Dual Nature of Matter and Radiation Flashcards
what is electron emision
The ejection of e-s from the surface of the metal on the application of suitable amount of energy.
what is work function
A certain minimum amount of energy is required to be given to an electron to pull it out from the surface of the metal. This
minimum energy required by an electron to escape from the metal surface
is called the work function of the metal. It is generally denoted by f0 and
measured in eV (electron volt).
what is 1eV
One electron volt is the energy gained by an
electron when it has been accelerated by a potential difference of 1 volt, so
that 1 eV = 1.602 ×10
–19 J.
types of electron emission
(i) Thermionic emission: By suitably heating, sufficient thermal energy
can be imparted to the free electrons to enable them to come out of the
metal.
(ii) Field emission: By applying a very strong electric field (of the order of
108 V m–1) to a metal, electrons can be pulled out of the metal, as in a
spark plug.
(iii) Photoelectric emission: When light of suitable frequency illuminates a metal surface, electrons are emitted from the metal surface. These
photo(light)-generated electrons are called photoelectrons.
Hertz observations
In his experimental investigation on the production of electromagnetic
waves by means of a spark discharge, Hertz observed that high voltage
sparks across the detector loop were enhanced when the emitter plate
was illuminated by ultraviolet light from an arc lamp.
Light shining on the metal surface somehow facilitated the escape of
free, charged particles which we now know as electrons.
When light falls
on a metal surface, some electrons near the surface absorb enough energy
from the incident radiation to overcome the attraction of the positive ions
in the material of the surface.
After gaining sufficient energy from the
incident light, the electrons escape from the surface of the metal into the
surrounding space.
lenards
Lenard (1862-1947) observed that when ultraviolet radiations were
allowed to fall on the emitter plate of an evacuated glass tube enclosing
two electrodes (metal plates), current flows in the circuit (Fig. 11.1). As
soon as the ultraviolet radiations were stopped, the current flow also
stopped. These observations indicate that when ultraviolet radiations fall
on the emitter plate C, electrons are ejected from it which are attracted
towards the positive, collector plate A by the electric field. The electrons
flow through the evacuated glass tube, resulting in the current flow. Thus,
light falling on the surface of the emitter causes current in the external
circuit. Hallwachs and Lenard studied how this photo current varied with
collector plate potential, and with frequency and intensity of incident light.
hallwachs expt
Hallwachs, in 1888, undertook the study further and connected a
negatively charged zinc plate to an electroscope. He observed that the
zinc plate lost its charge when it was illuminated by ultraviolet light.
Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light. Positive charge on a positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light. From these observations he concluded that negatively
charged particles were emitted from the zinc plate under the action of
ultraviolet light.
what is photoelectric effect
It is the phenomenon of emission of electrons from the surface of a photosensitive substance, when the radiations of suitable frequency fall on it.
what is threshold frequency
It is the minimum frequency of incident radiation required to emit electrons from an emitter without any kinetic energy.
experimental set up
read only
It consists of an evacuated
glass/quartz tube having a thin photosensitive plate C and another metal
plate A.
Monochromatic light from the source S of sufficiently short
wavelength passes through the window W and falls on the photosensitive plate C (emitter).
A transparent quartz window is sealed on to the glass
tube, which permits ultraviolet radiation to pass through it and irradiate
the photosensitive plate C.
The electrons are emitted by the plate C and
are collected by the plate A (collector), by the electric field created by the
battery. The battery maintains the potential difference between the plates
C and A, that can be varied.
The polarity of the plates C and A can be
reversed by a commutator.
Thus, the plate A can be maintained at a desired
positive or negative potential with respect to emitter C.
When the collector plate A is positive with respect to the
emitter plate C, the electrons are attracted to it. The
emission of electrons causes flow of electric current in
the circuit. The potential difference between the emitter
and collector plates is measured by a voltmeter (V)
whereas the resulting photo current flowing in the circuit
is measured by a microammeter (mA).
Effect of intensity of light on photocurrent
Keeping the frequency of the
incident radiation and the potential fixed, the intensity of
light is varied and the resulting photoelectric current is
measured each time. It is found that the photocurrent
increases linearly with intensity of incident light as shown
graphically
This implies that the number of photoelectrons
emitted per second is directly proportional to the intensity
of incident radiation
saturation current
At some stage, for a certain positive potential of plate A, all the
emitted electrons are collected by the plate A and the photoelectric current
becomes maximum or saturates. If we increase the accelerating potential
of plate A further, the photocurrent does not increase. This maximum
value of the photoelectric current is called saturation current.
stopping potential
For a particular frequency
of incident radiation, the minimum
negative (retarding) potential V0
given to the plate A (collector) for which the photocurrent
stops or becomes zero is called the cutoff or stopping potential.
Photoelectric current is zero when the stopping potential is
sufficient to repel even the most energetic photoelectrons, with the maximum kinetic energy (Kmax)
stopping potential variation with intensity/ saturation current variation with intensity
with incident radiation of the same
frequency but of higher intensity I2
and I3
(I3 > I2 > I1
). We note that the
saturation currents are now found to be at higher values. This shows
that more electrons are being emitted per second, proportional to the
intensity of incident radiation. But the stopping potential remains the
same as that for the incident radiation of intensity I1
, as shown graphically
in Fig. 11.3.
Thus, for a given frequency of the incident radiation, the
stopping potential is independent of its intensity. In other words, the maximum kinetic energy of photoelectrons depends on the light source and the emitter plate material, but is independent of intensity of incident
radiatio
variation of stopping potential with frequency
(i) the stopping potential V0
varies linearly with
the frequency of incident radiation for a given
photosensitive material.
(ii) there exists a certain minimum cut-off
frequency n0 for which the stopping potential
is zero.
iiii) The higher the work function of the photosensitive material, the greater is the value of the threshold frequency
