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Types of Spectra
Spectra
► Most
of you have seen light passed through a
prism and separated into a rainbow of colors.
► Today
you will use a different device to get a
separation of colors.
► This
device is known as a diffraction grating.
► The
diffraction grating is in the spectroscope
that will be used to analyze the colors in various
light sources.
► Your
lab instructor will now demonstrate the
use of a diffraction grating.
►A
picture of the spectroscope that you
will use in this lab appears below. Also
shown is a view of the interior.
Slit
Wavelength Scale
Top
Bottom
Diffraction Grating
► When
white light is viewed through a
diffraction grating, a continuous
spectrum will be produced.
► It looks like this
Continuous Spectrum
► It
is continuous because all visible
frequencies of light are present.
► Continuous spectra arise from
incandescent solids, liquids, and even
gases when at high pressure.
► Incandescent and fluorescent lights
produce continuous spectra.
A glowing, low density gas, such as a neon sign,
produces a discrete emission spectrum.
This type of spectra is seen as having individual
lines of different colors.
The colors that are emitted by an excited atom
are characteristic of that particular kind of
atom.
They represent a unique set of energy levels
(electron orbits) for that kind of atom.
These unique sets of energy levels are predicted
by quantum mechanics, not classical mechanics.
As an example of the kind of spectra that you will
see today, a discrete spectrum appears on the
next slide.
Discrete Emission Spectrum
400 nm
500 nm
600 nm
700 nm
(Nanometer wavelength scale is shown)
Continuous Emission Spectrum
400 nm
500 nm
600 nm
700 nm
Fluorescence
► Some
materials that absorb ultraviolet
light will emit visible light.
► To
understand this process consider the
Bohr model of the atom.
Here comes a nucleus
With possible orbits for electrons
e-
In this configuration the atom is not excited.
Here comes a UV photon
e-
e-
Now the atom is said to be excited because
the electron is in a higher than normal orbit.
e-
The electron can jump back to its original orbit,
and in doing so it will emit a photon of the same
wavelength as the original excitation UV photon.
e-
For some materials the electron might jump to an
intermediate orbit first. This would result in the
emission a photon of lesser energy than the original
UV photon and it would be visible.
e-
e-
The atom is still excited and the electron would move
to a lower orbit giving off another visible photon.
e-
Now the atom is in a non-excited state again.
Fluorescence
► In
conclusion, this simple model shows
how UV radiation can stimulate some
materials to emit visible light.
Fluorescent Lamps
► Primary
excitation - electron collisions with
low pressure Hg vapor, UV given off
► Secondary
excitation - UV photons absorbed
by phosphors. Phosphors fluoresce emitting
visible light.
► Remember
that the visible light from the
excited mercury vapor is also emitted.
► You
will now receive further explanation
on the use of the spectroscope and the
procedures of today’s experiment.