Properties of laser light

We know that laser generates light that has all the characteristics of ordinary light, However there are three important distinct characteristics of laser light that make it different from other ordinary light. These characteristics are

  • Monochromaticity
  • Divergence
  • Coherence
  1. Monochromaticity

    The laser light is monochromatic means colored. Thus laser light is monochromatic light means that laser light is only a single color light.

    The monochromaticity is the major characteristic of laser making it different from ordinary light. To prove the monocromaticity of laser light, we can use prism. The glass separate wave lengths by bending each wavelength at different angle. When we pass the standard white color (which consist of all colors) through prism then it break white color in to component colors with gradual bending from short wavelength to long wavelength, as shown in Figure 1.

    Light Through Prism
    Figure 1: Light Through Prism

    Now on other hand if we pass beam of laser light through the prism then beam is only changed in direction but it is not separated into different colors, as show in Figure 2.

    Laser Through Prism
    Figure 2: Laser Through Prism​​​​​​


  2. Divergence

    Another important characteristic of laser is that laser light does not spread out or diverge, but it stays together in a beam.

    We know that when light travels then it tends to spread out this spreading of light is called divergence and the angle at which the light spreads is called angle of divergence, denoted by phi (ɸ).

    Divergence is more in ordinary light as compared to ordinary light. Therefore the angle of divergence will be greater in ordinary light case while will be lesser in laser light case.

    Usually a laser generates a beam of less than one mille radian (0.001 rad). This means that a beam from the laser will spread to less than one foot diameter circle at a distance of 1000 feet from the laser. When a laser beam is sent through special optical devices then its divergence can be reduced even further. Due to divergence characteristic the major application of laser light is measuring the long distances such as distance between earth and moon can be measured by using laser light.

    If we sent a laser beam through telescope from earth to moon then when it reaches to moon, it covers approximately 2.5 miles diameter circle. Now what will be the divergence angle?

    Since we know that distance to the moon is approximately 250000 miles therefore

    Divergence angle = ɸ = 1.25 miles / 250000 miles = Radius of beam / distance covered.

    = 0.000005 radians

    = 5 rad

  3. Coherence

    The 3rd important characteristic of laser light that makes it unique is the coherence. Coherence means that the light waves are in phase. Laser light is much more coherent than ordinary light. It is impossible to detect this property with our eyes and therefore its important is some time over lacked.

    The incoherent waves have no relationship to each other. They don’t start at the same point in time and space, nor do they have same wavelength, as shown in Figure 3.

    Incoherent Light Waves
    Figure 3: Incoherent Light Waves

    On the other hand, coherent waves are locked together both in space and time,

    Coherent Light Waves
    Figure 4: Coherent Light Waves​​​​​​


Two points keep in mind for coherence are

  1. If two waves are not perfectly monochromatic means not of identical wavelengths, then they will not remain coherent, as illustrated.Not Coherent Light Waves
  2. If the wave is moving w.r.t each other even though they have identical wave lengths. They cannot remain coherent, as illustrated.Not Coherent Light Waves

Note: - The laser as good as it is cannot perfectly coherent, monochromatic and have no divergence.