PHYSICS ASSIGNMENT ON

DIFFRACTION GRATING

DONE BY: ARUNDHATHI RAJU

XII C
 ACKNOWLEDGMENT
This project is an outcome of ideas, comments and suggestions
from a number of people. We would like to extend our sincere
thanks and gratitude to the Pricipal, Mr. Srinivas K. Naidu, Vice-
Principal, Mr. Deep Wilson and the school authorities for enabling
us to undertake this project and for providing excellent facilities
that greatly contributed to the success of this project. We would
like to place on record the invaluable advice of Mr. Vetriselvan, our
physics teacher and the head of the department, and Mrs. Pratibha,
our lab teacher whose encouragement, guidance and support from
the initial to the final level enabled us to develop an understanding
of the subject. I also wish to thank my team mates for gathering
most useful information and thereby help me complete the project
successfully.
S.No Topic
1. What is Diffraction Grating?
2. Characteristics
3. Applications
4. Aim
5. Apparatus
6. Principle
7. Preliminary Observation
8. Observation
9. Result
10. Precautions
11. Sources of Error
12. Bibliography
 What is diffraction grating?

In optics, a diffraction grating is an optical component with a periodic

structure, which splits and diffracts light into several beams travelling in
different directions. The emerging coloration is a form of structural
coloration. The directions of these beams depend on the spacing of the
grating and the wavelength of the light so that the grating acts as
the dispersive element. Because of this, gratings are commonly used
in monochromators and spectrometers.
For practical applications, gratings generally have ridges or rulings on
their surface rather than dark lines. Such gratings can be either
transmissive or reflective. Gratings which modulate the phase rather
than the amplitude of the incident light are also produced, frequently
using holography.
The principles of diffraction gratings were discovered by James Gregory,
about a year after Newton's prism experiments, initially with items such
as bird feathers. The first man-made diffraction grating was made
around 1785 by Philadelphia inventor David Rittenhouse, who strung
hairs between two finely threaded screws.
Diffraction can create "rainbow" colors when illuminated by a
wide spectrum (e.g., continuous) light source. The
sparkling effects from the closely spaced narrow
tracks on optical storage
disks such as CD's or DVDs
and the diffraction of light
by a series of thousands of
scored lines in a glass plate
into a rainbow of
colours are an example. A grating has parallel lines, while a CD has a
spiral of finely-spaced data tracks.

EXAMPLE

The tracks of a compact disc act as a diffraction

grating, producing a separation of the colors of
white light. The nominal track separation on a CD
is 1.6 micrometers, corresponding to about 625
tracks per millimeter. This is in the range of
ordinary laboratory diffraction gratings. For red
light of wavelength 600 nm, this would give a first
order diffraction maximum at about 22° .
Characteristics

Diffraction is an optical device that consists of

a large number of parallel, equidistant, and
identically shaped lines marked on a flat or
concave optical surface. A diffraction grating
is a periodic structure: the lines, whose shape
is definite and constant for a given grating,
repeat over a strictly identical interval d,
known as the period of the grating. The main
property of a diffraction grating is the ability
to resolve an incident beam of light by
wavelengths (ie. into a spectrum); this
property is used in spectral apparatus. A plane
diffraction grating has lines marked on a plane
surface; a concave grating has lines marked on
a concave, usually spherical surface.
Diffraction grating may also be classified as
reflective and transmitting. The lines of
reflective gratings are marked on a mirror
surface (usually metal), and observations are
made in reflected light. The lines of
transmission gratings are marked on the
surface of a transparent plate (usually made of
glass), or they may be narrow slits in an
opaque screen; observations are made in the
transmitted light. Reflective diffraction
gratings are usually used in modern spectral
instruments.

HISTORY

Born in the eighteenth century (Rittenhouse,

1786), the diffraction grating has been one of
the most valuable instruments in the history
of science and technology. At one extreme, it
has enabled the study of celestial bodies. On
the other hand, it has been a crucial tool in the
study of atomic and molecular structures.
These outstanding capabilities are the
consequence of one basic property: it can
disperse light, i.e. separate the frequencies
contained in light radiation, thus allowing the
measurement of relative intensities.
APPLICATIONS
Diffraction gratings are used not only in spectral instru
ments but also as optical sensors of linear and angular d
isplacement (measuring diffraction gratings),
as polarizers and filters of infrared radiation,
and as beam splitters in interferometers. Reflective
Gratings are wavelength-selective filters. Other
examples of filters are Fiber Bragg grating, Fabry-
Perot, Mach-Zehnder, etc. In optical communications,
they are used for 1. Wavelength Selection: Splitting
and/or combining optical signals 2. Pulse Compression:
Normally as reflectors in external cavity DBR lasers. It
has many other applications in science and technology
such as wavelength selectors for tuneable lasers,
selective surfaces for solar energy, masks
photolithography, beam sampling mirrors for high
power lasers, spectrometers in extreme UV and X-ray
regions for space optics, metrology, phase measuring
interferometry and pattern recognition, etc.
PRINCIPLE
Diagram of the formation of spectra of
a transmission diffraction grating
consisting of slits upon illiumination by
monochromatic light (M1) and light of
complex spectral composition (M2)
The principle of operation of a diffraction g
rating is most clearly shown by a transmiss
ion grating, when a monochromatic, parallel beam oflig
ht of wavelength λ is incident on the diffraction grating
at an angle α. The diffraction grating consists of slits of
width ‘b’ separated byopaque intervals; interference of
the light emanating from the individual slits occurs. As
a result, after focusing on a screen, the location ofthe
maximums (Figure 1) may be determined by the equati
on d (sin α + sin β)= mλ,
where β is the angle between the direction normal to t
hegrating and the direction of propagation of the beam
(the diffraction angle); the integer m = 0, ±1, ±2, ±3, … i
s equal to the number ofwavelengths by which a wave
from some element of a given slit of the diffraction gra
ting lags behind or leads the wave emanating from thes
ame element of an adjacent slit. Monochromatic beam
s corresponding to the different values of m are called
spectral orders, and theimages of the entrance slits pro
jected by the beams are called spectral lines. All orders
that correspond to the positive and negative valuesof
m are located sym metrically with respect
to the zero order. The spectral lines beco
me narrower and sharper as the number of slits isincre
ased. If the radiation incident on a diffraction grating h
as a complex spectral composition, each wavelength wi
ll have its own set ofspectral lines, and consequently, t
he radiation will be resolved into spectra according to t
he number of possible values for m. The relativeintensi
ty of the lines is determined by the energy distribution
function for a particular slit.
The two main characteristics of a diffraction grating are
its angular dispersion and its resolving power. Angular
dispersion, which determinesthe angular width of the s
pectrum, depends on the difference ratio of diffraction
angles for two wavelengths:

Thus, the angular width of the spectrum varies approxi

mately proportional to the order number of the spectr
um. The resolving power R isdefined by the ratio of the
wavelength to the smallest wavelength interval that sti
ll can by separated by the grating:

where N is the number of slits in the diffraction grating

and W is the width of the hatched surface. The resolvin
g power for given angles can
be increased only by increasing the width of the grating
.
AIM: To determine the grating
element of a diffraction grating
using laser source of known
wavelength.

Apparatus: laser source of known

wavelength , a diffraction grating
screen,
 Procedure:
The simulation virtualizes the Mercury spectrum experiment. The user can use a
grating spectrometer to measure the wavelengths of Yellow, Green, Violet and
Red lines in the visible spectrum of Mercury.
1. Telescope Calibrate Slider : This slider helps the user to change the focus of
telescope.
2. Start Button : Helps the user to start the experiment after setting the focus of
telescope. The Start Button can be activated only if the focus of the telescope is
proper.
3. Light Toggle Button : Helps the user to switch the lamp ON or OFF.
4. Grating Toggle Button : Helps the user to place or remove the grating.
5. Telescope Angle Slider : This slider helps the user to change the angle of
telescope.
6. Vernier Angle Slider : This slider helps the user to change the angle of the
Vernier.
7. Telescope Angle Slider : Helps make minute changes of the telescope angle.
8. Calibrating Telescope Button : Helps the user to calibrate the telescope after
starting the experiment if needed.

To standardise the grating:

 Turn the telescope to obtain the image of the slit.

 Turn the telescope to both sides to obtain green lines.Note the reading of both
the verniers.

 Calculate the difference in the reading to obtain the diffraction angle. Then from
the equation, number of lines per unit length of the grating can be calculated .

To calculate the wavelegth of different lines

 Obtain the direct image.
 Telescope is moved to make the cross-wire coincide with each line of the
spectrum.
 Note the readings on the verniers and calculate the diffraction angle.
 Then calculate the wavelength of each colour.
Observation
The telescope is then released and is brought to observe the direct image. On
the either side of the direct image, the diffraction spectra are seen.The telescope
is turned slowly towards the left so that the vertical cross wire coincides with
the violet lines of the first order. The readings of the vernier are taken.

Calculation:

For green light, λ = 546.1nm

Determination of wavelength for prominent lines

Results

The wavelegth of Yellow I = .....................nm

The wavelegth of Yellow II = .....................nm
The wavelegth of Blue-green = .....................nm
The wavelegth of Violet I = .....................nm
The wavelegth of Violet II = .....................nm
Sources of error and Precautions:
(1)Before performing the experiment, the spectrometer should be
adjusted.
(2)Grating should be set normal to the incident light.
(3)While taking observation, telescope and prism table should be kept
fixed.

BIBLIOGRAPHY
Wikipedia
Google