8.1 Light – Physical Properties and Sources


Light is a type of electromagnetic wave within a certain part of the electromagnetic spectrum. It arises in the electron shells of atoms during the transition of one or more electrons from a higher energy level to a lower energy level.

Light is of a double nature, as proven in the last century. It also behaves as a stream of quanta of electromagnetic energy, on top of behaving as a transverse wave.

It can be proven that light is a transverse wave by polarising it.

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Speed of light in vacuum/air is c= 2.9979 x 10^8 m/s

In any environment other than air/vacuum, the speed of light is lower. The absolute refractive index(n) of a medium specifies how many times the speed of light in the given medium is lower compared to a vacuum.

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In an optically denser medium light propagates with a lower speed than in the reference medium.


Sources of Light:

The sources of optical radiation are natural and artificial.
The only important natural source, the radiation of which covers the whole spectrum, is the Sun.
There are two important artificial sources of optical radiation:

  • Incandescent – In principle, any body with a temperature higher than 0 K is a heat source. Incandescent light is the radiation emitted by a body whose temperature is higher than 0K. The intensity of the radiation depends on the temperature.
  • Luminescent – is radiation emitted due to the excitation of atoms or molecules.

Fluorescent-vs.-Incandescent-Bulbs

At present, the only source of coherent radiation is lasers – also called quantum generators – as they use the quantum properties of atoms of the active substance.


 

The foundation of geometrical optics are three laws:

  • The law of rectilinear propagation – which states that light will propagate between any two points in a medium so that the path travelled is the direct path (this law does not apply when light travels through more media).
  • The law of reflection – which states that the angle of reflection α1 = the angle of incidence α (These angles will always be measured from the normal). The reflected ray remains in the same medium with the incident ray. 2000px-Reflection_angles.svg
  • The law of refraction (Snell’s law) – which states that the ratio of the sinuses of the incident angle (θ1) and the refraction angle(θ2) equal to the ratio of the speed of light in the two media (v1 and v2).
    Also, when light passes from an optically denser medium to an optically less dense medium the light will bend away from the normal. θ2 > θ1.
    If θ2 > 90 then no light passes into the second medium because of total reflection of light. Bear in mind that sin90 = 1. This principle is taken advantage of in light-conducting fibres (optical fibres).snellformulasnell01
Refraction
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Wave Optics studies phenomena providing evidence of the wave nature of light. These include mainly light interference, diffraction, polarisation and light dispersion (the dependence of the phase speed of light in a given medium on its frequency or wavelength).

Interference describes the combining of superimposed waves. Whether the interference is constructive or destructive depends on the path difference of the two waves(Δx =δx).

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For light, originating in the electron shells of atoms, to achieve a stable interference pattern (be coherent) the following conditions must be met :

  • Light must be polarised in a single plane
  • The wavelength(λ) or frequency(f) must be the same – monofrequency light
  • Have the same phase difference over time

Light diffraction is only experienced in the immediate vicinity of small obstacles, openings or slits, as long as the space between the slits is equal to the wavelength of light.

a14cf10a5583d19f7cfdebd63cf64382_electromagnetic-spectrum

Based on the composition of its spectrum, light can be divided into

  • monochromatic light – contains rays of light, all having the same wavelength
  • polychromatic light – contains rays of light of different wavelengths

Each source of light is characterised by an intensity. The intensity is called luminous intensity(I), [I]S.I. = candela (cd).

One candela is defined as the black-body1 radiation emitted by 1/600000 part of one metre squared of Pt at its melting point. T=2042 K, P=101.325 kPa= 1atm.

The light energy radiated by a point source over a solid angle(Ω)2 determines luminous flux(ΔΦ), [Φ]S.I. = lumen (lm).

luminous_intensityThe luminous flux incident on area A determines its illuminance (E), [E]S.I. = lux (lx).

Illuminance

The time factor of the effect of light is contained in the notion of exposure(H), [H]S.I. = lx*s.

H= E*t

Radiant energy(We) is the energy transmitted by the electromagnetic waves. Radiant flux(Φe), [Φe]S.I. = Watt (W) is the power transmitted by the radiation and it is determined by a differential ratio of the radiant energy(We) landing on a particular surface, and time t.

Φe = ΔWe/Δt

The amount of radiation passing in all directions through a place in space determines the irradiance. It is analogous to irradiation intensity (Ee), [Ee]S.I.=W/m^2, which is determined by the ratio of the radiat fluxe) and area A.

Ee = ΔΦe/ΔA


According to Planck’s quantum hypothesis, energy is not radiated or absorbed continually but rather in quanta of energy. The basic quantum of energy is ε=h*f where h is Plank’s constant, h=6.6/10^34 J.s, and f is the frequency of the wave.

The Photoelectric Effect, explained by Albert Einstein, states that when matter is exposed to radiation photons “collide” with the electrons of the object they’re being shot at. As the photons pass their energy to the electrons they vanish. If the energy gained by one electron is greater the work that needs to be done for the electron to be promoted to the next energy level, then this electron will be “ejected” away from the object with some kinetic energy.

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Photosynthesis works based on the photoelectric effect.


  1. Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. The radiation has a specific spectrum and intensity that depends only on the temperature of the body. 
  2. In geometry, a solid angle (symbol: Ω) is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is expressed in a dimensionless unit called a steradian (symbol: sr). 
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