithm of the fraction of radiation (or light) that IS NOT scattered or absorbed (i.e., IS transmitted) on a path." —Preceding unsigned comment added by Random25 (talk • contribs) 18:47, 25 April 2009 (UTC)[reply]

Nuclear Trivia

For what it's worth: optical thickness (depth, whatever) is an important concept in understanding the design of thermonuclear weapons.

Basesurge (talk) 07:41, 22 March 2010 (UTC)[reply]

Optical Depth/Optical thickness

Is there no difference between the 2 concepts in English stellar physics ? I can't be sure this is a mistake, I am neither an English native-speaker nor a stellar physicist, so... Anyway, French physicists would see a big difference between optical depth (profondeur optique) and optical thickness (épaisseur optique), although the terms are often subject to confusion and related mistakes. In stellar physics :

Optical depth deals with intensity of light generally emitted by a star,
Optical thickness deals with transparency of a sheet of material.

Same in English ? Should it be clearly mentioned both here (the article I have been partially translating) and in Optical depth ?
Mianreg (talk) 02:39, 25 August 2010 (UTC)[reply]

Clarification of definition

There is one thing I do not understand with this concept. According to the scattering properties of light, it can be influenced even in a far distance from a small scatterer (but this influence is very small). So if we say that light which is scattered (even a little bit) has been 'influenced' and is taken out from the original light beam, than optical thickness would be infinite everywhere (in our universe). So what is the minimum scattering angle or minimum momentum transfer until which scattering has not to be taken into account for optical depth? Or is optical depth dependent on observer properties like a camera resolution (and this would define the minimum angle not to care about)?