# What is optical density?

1.the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before re-emitting it as a new electromagnetic disturbance.

2.the ability of a medium to change the path of light

so what is the relation between these two definitions??

One more question:Is there a relation between the physical density and the optical density?Is it a direct relation ??

Related Other Physics Topics News on Phys.org
Integral
Staff Emeritus
Gold Member
First find better sources neither of those definitions are very good.

No, Optical Density is not related to mass density. Think of OD as a measure of a materials ability to pass light. The higher the OD the lower the amount of light which gets through. This will vary with wavelength for any given material.

A pair of laser safety glasses I have is rated; O.D. 7 @ 190-400nm; 1.3+ @488nm ; 5 @10.4nm.

They are the safety glasses you would wear for working around a UV laser.

First find better sources neither of those definitions are very good.
I find the first one good so what's wrong with these definitions?
Think of OD as a measure of a materials ability to pass light.
transparency?

The higher the OD the lower the amount of light which gets through
how?I can't Imagine this

Well yeah, basically it's transparency. Not exactly, and Integral's definition is better, but essentially.

So your third doubt should disappear.

Integral
Staff Emeritus
Gold Member
What is wrong with the definitions you gave? If they were good you would not be here asking this question.

They are both way to general. Neither are direct definitions of OD, All transparent material effect the passage of light. The place where you will encounter OD in daily life is a pair of sunglasses. A very dark pair has a High OD thus letting through less light then a light pair. OD is a measure of how much light is adsorbed by the material. The more light (high OD) adsorbed by your glasses means less gets to your eyes.

uart

1.the sluggish tendency of the atoms of a material to maintain the absorbed energy of an electromagnetic wave in the form of vibrating electrons before re-emitting it as a new electromagnetic disturbance.

2.the ability of a medium to change the path of light

so what is the relation between these two definitions??

One more question:Is there a relation between the physical density and the optical density?Is it a direct relation ??
Hi Misr. Googling "optical density" I see that unfortunately there are two (conflicting) definitions in common use. One (as per Integrals usage) which is synonymous with opacity (or absorbance) and another which is synonymous with refractive index. In other words, one definition refers to an optical material's ability to block light while the other refers to its ability to slow and bend light.

Both of your definitions however refer to the latter. The first is emphasizing the mechanism by which the light is slowed and the second is emphasizing that the ability to slow the light is also an indication of its ability to refract light (as per Snells Law)

Last edited:
Let me provide an operational definition of optical density:
Write the number 1 and follow it with as many zeroes as the optical density. The number you get is the optical attenuation.
Example: after passing a filter of optical density 6, the outgoing light has been attenuated one million times.
Hope this is clear enough.

What is wrong with the definitions you gave? If they were good you would not be here asking this question.

They are both way to general. Neither are direct definitions of OD, All transparent material effect the passage of light. The place where you will encounter OD in daily life is a pair of sunglasses. A very dark pair has a High OD thus letting through less light then a light pair. OD is a measure of how much light is adsorbed by the material. The more light (high OD) adsorbed by your glasses means less gets to your eyes.
That's very simple but what about the relation between the OD and the speed?
as OD increases speed decreases ?

uart
That's very simple but what about the relation between the OD and the speed?
as OD increases speed decreases ?

the second is emphasizing that the ability to slow the light is also an indication of its ability to refract light (as per Snells Law)
how?that's confusing me :(

hello there??

No, Optical Density is not related to mass density. Think of OD as a measure of a materials ability to pass light. The higher the OD the lower the amount of light which gets through. This will vary with wavelength for any given material.
http://www.physicsclassroom.com/mmedia/waves/em.cfm
The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.
doesn't this mean that there's a certain relation between the optical density and the physical density?
I know that oil is more optically dense than water although it has lower density
but I'm very confused :(

can u plz give me a proper definition of optical density . i will be thankful

OD=lg(1/T) where T-transmission
But sometimes another definition is used OD=ln(1/T). So there may be confusion. To avoid it the base of logarithm should be specified.

It's base 10.

I met with use OD under e base also. May be it was inaccurate use.

The base depends on the materials.

By liquids, the base is 10. By gases the base is natural. Check wikipedia, searching for transmittance....