Wave characterisation ofelectromagnetic radiation        COMPILED BY TANVEER AHMED   1
INTRO• All electromagnetic radiation, including light,  travels through a vacuum with a velocity• c of 3 X 10 8m s–1 (abou...
Wavelength• Treating electromagnetic radiation as a• transverse wave motion,• the different forms can be distinguished by ...
Frequency• An alternative parameter is the frequency v, defined as• the number of complete waves from a single wave train ...
The wavelength andfrequency of radiation are related to its velocity by          Eqn 1.3:         COMPILED BY TANVEER AHME...
COMPILED BY TANVEER AHMED   6
Radiation in the UV and the visible regions of the electromagneticspectrum is usuallydescribed in terms of its wavelength,...
The unit of length• The unit of length that is chosen for wavelength depends  largely on the region of         –the spectr...
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 tot...
Energy content of      radiationMany properties of light, particularly those relating to        absorption and emission of...
QUANTUM THEORY    • The alternative (quantum) view that electromagnetic      radiation exists as a    • series of energy p...
QUANTUM THEORY• The equivalent amount of energy absorbed by a  mole of chemical material• (assuming complete absorption at...
QUANTUM THEORY• or Emol = 199 409 joules per mole (about 200 kJ  mol –1).This is a large amount of energy• and can, in pri...
COMPILED BY TANVEER AHMED   15
Measurements of light     intensity    In order to quantify the amount of radiation                emitted by a source    ...
Measurement of light intensity• Our choice of unit depends on whether we  consider all the radiation emitted by the• sourc...
Measurement of light intensity• We can imagine the radiation as a   –   series of particles or photons   –    travelling b...
Measurement of light intensity• The projector is designed,   – using mirrors and lenses,   – to throw the radiation   – in...
Measurement of light intensity• For a given distance of the screen we can  measure the• irradiance or flux per unit area (...
In this discussion we have considered the total radiant emissionwhich, for a typical projector, will include both    visi...
Measurement of light intensity• The fundamental relationship between radiant and luminous quantities is  incorporated in t...
Measurement of light intensity         COMPILED BY TANVEER AHMED   23
Upcoming SlideShare
Loading in …5
×

1.2.1 weave characterization of electro megnetic radiation

589 views

Published on

1 Comment
1 Like
Statistics
Notes
No Downloads
Views
Total views
589
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
14
Comments
1
Likes
1
Embeds 0
No embeds

No notes for slide

1.2.1 weave characterization of electro megnetic radiation

  1. 1. Wave characterisation ofelectromagnetic radiation COMPILED BY TANVEER AHMED 1
  2. 2. 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: COMPILED BY TANVEER AHMED 2
  3. 3. 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. COMPILED BY TANVEER AHMED 3
  4. 4. 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): COMPILED BY TANVEER AHMED 4
  5. 5. The wavelength andfrequency of radiation are related to its velocity by Eqn 1.3: COMPILED BY TANVEER AHMED 5
  6. 6. COMPILED BY TANVEER AHMED 6
  7. 7. Radiation in the UV and the visible regions of the electromagneticspectrum is usuallydescribed in terms of its wavelength,whereas in the IR bothwavelength units andwavenumber unitsare in common use.wavenumber v – , which is thenumber of wavelengths per metre, i.e. thereciprocal of the wavelength (Eqn 1.4): COMPILED BY TANVEER AHMED 7
  8. 8. 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. COMPILED BY TANVEER AHMED 8
  9. 9. The usual rule is,where possible, to choose a unit of wavelengthwhich keeps the numbers in the range 1 to 1000. COMPILED BY TANVEER AHMED 9
  10. 10. 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, COMPILED BY TANVEER AHMED 10
  11. 11. Energy content of radiationMany properties of light, particularly those relating to absorption and emission of energy by atoms and molecules,cannot be fully explained by the wave theory of light. COMPILED BY TANVEER AHMED 11
  12. 12. 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 ofenergy in a quantum of radiation, and is typically the amount ofenergy absorbed by a single atom or molecule. COMPILED BY TANVEER AHMED 12
  13. 13. 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: COMPILED BY TANVEER AHMED 13
  14. 14. 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. COMPILED BY TANVEER AHMED 14
  15. 15. COMPILED BY TANVEER AHMED 15
  16. 16. 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 COMPILED BY TANVEER AHMED 16
  17. 17. 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. COMPILED BY TANVEER AHMED 17
  18. 18. Measurement of light intensity• 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. COMPILED BY TANVEER AHMED 18
  19. 19. Measurement of light intensity• 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. COMPILED BY TANVEER AHMED 19
  20. 20. Measurement of light intensity• 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. COMPILED BY TANVEER AHMED 20
  21. 21. In this discussion we have considered the total radiant emissionwhich, for a typical projector, will include both visible (light) and IR (heat) emissions.In terms of viewing a projector screen what is important is theoverall 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. COMPILED BY TANVEER AHMED 21
  22. 22. Measurement of light intensity• 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 COMPILED BY TANVEER AHMED 22
  23. 23. Measurement of light intensity COMPILED BY TANVEER AHMED 23

×