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WAVE MOTION

 

Electromagnetic Waves

 

e/m spectrum

absorption & emission spectra

line spectra

band spectra

continuous spectra

 

 

 

Electromagnetic Spectrum

 

 

electromagnetic spectrum - UV to gamma rays

 

 

electromagnetic spectrum - visible light

 

 

electromagnetic spectrum - IR to radio waves

 

 

 

γ(gamma)-rays

generation :


1. nuclear fission
2. nuclear fusion
3. radioactive decay
4. elementary particle interactions

properties :


1. very penetrating
2. produce weak ionization
3. produce weak fluorescence
4. affect photo film

detection :


1. Geiger-Muller tube
2. scintillation counter
3. solid state detectors
4. photo film

 

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X-rays

generation :


1. electron deceleration
2. electron energy level changes in atoms

properties :


1. ionizing
2. affect photo film
3. penetrating
4. produces fluorescence
5. can produce photoelectric emission

detection :
1. Geiger-Muller tube
2. scintillation counter
3. solid state detectors
4. photo film

 

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ultraviolet

generation :


electron energy level changes in atoms

properties :


1. produces ionisation, fluorescence
2. initiates chemical reactions
3. absorbed by plate glass
4. produces the photoelectric effect
5. affects photo film

detection


1. photo film
2. photoelectric cell
3. fluorescent materials

 

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visible

generation :


electron energy level changes in atoms

properties :


1. starts chemical reactions (eg photosynthesis)
2. affects photo film

detection :


1. photoelectric cell
2. photo film
3. solid state detectors (eg CCD)
4. vision
5. light dependent resistor (LDR)

 

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infrared

generation :


1. molecular vibration
2. electron energy level changes in atoms

properties :


1. transfer of heat energy to materials
2. modulation for short distance control (eg TV remotes)

detection :


1. CCD devices
2. thermopile
3. special photo film

 

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microwaves

generation :


1. from magnetrons, klystrons & masers
2. from red-shifted light from stars & galaxies

properties :


1. modulation of waves for communication
2. resonance with molecules, producing heat

detection :


1. directional aerials, parabolic dishes
2. solid state arrays

 

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radio waves

generation :


1. electrons oscillating
2. red-shifted lower wavelengths from stars etc.

properties :


waves can be modulated for communication

detection :


1. aerials, parabolic dishes
2. solid state arrays

 

 

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Absorption & Emission Spectra

 

Very simply, emission spectra are obtained by viewing light directly. The light is made entirely of emission spectra. That is spectra that are either continuous bands of colour, lines or bunched lines of colour.

 

On the other hand, absorption spectra are obtained by viewing light through an intervening, translucent substance.

 

Absorption spectra are simply emission spectra with discrete vertical black lines across them.

 

 

The explanation is to do with electron energy levels within atoms.


Emission spectra are simply discrete wavelengths emitted by atoms when excited electrons fall to lower energy levels.

 

 

explanation of emission spectra

 

 

The wavelength λ is given in terms of the energy level change E2 - E1 by the equation:

 

 

emission line equation

 

where h is Planck's constant.

 

 

Note that the energy level change is inversely proportional to the wavelength.

 

In other words, a large energy change produces a short wavelength and vice versa.

 

 

When a light beam passes through say a cloud of gas, the electrons in atoms of the gas absorb some of its energy at particular wavelengths. The electrons are then excited to higher energy levels.

 

 

absorption lines explanation

 

 

Viewing the light after it has passed through the gas reveals black absorption lines. These are evidence of the missing wavelengths of light that were absorbed by the gas atoms.

 

 

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Line Spectra

 

Line spectra are produced by low density monatomic (single atom) gases and vapours. In this scenario there are no interactions between neighbouring atoms.


Both emission and absorption spectra, as described earlier, are examples of line spectra.

 

 

emission spectrum of mercury vapour

a mercury vapour emission spectrum

 

 

The black absorption spectra are sometime called Fraunhofer lines after Joseph Von Fraunhofer, who first discovered them in the solar spectrum.

 

 

fraunhofer lines in the solar spectrum

a section of the solar spectrum

 

 

 

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Band Spectra

 

 

band spectrum of air

spectrum of air

 

 

Bands are groups of spectral lines, staggered so they are closer on one side than the other.
Unlike line spectra, bands are produced by molecules, not single atoms. Like line spectra, the material must be in the gaseous or vapour state.

 

The relative molecular mass (RMM) of the material has some bearing on how close together the lines are.


A low RMM has the effect of spreading lines out.
A high RMM tends to compress them.

 

 

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Continuous Spectra

 

Hot gases emit many wavelengths because they often contain many different kinds of atoms and these are all in different excited states.

 

Atoms also interact between each other as a result of their close proximity. So all the individual emission lines merge to appear as one continuous band of colour.

 

 

variation of intensity of sunlight with wavelength - image courtesy of Reef Keeping Fever

 

image courtesy of Reef Keeping Fever

 

 

Note the jagged nature of the curve. This is a result of both absorption in the Sun's corona (atmosphere) and by the Earth's atmosphere.

 

Interpolating the results, the most intense colour is green. So not surprisingly our eyes are more adapted to that colour.

 

The overall shape of the curve is that of a 'black body' radiator.

 

 

 

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