Optical Density

Optical density is the process of transmission of light or other electromagnetic radiation by matter. The process of emission and absorption depends on the radiation’s wavelength, which includes the interaction between fundamental particles like electrons, atoms, ions etc.

When a beam of light is interacted with absorbing atoms then the absorption takes place. It is depended on the sample’s thickness and the concentration of the absorbing atoms.

Optical Density Definition

It is often said to be identical with the absorbance. It is a logarithmic ratio of the falling radiation to the transmitted radiation through a material.

For a  given wavelength, the expression of optical element transmittance is expressed as:

Log10 (1/T)

Where T is transmittance.

Few things to note:

  • The higher the optical density, the lower the transmittance.
  • The ten times of this density is equal to transmission loss expressed in decibels. For example, the density of 0.3 corresponds to a transmission loss of 3dB.

Optical Density Units

It is measured in the moles per liter and the length of path denotes in centimeter.

So the unit is denoted as:

\(\large \frac{liters}{(mole\times cm)}\)

Optical Density Measurement

The measurement is done at maxima of the absorbance spectra as there is least change of absorbance with the change in the wavelength. The measurement is a common method to quantify various important parameters like concentration of cell, production of biomass and much more.

\(\large Absorbance\; A = -log \left ( \frac{I}{I_{0}} \right )\)


I is the intensity of light passes through the sample

I0 is the initial light intensity.

Optical Density

Optical Density of Spectrophotometer

The formula for spectrophotometer is:

\(\large OD = \frac{A}{L}\)


L = the thickness of the sample.

Stay tuned with BYJU’S to learn more about electromagnets, the atom, the wavelength of light and many more.

Practise This Question

A bar magnet is held perpendicular to a uniform magnetic field. If the couple acting on the magnet is to be halved by rotating it, then the angle by which it is to be rotated is