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Differences between laser light and ordinary light.
Coherence: Laser light is coherent (the photons in a cross section of the beam are in phase with one another); ordinary light is not (photons possess random phases relative to other photons).
Shape: Laser light is a narrow intense beam, almost an ideal plane wave. Ordinary light is emitted isotropically, in a spherically symmetric way.
Intensity: A laser’s intensity remains approximately constant as one moves away from the source. Ordinary light’s intensity falls off as $1/r^2$.
Color: Laser light is always monochromatic (one frequency). Ordinary light can be monochromatic, but isn’t required to be.
Similarities between laser light and ordinary light.
Identity, Speed: Laser light and ordinary light are electromagnetic waves, and both travel at c.
Wave-particle duality: Laser light and ordinary light behave as both a particle and a wave.
Origin: Laser light and ordinary light are emitted when electrons in excited atoms fall to lower energy levels.
