1.2.1 weave characterization of electro megnetic radiation

COMPILED BY TANVEER AHMED 1

Wave characterisation of electromagnetic radiation


INTRO• All electromagnetic radiation, including

light, travels through a vacuum with a velocity

• c of 3 X 10 8m s–1 (about 186 000 miles per second).

• This value is constant for all types of radiation, and whatever its intensity.

• When light has to travel through a medium of refractive index n, however, its speed is given by Eqn 1.1:


Wavelength• Treating electromagnetic radiation as a• transverse wave motion, • the different forms can be distinguished by

their wavelength λ, defined in Figure 1.2 by the distance AB.


Frequency• An alternative parameter is the frequency v, defined

as • the number of complete waves from a single wave

train passing a given point in space in one second • (the unit of frequency is known as the hertz and has

dimensions of s–1). • Frequency is also measured as• the reciprocal of the time taken for one wave to pass

the given point in space (the time period T of the radiation, Figure 1.3) (Eqn 1.2):


The wavelength and frequency of radiation

are related to its velocity by Eqn 1.3:



Radiation in the UV and the visible regions of the electromagnetic spectrum is usuallydescribed in terms of its wavelength,

whereas in the IR both wavelength units andwavenumber units are in common use.

wavenumber v – , which is thenumber of wavelengths per metre, i.e. the reciprocal of the wavelength (Eqn 1.4):


The unit of length• The unit of length that is chosen for wavelength

depends largely on the region of–the spectrum

• that is being studied. • Thus yellow-green light near the centre of the visible

spectrum is said to have a – wavelength of 550 nanometres – (1 nm = 1 x 10 –9 m).

• This is preferable to describing the wavelength as • 0.000, 000,550 m, which is cumbersome.• Similarly we might describe radiation in the mid-IR

range as having a wavelength of 10 mm (or a wavenumber of 1000 cm–1) rather than a wavelength of 10 000 nm or even 0.000, 01 m.


The usual rule is, where possible, to choose a unit of

wavelengthwhich keeps the numbers in the

range 1 to 1000.


The visible region• The visible region of the electromagnetic spectrum makes up a

very small part of– the total spectrum

• Averaged visual assessments• have placed a• with no true red appearing in the spectrum (long-wave red has

a yellowish cast).

pure (or unitary) blue hue at 436 nm, pure green at 517 nm, pure yellow at 577 nm,


Energy content of radiation

Many properties of light, particularly those relating to

absorption and emission of energyby atoms and molecules,

cannot be fully explained by the wave theory of light.


QUANTUM THEORY• The alternative (quantum) view that

electromagnetic radiation exists as a• series of energy packets called photons • is accepted as the best model for understanding the

energetics of the processes of light absorption and emission.

The photon energy depends• directly on the frequency of the radiation involved,

as given by Planck’s relationship (Eqn 1.5):

The energy given by Eqn 1.5 represents the small amount of energy in a quantum of radiation, and is typically the amount of energy absorbed by a single atom or molecule.


QUANTUM THEORY• The equivalent amount of energy absorbed

by a mole of chemical material• (assuming complete absorption at the

frequency or wavelength concerned)

• is obtained by multiplying the value of E in Eq 1.5 by Avogadro’s number (N = 6.023 X10 23).

• Suppose, for example, that a blue dye molecule absorbs strongly at 600 nm in the orange/red region.

• This corresponds to energy absorption Emol given by Eqn 1.6:


QUANTUM THEORY• or Emol = 199 409 joules per mole (about

200 kJ mol –1).

This is a large amount of energy• and can, in principle, lead to breakdown of

chemical bonds• and destruction of the dye molecule.


UV radiation at 300 nm wavelength has twice the energy, and its absorption is even more likely to lead to bond scission and hence colorant

destruction.

The intrinsic highly energetic character of low-wavelength radiation, which can leadto damage to the biological molecules in living cells,

is the reason why human beings must be screened from UV and other high-energy radiation.


Measurements of light intensity

In order to quantify the amount of radiation emitted by a source

and travelling through space to fall on a surface,

we need to specifiy the radiation amount in different units.


Measurement of light intensity

• Our choice of unit depends on whether we consider all the radiation emitted by the

• source, only that emitted– in a certain direction– (or over a certain solid angle),– or that falling on a given area of the

surface• The familiar slide projector and screen set-

up is a good illustration of the quantities• that need to be considered. • We would first of all be interested in the

– total power emitted by the projector, which is measured in watts.



• We can imagine the radiation as a– series of particles or photons– travelling between– the light source and the screen – And hitting the screen at a certain rate,

• that is, with a definite number of collisions per

• second (in wave terminology, we are considering the amplitude of the wave here).

• The total radiant flux hitting the screen will be measured in joules per second or in watts.



• The projector is designed,– using mirrors and lenses, – to throw the radiation– in a certain direction

• so that what is important is that the radiant intensity or the number of photons per unit solid angle per second (i.e. the flux per unit solid angle) is reasonably

• constant over the illuminated screen area.

• If, however, the screen is moved closer to the projector

• the radiance or intensity per unit area increases, • whilst if it is moved further away the radiance

decreases, even although the radiant intensity is constant by definition.



• For a given distance of the screen we can measure the

• irradiance or flux per unit area (in watts per square metre, for example),

• and if we allow the radiation to fall• on the screen for a known time

– we can then compute the total radiant exposure in

joules per square metre.


In this discussion we have considered the total radiant emission which, for a typical projector, will include both

visible (light) and IR (heat) emissions.

In terms of viewing a projector screen what is important is the overall effect on the human eye, whichmeans we should concern ourselves

with only the luminous radiation and define appropriate quantities in terms of the

visual effect of the radiation. Such terms and the relevant units are indicated in Table 1.1.



• The fundamental relationship between radiant and luminous quantities is incorporated in the definition of

• the lumen, which is defined as the luminous • flux of a beam of yellow-green monochromatic radiation• whose frequency is 540 x 1012 Hz• (equivalent to a wavelength of about 555 nm) • and whose radiant flux is 1/683 W.

• The relationship is of course wavelength-dependent, since radiation outside the visible spectrum has – zero luminous contribution.

• The variation with wavelength is defined by the so-called Vλcurve,

• which has a maximum at 555 nm and decreases to zero at the ends

• of the visible spectrum. • The use of Vλ values and their variation with wavelength are

discussed in section 1.6



1.2.1 weave characterization of electro megnetic radiation

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