Download Measurement of the Wavelength by Diffraction Gratings

11TH CLASSES PHYSICS
SUBJECT: Single Slit Diffraction
AIM: To understand the diffraction of light waves
DURATION: 120 min.
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parallel light beams emerging from the grating and they
are brought to a focus at point P (Figx). A clear image of
the illuminated slit normal to the grating is formed on
the screen (Fig11a, b). The kind of the interference
depends on the path difference of two parallel rays
Diffraction: The spreading of light into a region
emerging from two adjacent slits of the grating. If
behind an obstruction in path of the light is called
the optical path difference between the two rays is
diffraction. A slit opening, a fine wire, a sharp edged
integer
object or a pinhole may serve as a suitable obstruction
interference takes place, and
in the path of a beam of light from a point source.
formed. If the path difference is odd multiplies of half of
Diffraction Gratings
the wavelength destructive interference takes place, and
multiples
of
the
wavelength
constructive
a bright fringe is
We have seen that the wavelength of light could
a dark fringe is formed. The wavelength of the light
be measured by making use of Young’s double slit
used can be calculated as it is calculated in Young’s
experiment. However since the fringes formed in this
double slit experiment
experiment are wide and their intensities are

weak, in this method the wavelength cannot be
measured accurately. By increasing the number of
slits to a very large number narrow fringes (lines)
of higher intensities can be produced (Fig11).
Moreover the fringes are formed far apart from one
another. For this purpose Fraunhofer made the first
grating. An ideal diffraction grating consists of a large
number of fine equidistant parallel grooves ruled on a
plate of glass. The region between two grooves is
transparent and allows the light to pass through,
while the grooves themselves do not allow the
light to pass through. The grooves are usually ruled
on the glass plate by a sharp diamond point. The regions
between any two grooves behave like equally spaced
parallel slits.
Standard
gratings
may
have
1000-10000
grooves or slits per centimeter. The distance between
the centers of two adjacent slits of a grating is
called the grating constant. The grating constant of a
grating containing 6000 lines per centimeter is equal to
1
d
cm/lines .
6000
Measurement of the Wavelength by Diffraction Gratings
A grating of width 2-5cm (Fig10) is placed on
d.sinθ
the order of the diffraction, is the angle of deviation
(diffraction angle) in Nth order, and d is the grating
constant.
When the angle  and the order N are known
the wavelength, can be easily calculated since the
grating constant, d is known for all diffraction
gratings.
When
a
light
and is incident perpendicularly on the grating. The light
propagates through the grating and the wavelets from
spectroscope
can
observe
this
coincide. The fringes of the shorter wavelength will be
closer to the central bright fringe.
When white light is used the central bright fringe
will be white, and a continuous spectrum of the rainbow
colors is formed on both sides of the central bright
fringe. However this spectrum is different from that
in Young’s double slit experiment where the image
is wide and the colors are mixed. As a matter of fact
a diffraction grating analysis the light to its component
colors very well (better than a prism).
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interference
pattern. The lens of the spectroscope collects the
TEACHER:
different
fringe where the central fringes of the two waves
pattern of diffraction fringes is formed. The telescope
the
two
each wavelength in both sides of the central bright
different slits interfere. As a result an interference
of
including
wavelengths is used, a set of fringes is formed for
spectroscope (see Ch9 Fig11). The collimator of the
wavelength, . Parallel light exists from the collimator
Eq8.9
Where  is the wavelength of the light used, N is
the spectroscope table instead of the prism of the
spectroscope is illuminated with monochromatic light of
N
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