egg DUAL NATURE OF RADIATION AND MATTER g@BEO⑧OBgg→••m
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Some Important Discoveries of the late 19th Century
- X-Rays were discovered by Roentgen
- Electron was discovered by J.J. Thomson
- Cathode rays were discovered by William Crookes and were confirmed by J.J. Thompson
- J.J. Thomson calculated the e/m ratio of cathode ray particles
- Charge on an electron was discovered by R.A. Millikan by his famous Oil-drop experiment. This established that
charge on an electron is quantized.
Baotou
Cathode Rays
If a gas is filled in a glass tube at extremely low pressure and very high voltage is applied then, a discharge takes
place between the two electrodes on applying electric field. A fluorescent glow appeared on the glass opposite to
the cathode, the color of which depends on the glass. This glow was attributed to the radiation coming from the
cathode which was named as cathode rays.
It was found out that the cathode rays consisted of a stream of negatively charged particles.
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e/m Ratio
By applying mutually perpendicular electric and magnetic fields across the discharge tube, J.J. Thomson was able to
calculate the specific charge (charge to mass ratio) for the cathode rays
- The value of e/m ratio was found to be independent of nature of material/metal used as the cathode and the
gas in the discharge tube
- The value of e/m ratio of cathode rays (electrons) is 1.76 × 1011 C/kg
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Electron Emission
We know that metals have free electrons but these free electrons cannot escape the metal because when an
electron escapes the metal, the metal surface becomes positively charged and it pulls back the electron. So, an
electron can only escape a metal if it has a minimum amount of energy.
Work function (ɸ˳) – The minimum energy required by an electron to escape from the metal surface is called the
work function of the metal. It is generally measured in eV (electron volt). The work function of the metal depends on
its properties and the nature of its surface.
The energy required for electron emission can be supplied by-
1. Thermionic Emission- By suitably heating, sufficient energy can be supplied to the electrons to enable them to
come out of the metal
2. Field emission- By applying very strong electric field to a metal, electrons can be pulled out of the metal, as in a
spark plug.
3. Photo-electric emission- When light of suitable frequency illuminates a metal surface, electrons are emitted
from the metal surface. The photo (light) generated electrons are called photo electrons.
Biotope
Photoelectric Effect
Hertz’ Observations
The phenomenon of photoelectric emission was discovered by Heinrich Hertz.
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.
, Hallwachs’ and Lenard’s Observations
- Lenard 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.
- 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, 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.
Bogota
Experimental Study of Photoelectric effect
The Figure depicts a schematic view of the arrangement used for the experimental study of the photoelectric effect.
Construction-
It consists of an evacuated glass/quartz tube having a photosensitive plate
C and another metal plate A.
Working-
- 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.
Bogota
Effect of Intensity of light on photocurrent
It is found that the photocurrent increases linearly with intensity of
incident light as shown graphically. The photocurrent is directly
proportional to the number of photoelectrons emitted per second.
This implies that the number of photoelectrons emitted per second
is directly proportional to the intensity of incident radiation.
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AMEBLO.gga.am
Some Important Discoveries of the late 19th Century
- X-Rays were discovered by Roentgen
- Electron was discovered by J.J. Thomson
- Cathode rays were discovered by William Crookes and were confirmed by J.J. Thompson
- J.J. Thomson calculated the e/m ratio of cathode ray particles
- Charge on an electron was discovered by R.A. Millikan by his famous Oil-drop experiment. This established that
charge on an electron is quantized.
Baotou
Cathode Rays
If a gas is filled in a glass tube at extremely low pressure and very high voltage is applied then, a discharge takes
place between the two electrodes on applying electric field. A fluorescent glow appeared on the glass opposite to
the cathode, the color of which depends on the glass. This glow was attributed to the radiation coming from the
cathode which was named as cathode rays.
It was found out that the cathode rays consisted of a stream of negatively charged particles.
%aEotoo.am
e/m Ratio
By applying mutually perpendicular electric and magnetic fields across the discharge tube, J.J. Thomson was able to
calculate the specific charge (charge to mass ratio) for the cathode rays
- The value of e/m ratio was found to be independent of nature of material/metal used as the cathode and the
gas in the discharge tube
- The value of e/m ratio of cathode rays (electrons) is 1.76 × 1011 C/kg
BBBBBEqa.am
Electron Emission
We know that metals have free electrons but these free electrons cannot escape the metal because when an
electron escapes the metal, the metal surface becomes positively charged and it pulls back the electron. So, an
electron can only escape a metal if it has a minimum amount of energy.
Work function (ɸ˳) – The minimum energy required by an electron to escape from the metal surface is called the
work function of the metal. It is generally measured in eV (electron volt). The work function of the metal depends on
its properties and the nature of its surface.
The energy required for electron emission can be supplied by-
1. Thermionic Emission- By suitably heating, sufficient energy can be supplied to the electrons to enable them to
come out of the metal
2. Field emission- By applying very strong electric field to a metal, electrons can be pulled out of the metal, as in a
spark plug.
3. Photo-electric emission- When light of suitable frequency illuminates a metal surface, electrons are emitted
from the metal surface. The photo (light) generated electrons are called photo electrons.
Biotope
Photoelectric Effect
Hertz’ Observations
The phenomenon of photoelectric emission was discovered by Heinrich Hertz.
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.
, Hallwachs’ and Lenard’s Observations
- Lenard 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.
- 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, 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.
Bogota
Experimental Study of Photoelectric effect
The Figure depicts a schematic view of the arrangement used for the experimental study of the photoelectric effect.
Construction-
It consists of an evacuated glass/quartz tube having a photosensitive plate
C and another metal plate A.
Working-
- 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.
Bogota
Effect of Intensity of light on photocurrent
It is found that the photocurrent increases linearly with intensity of
incident light as shown graphically. The photocurrent is directly
proportional to the number of photoelectrons emitted per second.
This implies that the number of photoelectrons emitted per second
is directly proportional to the intensity of incident radiation.
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