Saturday, June 27, 2009

7. Dual Nature of Radiation and Matter and Relativity
1.What is Photoelectric effect (or)photoelectric emission?
Photoelectric emission is the phenomena by which a good number
of substances, chiefly metals, emit electrons under the influence of
radiation such as γ rays, X-rays, ultraviolet and even visible light. This
effect was discovered by Heinrich Hertz.
2.Define saturation current.
When the positive potential of anode A is increased, the photoelectric current is also increased, if the positive potential is further increased such that it is large enough to collect all the photo electrons emitted from the cathode plate C,the photoelectric current reaches a certain maximum value and this current is known as saturation current.
3.Define stopping (or) cutoff potential.
If the negative or retarding potential applied to anodeis increased, the photo current decreases and finally becomes zero at a particular value. This minimum negative (retarding) potential given to the anode for which the photo electric current becomes zero is called the cut-off or stopping potential.
4.Define threshold frequency.
Threshold frequency is defined as the minimum frequency of incident radiation below which the photoelectric emission is not possible completely,however high the intensity of incident radiation may be. The threshold frequency is different for different metals.
5.Why photon is considered neither a particle nor a wave?
How photon has dual nature?
Photon is neither a particle nor a wave. In the phenomena like
interference, diffraction, polarisation, the photon behaves like a wave.
In the phenomena like emission, absorption and interaction with matter
(photo electric effect) photon behaves as a particle. Hence light photon
has a dual nature.
6.Define photoelectric work function.
A part of the energy of the photon falling on metal surfaceis used in extracting the electron from the surface of metal, since the electrons in the metal are bound to the nucleus. This energy W spent in releasing the photo electron is known as photoelectric work function of the metal.
7.What is photoelectric cells?State its types.
The photoelectric cell is a device which converts light energy
into electrical energy. The photo electric cells are of three types:
(i) Photo emissive cell (ii) Photo voltaic cell and (iii) Photo conductive cell
8.State four applications of photo electric cells.
(i) Photoelectric cells are used for reproducing sound in
(ii) They are used for controlling the temperature of
(iii) Photoelectric cells are used for automatic switching on
and off the street lights.
(iv) Photoelectric cells are used in the study of temperature
and spectra of stars.
9.How photoelectric cells used in burglar alarm?
In burglar alarm, ultraviolet light is continuously made to fall on the
photo-cell installed at the door-way. A person entering the door
interrupts the beam falling on the photo-cell. The abrupt change in
photocurrent is used to start an electric bell ringing.
10.How photoelectric cells used in fire alarm?
In fire alarm, a number of photo-cells are installed at suitable places in a building. In the event of breaking out of fire, light radiations fall upon the photocell.This completes the electric circuit through an electric bell or a siren which starts operating as a warning signal.

11.State the uses of electron microscope.
(i) It is used in the industry, to study the structure of textile
fibres, surface of metals, composition of paints etc.
(ii) In medicine and biology, it is used to study virus, and
(iii) In Physics, it has been used in the investigation of atomic
structure and structure of crystals in detail.
12.State the Limitations of electron microscope.
An electron microscope is operated only in high vacuum. This
prohibits the use of the microscope to study living organisms which
would evaporate and disintegrate under such conditions.
13.Write note on :Concept of Space
In classical mechanics, motion in absolute space led to two
useful results.
(i) Fixed frame of reference by which the position or motion of
any object in the universe could be measured.
(ii) The geometrical form of an object remains the same irrespective of changes in position or state of motion of the object or observer.

14.Write note on : Concept of time
According to classical mechanics,
(i) The time interval between two events has the same value for all
observers irrespective of their motion.
(ii) If two events are simultaneous for an observer, they are
simultaneous for all observers, irrespective of their position or motion.
This means simultaneity is absolute.
15.What is frame of reference?
A system of co-ordinate axes which defines the position of a
particle in two or three dimensional space is called a frame of reference.
The simplest frame of reference is the Cartesian co-ordinate
system in which the position of a particle is specified by three coordinates x,y and z. There are two types of frames of reference
(i) inertial and (ii) non – inertial frames.
16.Define Inertial (or) unaccelerated frames.
A frame of reference is said to be inertial, when the bodies in
this frame obey Newton’s law of intertia and other laws of Newtonian
mechanics. In this frame, a body remains at rest or in continuous
motion unless acted upon by an external force.
17.Define : Non-inertial (or) accelerated frames
A frame of reference is said to be a non-intertial frame, when a
body not acted upon by an external force, is accelerated. In this frame,
Newton’s laws are not valid.
18.State postulates of special theory of relativity.
The two fundamental postulates of the special theory of relativity
are : (i) The laws of Physics are the same in all inertial frames of
reference. (ii) The velocity of light in free space is a constant in all the
frames of reference.
19.Write note on Length contraction (or) Lorentz-Fitzgerald contraction.
The length of the rod as measured by the observer at rest is lo, and the length of the rod is measured as l by the moving observer. (l < lo).
The length of the rod moving with a velocity v relative to the observer at rest is contracted by a factor.This is known as Lorentz – Fitzgerald contraction.Example : A circular object will appear as an ellipse for a fast moving observer as shown

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