The previous video discussed
the different photosensitive detectors used in
analytical instruments.
This video covers the optical elements used to isolate the wavelengths of radiation and
obtain monochromatic light.
These optical elements are called filters or monochromators.
The first element is an optical filter.
The optical filters are further classified
as absorption filters and interference filters.
The absorption filters absorb light of certain wavelengths
and only allows radiation of a
particular wavelength to pass.
They are made of materials like coloured glass,
coloured gelatin papers or coloured solutions.
The light allowed to pass depends on material colour.
Red will allow radiations in red region to pass.
The selectivity in such filters is very low,
and the emissions exiting these filters are
not highly monochromatic.
One can increase the selectivity of these filters
by stacking them one after the another.
However, this method can affect the intensity of radiation and reduce it.
The next type of filter is interference filter.
For this, we need to understand the concept of interference.
Consider two sine waves.
If the crest of one wave matches with the crest of another wave, the addition of these
two waves creates a wave having the maximum amplitude
This phenomenon is called constructive interference.
Similarly, if crest of one wave aligns with the trough of another wave, the waves cancel
out each other, and the final wave has zero intensity.
This is called destructive interference.
Using this phenomenon of interference, we can obtain light of the desired wavelength.
The wavelength required undergoes constructive interference while the radiations of other
wavelengths are cancelled out by destructive interference.
The filter using this property of interference is called as an interference filter.
It is constructed using two semitransparent layers of silver deposited on a transparent
glass plate.
A transparent dielectric film separates these layers.
This spacer film has a low refractive index, and the thickness of spacer film
determines the wavelength of light transmitted.
The light transmitted by the first layer of silver
reaches the second silver screen where
it gets transmitted, and some part gets reflected towards the first layer.
When the reflected wave strikes the first silver screen, it undergoes reflection,
and the wave moves towards the second screen.
On hitting the second silver layer,
some part gets transmitted.
If this transmitted wave and the earlier transmitted wave are in phase and their troughs match,
a constructive interference takes place.
Radiations of other wavelengths undergo destructive interference,
and those waves get diminished.
The formula connecting the wavelength that undergoes constructive interference is
m(lambda)= 2dSin(theta)
Here the incident radiations are perpendicular to surface.
So sin 90 = 1,s.
d is the thickness of the spacer film or distance between two silver layer
By adjusting the thickness, one can select the required wavelength.
Suppose, one needs to design a filter that allows the radiation of 400 nm to pass.
If the order of interference, 'm' is considered 1, then the thickness, 'd' of the spacer film
should be 200 nm.
The isolation in interference filters is better compared to absorption filters.
The next optical element that can be employed to isolate the desired wavelength is the
prism monochromator.
This device has a prism and set of mirrors which focus and collimate light from input
slit to prism and from the prism to output slit.
The property of prism to disperse light according to its wavelength is used in the prism monochromator.
When light falls on the prism,
it gets separated into VIBGYOR components.
Different wavelengths of radiations get refracted at different angles when they change their
travel medium.
Looking at the prism monochromator, we can understand its working.
Light from the source enters the monochromator through the input slit.
This light is focussed on a collimating mirror.
which renders the light beam parallel
This light is composed of different wavelengths.
When the radiation falls on the prism surface
different wavelengths present in the radiation
bend at different angles and get separated.
These separated radiations then reflect and are focused on the output slit.
Since the wavelengths are separated, they travel in different paths towards the slit
and are focussed at different locations.
By moving the output slit,
one can select the desired wavelength of light
required for the application.
The diffraction grating is also used to separate the wavelengths of light.
The diffraction grating is constructed by etching closely spaced parallel grooves on
a reflecting surface.
When light falls on these grooved surface, it acts as narrow mirror and light diffracts.
The radiation overlaps radiation from other grooves and interference occurs.
Depending upon the separation of grooves as seen by light
a certain wavelength of light
can be in phase or out of phase.
If the crests match, constructive interference occurs,
and light of that particular wavelength
is obtained at the output of grating.
Similarly, if crest of wave matches with its trough,
destructive interference occurs
and light of that particular wavelength gets diminished.
The wavelength that will be obtained is given by the mathematical expression
m(lambda)=2dsin(theta).
For a fixed set of grooves with a separation distance of d
one can get the wavelength
by rotating the grooved surface.
i.e.) by changing the angle theta perceived by the light.
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