a)
State plank’s quantum theory of radiation?
Solution:
By
Planck’s quantum theory,
- Different atoms and molecules can emit or absorb energy in discrete quantities only. The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum.
- The energy of the radiation absorbed or emitted is directly proportional to the frequency of the radiation.
b)
Write Einstein’s photoelectric equation and explain how it deals with the law
of conservation of energy?
Solution:
When
the light means photons incident on a metal surface, the electrons are emitting
from metal surface, called photoelectric effect.
The
Einstein's Photo-Electric equation: KEmax=hf−W
Einstein's
photoelectric equation is based on the law of conservation of energy. Einstein
resolved this problem using Plank's revolutionary idea that light was a
particle. The energy carried by the each particle of light (photon) is
dependent on the light's frequency.
c) In an experiment on photoelectric effect, maximum K.E. (Ek) and frequency (f) is found to be straight line as shown in the figure. Compare the value of threshold frequency and Plank’s Constant.
Solution:
Einstein's
equation for photoelectric effect is given by,
Ek(max)=hf−W
where W= work
function for metal and
h= Planck's
constant and
f= frequency
of incident photon.
So,
slope of the given line will be h and intercept will be −W.
From
the graph,
$h
= \frac{{2.4 \times {{10}^{ - 15}}}}{{(6 - 2) \times {{10}^{ - 18}}}} = 6
\times {10^{ - 34}}J.s$
and W=2.4×10−15
Also,
W=hf0
So,
Threshold frequency (f0) = $\frac{W}{h} = \frac{{2.4 \times {{10}^{
- 15}}}}{{6 \times {{10}^{ - 34}}}}$
$=
3.33 \times {10^{18}}{S^ - }$
The figure shows a graph between stopping potential and frequency of incident radiation for some metal surface. Study the graph and answer the following questions. a. What are the values of threshold frequency and threshold wavelength? [use c = 3 x108 m/s] b. Obtain the value of Planck's constant from the graph. c. The metal surface is illuminated by the radiation of frequency 1015 Hz. What would be the maximum kinetic energy of the emitted electrons?