Main Difference – Ordinary Light vs Laser Light
Both ordinary light and laser light are electromagnetic waves. Therefore, both travel with the velocity of light in vacuum. However, laser light has very important and unique properties that cannot be seen in nature. Ordinary light is divergent and incoherent whereas laser light is highly directional and coherent. Ordinary light is a mixture of electromagnetic waves having different wavelengths. Laser light, on the hand, is monochromatic. This is the main difference between ordinary light and laser light. This article focuses on the differences between ordinary light and laser light.
What is Ordinary Light
The sunlight, fluorescent bulbs and incandescent bulbs (Tungsten filament bulbs) are the most useful ordinary light sources.
According to theories, any object with a temperature greater than the absolute zero (0K) emits electromagnetic radiation. This is the basic concept used in incandescent bulbs. An incandescent bulb has a Tungsten filament. When the bulb is switched on, the applied potential difference, causes the electrons to accelerate. But these electrons collide with atomic cores within shorter distances as Tungsten has a high electrical resistance. As a result of electron- atomic core collisions, the momentum of the electrons change, transferring some of their energy to the atomic cores. So, the Tungsten filament heats up. The heated filament acts as a blackbody and emits electromagnetic waves covering a wide range of frequency. It emits microwaves, IR, visible waves, etc. Only the visible part of its spectrum is useful to us.
The sun is a super-heated blackbody. Therefore, it emits a tremendous amount of energy in the form of electromagnetic waves, covering a wide range of frequency from radio waves to gamma rays. In addition, any heated body emits radiation including light waves. The wavelength corresponding to the highest intensity of a blackbody at a given temperature is given by the Wien’s displacement law. According to the Wien’s displacement law, the wavelength corresponding to the highest intensity decreases as the temperature increases. At the room temperature, the wavelength corresponding to the highest intensity of an object falls into the IR region. However, the wavelength corresponding to the highest intensity can be adjusted by increasing the temperature of the body. But, we can’t stop the emission of electromagnetic waves having other frequencies. Therefore, such waves are not monochromatic.
Normally, all the ordinary light sources are divergent. In other words, ordinary light sources emit electromagnetic waves to all the directions randomly. There is also no relationship between the phases of emitted photons. So, they are incoherent light sources.
In general, the waves emitted by ordinary light sources are polychromatic (Waves having many wavelengths).
What is Laser Light
The term “LASER” is an acronym for Light Amplification by the Stimulated Emission of Radiation.
In general, most of the atoms in a material medium stay in their ground states as ground states are the most stable states. However, a small percentage of the atoms exist at excited or higher energy states. The percentage of the atoms exist at higher energy states depend on the temperature. Higher the temperature, higher the number of atoms exists at a given excited energy level. Excited states are very unstable. So, the lifetimes of the excited states are very short. Therefore, excited atoms de-excite to their ground states immediately releasing their excess energy as photons. These transitions are probabilistic and do not need any stimulus from the outside. No one can say when a given excited atom or molecule is going to de-excite. The phase of the photons emitted is random as the transition process is also random. Simply, the emission is spontaneous, and photons emitted when transitions occur are out of phase (incoherent).
However, some materials have higher energy states with higher lifetimes (Such energy states are referred to as metastable states.). Therefore, an atom or molecule promoted to a metastable state doesn’t return to its ground state immediately. Atoms or molecules can be pumped to their metastable states by supplying energy from the outside. Once pumped to a metastable state, they exist for a long time without returning to the ground. So, the percentage of the atoms that exist at the metastable state can be largely increased by pumping more and more atoms or molecules to the metastable state from the ground state. This situation is completely opposite to the normal situation. So, this situation is called population inversion.
However, an atom that exists in a metastable state can be stimulated to de-excite by an incident photon. During the transition, a new photon is emitted. If the incoming photon’s energy is exactly equal to the energy difference between the metastable state and the ground state, the phase, direction, energy and the frequency of the new photo will be identical to those of the incident photon. If the material medium is in the population inversion state, the new photon will stimulate another excited atom. Eventually, the process will become a chain reaction emitting a flood of identical photons. They are coherent (in phase), monochromatic (single color) and directional (travels in the same direction). This is the basic laser action.
The unique properties of laser light such as coherence, directionality, and narrow frequency range are the key advantages used in laser applications. Based on the type of lasing mediums, there are several types of lasers namely solid state lasers, gas lasers, dye lasers and semiconductor lasers.
Today, lasers are being used in many different applications while more new applications are being developed.
Difference Between Ordinary Light and Laser Light
Nature of Emission:
Ordinary light is a spontaneous emission.
Laser light is a stimulated emission.
Coherence:
Ordinary light is incoherent. (Photons emitted by an ordinary light source are out of phase.)
Laser light is coherent. (Photons emitted by a laser light source are in phase.)
Directionality:
Ordinary light is divergent.
Laser light is highly directional.
Monochromatic/Polychromatic:
Ordinary light is polychromatic. It covers a wide range of frequencies. (A mixture of waves having different frequencies).
Laser light is monochromatic. (Covers a very narrow range of frequencies.)
Applications:
Ordinary light is used in lighting a small area. (Where divergence of the light sources is very important).
Laser light is used in eye surgery, tattoo removal, metal cutting machines, CD players, in nuclear fusion reactors, laser printing, barcode readers, laser cooling, holography, fiber optic communication, etc.
Focusing:
Ordinary light cannot be focused to a sharp spot as ordinary light is divergent.
Laser light can be focused to a very sharp spot as laser light is highly directional.
