Optical Density

Optical Density Definition

Basically, the physical density would be the ratio of mass to the volume, and optical density measures the speed of light while passing through an optically dense medium. 

The optical density is a property of a transparent material that measures the speed of the light through the material. The extent to which any optically dense medium bends transmitted light rays towards or away from the normal is called the optical density.  

The light passing via an optically dense medium bends towards the normal and if the same light passes via any rarer medium such as air, it bends away from the normal.

Optical Density

Now, let us understand the phenomenon of optical density and its effect on the light passing via a medium by comparing the two media.

Consider two mediums: glass and air. When a beam of light passes from air to glass. The speed of the light decreases in the glass. Therefore, the light bends towards the normal as you can see in the image below:

(image will be uploaded soon)

It tells us that the glass is optically denser than air.

Which means,

The velocity of light in glass (denser medium) is less than the velocity of it in rarer medium (air).

If the same light passes from glass to air. The speed of light increases. Therefore, the light bends away from the normal as you can see in the image below:

(image will be uploaded soon)

Which means,

The velocity of light in the air (rarer medium) is less than the velocity of it in glass (denser medium).

What is Transmittance?

The transmittance of a medium or a material is defined as the constituent of the light that moves via the other side of the medium or the ratio of the light energy falling on it to the light transmitted through it. When light passes through any medium, it can be transmitted, reflected, or absorbed.

The transmittance of the light can be defined as the ratio of the intensity of incident light (Io) to the amount of intensity (Ia)  passing through the medium. It is denoted by T.


T =  Ia /Io

The transmittance has no unit.

The ratio of radiant flux transmitted by the material (Φt) to the radiant flux (Φi) received by that surface known as the Transmittance (T). 

Transmittance Formula

T = Φ t /  Φ i is the transmittance of the material.

Here, the radiant flux is the radiant energy emitted, reflected, transmitted, or received, per unit time. Its unit is Watt and the SI unit is Joule per second (J/s)

Optical density formula

An optical density is defined as an equation given by, 

Optical density (O.D.) =  log₁₀ Io / It

The optical density of any medium is defined as the logarithmic ratio of the intensity of incident light (Io) to the intensity of the transmitted light (It) passing through that medium.

(image will be uploaded soon)

The O.D value for absorbance can be computed by the formula given by,

O.D. Value

Absorbance (A)  = log₁₀ (100) / (%T)

Where, Io is the intensity of visible light incident upon a small area of the film and It, the intensity of light transmitted by that region. T% is the percent of transmission.

The Relation Between Optical Density and Absorbance

The optical density and absorbance both measure the absorption of light when that light passes through an optical medium however they both are not the same.

Optical density measures the ability of an object to slow or delay the transmission of light. It measures the speed of light via a substance which is affected mainly by the wavelength of the given light wave.

Absorbance, in a wave motion, is the transfer of the energy of a wave to matter as the wave passes via it. If there is only a fragmental absorption of energy, the medium is said to be lucid to that particular radiation, but, if all the energy is lost, the medium is said to be opaque. Therefore, absorption of light occurs more in an optically dense medium.

Absorption Unit

The absorption unit is basically used in ultraviolet-visible spectroscopy where AU is a dimensionless quantity denoted by AU. 

Absorption of any material is taken as A given by,

AU = - log₁₀ (Φ t /  Φ i)  = - log₁₀ T…(1)

Where Φ t is the radiant flux transmitted by that material and, Φ i is the radiant flux received by that material. T is the transmittance.

FAQ (Frequently Asked Questions)

Q1: If the incident light intensity is 2000 units and the transmitted intensity is 1/200 of this, then calculate the density.

Ans: Optical density (O.D.) = log₁₀ Io/ It

= log₁₀ 2000/500 = log₁₀ 4

= log₁₀  ((10)2) …(1)

Applying the formula (log₁₀ (10) = 1) 

= 2 * log₁₀ (10) = 2*1

O.D. = 2 (Here, the optical density is equal to 2)

Optimal optical density (OD) values for reliable measurements must be lower than 2.

Q2: What does Higher Optical Density Mean?

Ans: The optical density of any material is directly related to the refractive index of that material. The more is the optical density, more will be the refractive index which means the speed of light also varies with the optical density. Hence the optical density is directly related to the refractive index of that material.

Q3: How do you Calculate Absorption?

Ans: The most common method to measure the absorption is to direct a generated beam at the body and detect the intensity of the radiation that passes via it. The basic example of absorption can be a blackbody that absorbs all the light on it and when heated they transmit the lights of varying wavelengths in the form of energy. The transmitted energy can be utilized to calculate the absorption.

Q4: What Does the Absorbance of 1 Mean?

Ans: Absorbance can range from 0 to infinity where the absorbance of 0 means the medium does not absorb any light, an absorbance of 1 means the medium or a material absorbs 90% of the light and absorbance of 2 means the material absorbs 99 % of the light.