Optical Phenomena and Properties of Materials 1

8/4/2019 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:

Optical Phenomena and Properties of Materials 1

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