This document describes using a diffraction grating to measure the wavelength of light. A diffraction grating consists of parallel lines that split light into different wavelengths at specific angles. The wavelength can be calculated using the grating's line spacing and the angle of the split beams. The procedure involves shining a laser through a grating and measuring the angle of the first diffraction pattern to calculate the wavelength using the diffraction grating equation. This allows verifying the laser's specified wavelength.

1. WAVELENGTH OF LIGHT USING
DIFFRACTION GRATTING .
INTRODUCTION :
Light is an electromagnetic wave, like a radio wave, but very high frequency and very short wavelength.
Different colors of light have different wavelengths. The eye can detect wavelengths ranging from about
400 nm (violet) to 700 nm (red).
The word “diffraction” refers to the spreading out of waves after passing through a small opening.
Diffraction effects are important when the size of the opening is comparable to or less than the
wavelength.
To measure wavelengths, we need a device that can split a beam of light up into different wavelengths.
Such a device is a diffraction grating. A transmission diffraction grating consists of a very large number
of equally spaced parallel lines scratched on a transparent surface. The diffraction gratings used in this
experiment are plastic replicas of a master grating, made by pressing the plastic against the master
grating, which acts as a mold. A diffraction grating behaves as if it were a series of slits in an opaque
screen.
DISCUSSION OF APPARATUS :
 Diffraction grating with lines of known separation
 Laser pointer with a known wavelength

2.  Meter stick
 Ruler
 Binder clips
 Index card
 Tape
 Protractor
THEORY AND BACKGROUND OF EXPERIMENT:
A diffraction grating is made by making many parallel scratches on the surface of a flat piece of some
transparent material. It is possible to put some large number of scratches per cm on the material. The
distribution of wavelengths of light given off by a particular source is called the spectrum of that source.
An incandescent lamp gives off a continuous spectrum containing all wavelengths in the visible part of it,
from red to violet. A laser emits light of a single wavelength. A spectral lamp that contains the excited
vapor of a particular element emits a spectrum that contains a few discrete wavelengths that are
characteristic of that atom. The object of this experiment is to measure some of these wavelengths, and
then to use observations of diffraction from a compact disc to infer spacing of information stored on the
CD.
Consider a diffraction grating consisting of a very large number of slits. When all the waves spreading out
from all the slits are added up, they cancel out everywhere except in certain directions along which all the
crests of all the waves exactly coincide and add up constructively. These particular directions are
determined by the wavelength of the light λ and the distance d, between centers of adjacent slits in the
grating, known as the grating spacing. An example of the constructive interference that arises for just two
slits.

3. PROCEDURE :
 Tape an index card to the wall so the blank side is facing you.
 Lay the meter stick on a table or the floor so the 0 meets the index card.
 Mount the later pointer at the end of the meter stick, pointing towards the index card.
 Mount the diffraction grating a few centimeters from the index card so the lines are vertical.
 Turn off the lights in the room and turn on the laser pointer.
 Use your protractor to measure the angle between the meter stick and the first order visible
band.
 Use the formula Where λ is the wavelength, in meters d is the distance in meters between
lines on the diffraction grating θ is the angle and n is the order. Unless the room is extremely
dark, you will only be able to see the first order, so n=1.
 Compare your calculated wavelength to the wavelength provided by the manufacturer of the
laser.
 Repeat for different lengths along the meter stick.
OBSERVATIONS :
The patterns that arise for more than two slits sharpen the directions of the brightest regions. For a
diffraction grating, the directions of maximum intensity can be specified by a series of angles. The angles
are measured relative to the line OP, which is parallel to the light falling on the diffraction grating
initially.

4. On either side of OP are two directions of maximum intensity (lines OA1 and OB1 ) known as the "first
order" maxima, at angle θ1 given by
Two other directions are lines OA2 and OB2 (the "second order" maximum) at angle θ2 given by
Higher order maxima may be observed at angles given by the general formula
The diffraction gratings used in this lab have 600 lines per mm. So the spacing d between lines is simply
RESULT ANALYSIS :
Diffraction gratings diffract, or split, light periodically, meaning the light splits into several beams with a
given angular separation. In this experiment, the first period, n=1, will be the brightest spot on the index
card (besides the straight path of the laser, of course) after the grating splits the rays from the laser
pointer. Using the formula above, We can verify the wavelength of light using what the manufacturer of
the laser pointer says it is. If the room is dark enough, we may even be able to measure the 2nd and 3rd
periods and plug n = 2 and n = 3 into the equation, respectively. It should yield the same result.