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Introduction to Light and Matter



Matter is a collection of charges. On average, matter is electrically neutral.



A beam of light can be approximated by a plane wave if it is monochromatic (of one color). The fields associated with a plane wave in free space are pictured below. The electric (E) and magnetic (B) fields are perpendicular to each other and to the direction of wave propagation, k. The fields are represented as vectors. The direction of the field is given by the direction of the arrow and the length of the arrow is proportional to the magnitude (strength) of the field at the base of the arrow. The figure below thus represents the electric and magnetic fields along the horizontal black line.

Light Fields

The wave propagates to the right along the k-vector. In a plane wave, the peaks of the waves are all in step (in phase) in a plane that is perpendicular to the k-vector. At a given point in space, the fields vary sinusoidally in time. Similarly, at a fixed time (as shown above, the fields vary sinusoidally in space.

Plane Waves

The figure below represents a plane wave. The three surfaces correspond to the planes on which the field strength is maximum. The distance between these planes is the wavelength.

Plane Waves

There are other types of light waves, such as twisted light, where the surface of constant phase is a corkscrew, as shown in the figure below.

Twisted light

When the light intensity gets low, the light appears to be made of quanta of energy, called photons. The photon has spin angular momentum given by Plancks constant. A twisted beam of light also has orbital angular moment given by an integer times Planck's constant.

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