# Physics Class 12 NCERT Solutions: Chapter 11 Dual Nature of Radiation and Matter Part 13 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q: 28. A mercury lamp is a convenient source for studying frequency dependence of photoelectric emission, since it gives a number of spectral lines ranging from the UV to the red end of the visible spectrum. In our experiment with rubidium photo-cell, the following lines from a mercury source were used:

The stopping voltages, respectively, were measured to be:

Determine the value of Planck՚s constant h, the threshold frequency and work function for the material.

[Note: You will notice that to get h from the data, you will need to know e (which you can take to be ) . Experiments of this kind on , etc. were performed by Millikan, who, using his own value of e (from the oil-drop experiment) confirmed Einstein՚s photoelectric equation and at the same time gave an independent estimate of the value of .]

Einstein՚s photoelectric equation is given as:

Where,

Stopping potential

Planck՚s constant

Charge on an electron

Work function of a material

It can be concluded from equation (1) that potential is directly proportional to frequency .

Frequency is also given by the relation:

This relation can be used to obtain the frequencies of the various lines of the given wavelengths.

The given quantities can be listed in tabular form as:

The following figure shows a graph between v and .