Experiment if planck's constant

Experiment if Planck's constant by using Wien’s radiation method.

Trisha Banerjee @ 2010

Diffraction grating

Spectrometer without telescope

Bifocal lens

LDR

Scale for reading

Dc power supply for LDR and tungsten

collimator

Whole experimental setup:-


Apparatus:- 12 V -35W Tungsten filament bulb , mercury vapor lamp , spectrometer (without telescope) , diffraction grating , 50 to 100 FL biconvex lens , LDR with appropriate housing and mount, two optical benches with stands, variable 12 V and 30 V dc power supplies for bulb and the LDR respectively, 100/500 micro amp range current meters, voltmeter of 50 V range.

procedure in brief:-

A tungsten lamp ( in its housing fitted with a convex lens) and a mercury vapor lamp are mounted over a small optical bench . In the first part of the expt , light from the tungsten lamp is made first parallel by a collimator and then it is allowed to be incident at a diffraction grating . A particular color of one of the first order spectrum formed by the grating is then focused at the LDR by a convex lens both mounted on a long optical bench. The resistance of the tungsten lamp can be determined by the readings of the current meter ( embedded in the 0-5 V power supply ) and a DVM connected with it .the current produced in LDR is to be read by the 500/100 micro amp current meter connected in series with it and another 0-5 V power supply . A 50 V range another voltmeter is permanently connected with the supply to show its voltage.Trisha Banerjee @ 2010

To determine the wavelength of the radiation s incident at the LDR , the mercury vapour lamp should be mounted just in front of the slit and its spectrum should be observed at a scale (mounted just behind the LDR). Observations of the positions of yellow and violet line and the LDR on this scale directly make it possible to calculate the wavelength of the radiation incident at LDR.

Precautions:-

1) after completing the observations with the mercury lamp mount it back at its original position and raise the height of the LDR at its original positions before you leave the table. 2) collimator is already focused for the parallel rays and the focusing screws should not be disturbed. 3) while mounting the mercury lamp in front of the collimator – slit care should be taken that the spectrometer , collimator , the convex lens is not disturbed at all. 4) The telescope of the spectrometer should not be displaced to such a position that it does not obstruct the path of the rays from grating to the LDR.


Direction of rays:-

At normal incident grating with prism table

Reading for optical bench

Current meter series with LDR


NOTE:-

1) during the working wit tungsten filament lamp the variable voltmeter at 0 V adjust and fall the green spectrum on the LDR and by the current meter of 500 micro amp find the maximum current position of the convex lens. 2) thereafter replace the micrometer should be replace by another micrometry and the lit with should be adjust at that range onwards. 3 ) by the another power supply and DVM find the resistance of the tungsten. 4) for the wavelength we use only mercury lamp just.

5) experiment should be done in dark as the any fluctuation could alter the experimental value.


Two different types of light sources:-

Mercury light source

Tungsten filament


Dc power supply for LDR and Tungsten

Variable voltmeter for LDR ( i.e. voltage is adjust at 10 V in this expt)


This distance should be 75 cm approx. Trisha Banerjee @ 2010

VsIs

Power supply for finding the resistance of the tungsten's filament lamp

Finding the voltage of the tungsten lamp i.e. Vs.


Some relations used in this experiment for determining the Planck's constant by Wien's radiation method are:-

R(t) = Vs / Is T = {R(t) /R(d)} X T(d)

wavelength= {wave(green) - wave ( yellow) / L y- Lv} *(L-Lg ) + wave ( green)

m = slope of log I versus 1/T x10(4)

h = (2.303 * wavelength X Kb * m )/c

where Kb is Boltzmann constant Ly position of yellow spectrum on LDR scale L position of LDR on LDR scale Lg position of green spectrum on LDR scale


Experiment if planck's constant

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