optical phenomena and properties of materials 1
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The Continuous Emission
Spectrum
This spectrum is obtained when purewhite light is passed through a
triangular glass prism.
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OPTICAL PHENOMENA AND
PROPERTIES OFMATERIALS
CONTINUOUS EMISSION SPECTRUM.
LINE EM
ISSIO
N SP
ECTR
UM
LINE ABSORPTION SPECTRUM
There are 3 different types of spectra
upon which we need to focus.
They are:
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When does a continuous emission spectrum
form?
When an object is heated the particles begin
to vibrate. The vibrating protons and
electrons in the atoms emit electromagnetic
radiation.
Initially red light with the lowest frequency in
the visible part of the spectrum is emitted.
The object appears to be red hot.
As the object gets hotter the particles vibrate
faster and emit light of higher frequencies.
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When the object is white hot, the atoms
are vibrating at all possible frequenciesand emit light of all the frequencies in the
visible spectrum.
The object appears to be white hot.
If this light is passed through a prism, a fullcontinuous emission spectrum is
observed.
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THE LINE EMISSION SPECTRUM
Only certain frequencies are present in a
line emission spectrum, compared to all
the frequencies in a continuous spectrum.When does a line emission spectrum
form?
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When a single element is heated until it emits light,
the light emitted forms a line emission spectrum
when passed through a prism, instead of acontinuous spectrum.
The line emission spectrum can be better observed
by passing the emitted light through a diffraction
grating
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Each colour in the line emission spectrumis the result of the emission of an
electromagnetic wave of a particularfrequency.
Each line represents photons of a differentenergy.
The line spectrum of an element ischaracteristic of that element and can beused to identify the element.
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The line emission spectrum of hydrogen.
Each line is a colour made up of waves of a different
frequency. Each colour has photons of a particular
energy, calculated according to Plancks equation.
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Flame tests
The flame test is a procedure used inchemistry to detect the presence of certain
metal ions, based on each elements
characteristic emission spectrum.
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Neon lights
Inside the glass tube there is a gas like neon,argon or krypton at low pressure. At both ends ofthe tube there are metal electrodes.When you
apply a high voltage to the electrodes, the neongas ionizes, and electrons flow through the gas.These electrons excite the neon atoms andcause them to emit light that we can see. Neonemits red light when energized in this way. Othergases emit other colors.
The emitted light forms a line emission spectrum.
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Fluorescent light
A fluorescent lamp is a gas dischargelamp that uses electricity to excite mercuryvapour in argon or neon gas, resulting in a
plasma that produces short-waveultraviolet light.
This light then causes a phosphor to
fluoresce, producing visible light.This results in a continuous emissionspectrum.
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Line absorption spectrum
A line absorption spectrum is formed when
white light is passed through a cold gas
before being shone through a prism or adiffraction grating.
The black lines represent wavelengths of
light that have been absorbed by the gas.
The frequency of the lines in the emission
spectrum of an element equals those of the
missing lines in the absorption spectrum.
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A summary:
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