This document summarizes an experiment to study the emission spectroscopy of low pressure hydrogen gas. The experiment uses a diffraction grating to separate the light from a hydrogen gas lamp into distinct color lines corresponding to the Balmer series. Measurements of the positions of the red, blue-green, and violet lines are used to calculate the wavelengths and energies of the photons emitted in the spectral lines. The results confirm that the hydrogen spectrum is an atomic line spectrum as predicted by Bohr's quantum model of the hydrogen atom.

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STUDY THE EMISSION OF
SPECTROSCOPY OF LOW PRESSURE
GAS( HYDROGEN ).
INTRODUCTION :
The branch of physics deal with the study of interaction b/w matter and electromagnetic radiation is called
spectroscopy. The spectrum which is produce by one atom (hydrogen atom) is called atomic spectrum, in
atomic spectrum (color) lines are separated by dark spaces. In this experiment we have to study the
emission of spectroscopy of low pressure gas (Hydrogen).
THEORY AND BACKGROUND OF EXPERIMENT :
The spectrum which is produce by single atom (hydrogen atom) is called atomic spectrum or line
spectrum. Hydrogen gas is filled in the discharge tube at very low pressure when the current is pass
through the discharge tube then the electrons jump to the higher energy state due to the application of
electric current after some time when electron come back to its original state it emit energy in the form of
photon. Color of photon depend upon the wavelength of emitted photon.as shown in figure. Different
color have different energy.
In this experiment we use diffraction grating instead of prism. A diffraction grating is a transparent film
ruled with a number of closely spaced grooves. It is used to separate the light from the emission lamp
according to its wavelengths. If light from an incandescent lamp is directed onto the grating, a continuous
spectrum of colors is formed. The grating produces an image of the light for each color emitted; because

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all colors are emitted, these images blur together and appear as a continuous band (like a rainbow). When
a hydrogen lamp is viewed through the grating, only three images of the light will appear, each in a
different color. These correspond to the individual emission lines of the Balmer series, each with a
different wavelength. Because they are separated in space, the images appear distinct. The wavelengths
of these emission lines are determined by the diffraction equation.
Where d is the separation between the grooves on the grating, and the angle θ is determined by the
geometry of the schematic as shown above. Measurements of the distances shown will provide the angle
θ, because tan θ = b/a.
As indicated above, the emission spectra of hydrogen atoms and “hydrogen-like” (one electron) ions
consist of a set of individual “lines” of specific wavelengths. The numerical values of the wavelengths of
these lines fit a particular mathematical pattern called a series. Rydberg determined that the equation
Could reproduce the pattern observed by Balmer for hydrogen. RH is the Rydberg constant, RH =
1.09737 x 107 m-1, and m and n are positive integers. In the Balmer series, m = 2 and n > m.
In 1905, Albert Einstein proposed that light was composed of bundles of energy called photons. Each
photon of light contains an energy, Ephoton, given by the relationship:

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Where ν is the frequency in s-1 and λ is the wavelength in meters. The constants are Planck’s constant, h
= 6.6261 x 10ˉ34 J·s, and the speed of light, c = 2.9979 x 108 m/s. So, once you have determined the
wavelengths of your emission lines, you can calculate the energies, Ephoton,of the photons that are emitted.
But Einstein’s picture of the nature of light occurring in photons did not answer the question as to how the
photons of light were produced. In 1913, the Danish physicist, Niels Bohr, proposed the quantum model
for the hydrogen atom. In Bohr’s model, the electron can only exist in certain energy levels given by the
equation:
Where n is an integer known as the quantum number for that particular energy level and 2.17 x 10-18 J is
the Bohr Model unit of energy.
DISCUSSION OF APPARATUS :
 Diffraction grating
o It consist of number of slits align with the same distance. As we know when light passes
through the slit it diffract the same phenomena occur hare. When light from hydrogen
lamp pass through the diffraction grating it (split) diffract in to three different color lines,
due to having different frequency and wavelength.
 Hydrogen gas lamp
o Is use to produce the light when current pass through it.
 Spectrometer
o It consist of colorimeter, telescope, and circular scale. Is is use to determine the distance,
angle and wavelength of diffracted light.
PROCEDURE :
o Record all readings, to the proper number of significant figures and with appropriate units, in the
Data Section.
o Make sure that the lamp is centered at 100 cm on the front rail track. Check that the diffraction
grating is centered at 100 cm on the back rail track. Check that the two rail tracks are 50 cm apart
at both ends.
o Turn off the overhead room lights; light from adjacent rooms should be sufficient. Look at the
images on the diffraction grating. Three visible lines should be present. Using your line of sight,
align the red line emission with the marker on the center of the diffraction grating. Move the
needle indicator on the front rail track to be in line with the emission line. Measure the distance of
the needle indicator from the lamp.
o Repeat for the Blue-Green and Violet lines.

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CALCULATION :
o Diffraction Grating Ruling ____600_____ lines / mm
o Number of separations/mm in diffraction grating ____________
o Calculate width of one separation, d, in nm ____________
o Lamp position along front rail track ____100.0 cm__
o Positions of images along front rail track red ____________
o blue-green ____________
o violet ____________
o Distance from front rail track to grating, a ____50.0 cm__
These all calculation is done by using formulas that are given bellow :
o Use to determine the distance d.
o Use to determine the wave length of spectrum color.
o This is use to determine the energy of different colors.
o Furthermore if we want to determine the frequency of different color we use this formula
RESULT ANALYS :
When we start our experiment we say that atomic spectrum is a line spectrum and is form by using single
atom when we conduct the experiment we observed that yes the spectrum that we get from the hydrogen
gas is the (line spectrum) atomic spectrum. We clearly observed that color lines are separated by dark
space. And the further by using apparatus we determent the wavelength and energy of different color, and
we also observed from this experiment different color have different wavelength and energy. Which is our
aim of experiment.