The speed of transmitted light depends upon the properties of the medium. So, the speed of the transmitting electromagnetic wave depends upon the optical density of that particular material through which it passes. The optical density is completely different form the physical density which is equal to mass by volume ratio while the optical density is related to the sluggish tendency of the absorbing atoms of the material. The sluggish tendency shows the maintenance of absorbed energy of an electromagnetic radiation in the form of vibrating electrons before the remittance of. the new electromagnetic disturbance. The high value of optical density of the material lowers the movement of wave through the material The vibrating electric charge is used to produce an electromagnetic wave or a light wave. The speed of wave in the vacuum of empty space is 3 x 108 m/s. When the wave passes through a particle of matter then the absorbed energy set electrons within the atoms into vibrational motion. If the resonant frequency of vibration of electron and electromagnetic wave frequency are not correlated, then the energy is remitted. The remitting of energy is in the form of an electromagnetic wave which has the same frequency to the original wave and it travels with the speed of wave in vacuum All the photons are behaved like a bundle of electromagnetic energy which travels in the void with the speed of wave in vacuum. Thus, the net speed of an electromagnetic wave in other medium than vacuum is less than the speed in a vacuum. The optical density of a material is indicated by the index of refraction value of the material. The Index of refraction values refers to the numerical index values. The index of refraction value of a material is indicated by the number of times slower that a light wave would be in that material than in a vacuum. The value of index of refraction is different for the different medium. The speed of transmitted light is slower in the most optically dense materials than the less dense materials. Thus, with the increase in the index of refraction value, the optical density increases which decreases the speed of light in that material
Spectroscopy is the branch of Physics which involves the study of the light absorption and emission by matter. Both the process of emission and absorption depends on the radiation’s wavelength. This also includes the interaction between fundamental particles like electrons, atoms, ions etc. and collision energy which is based on quantum mechanics and electrodynamics and relativity theory. Here, we are discussing about the transmission process of light or other electromagnetic radiations by matter. This is called optical density or transmission density. It is an absorbance which is used to measure absorbance in analytical chemistry. The absorption takes place when a beam of light is got interacted with absorbing atoms. The absorbance depends on sample’s thickness and the concentration of the absorbing atoms of the sample. Here we are discussing about optical density, it is calculation formula, and some examples based on it .