Laws of photoelectric emission

Laws  of  photoelectric  emission

The experimental observations on photoelectric effect may be summarized  as follows,which  are  known as the fundamental laws of photoelectric emission.

• There is a minimum frequency called the threshold frequency, for a given photo sensitive  material,below which emission of photoelectrons stops completely.
• For a given photosensitive material,the photo electric current is directly proportional to the intensity of the incident radiation,provided the frequency is greater than the threshold  frequency.
• The photoelectric emission is an instantaneous process. i.e. there is no time lag between the incidence of radiation and the emission of  photo electrons.
• The maximum kinetic energy of the photo electrons is directly proportional to the frequency of incident radiation,but is independent of  its intensity.

Hallwachs Experimental Setup and Explanation

Hallwachs Experimentset-up

Experimentset-up to study the photo electric effect is shown  in  Fig. It  consists  of  an  evacuated quartz  bulb  with  two  zinc  plates  cathode and  anode.  The  plates  are  connected  to  a battery  and  a  sensitive  galvanometer.  In  the absence   of   any   radiation   incident   on   the plates.There  is  no  flow  of  current  and  hence there  is  no  deflection  in  the  galvanometer. But,when  an  electro  magnetic  radiation  like ultraviolet  radiation  is  allowed  to  fall  on  the cathode plate which  is  connected  to  the  negative terminal  of  the  battery, a  current  begins  to flow,indicated  by  the deflection  in  the  galvanometer But,  when  ultraviolet  radiation  is made  to fall  on  anode there  is  no  deflection  in  the  galvanometer.  These observations reveal that the particles emitted by the cathode plate  due to the photoelectric  effect  are  negatively  charged.  These  particles  were  found to  be  electrons.  The  observed  current  known  as  the  photoelectric current  is  due  to  the  flow  of  electrons.After  the  study  of  photoelectric  effect  by  Hallwachs,  scientists J.J.Thomson, Lenard, Richardson, Compton did a series of experiments to  study  the relationship  between  photoelectric  current,  intensity  ofincident   radiation,   velocity   and   the   kinetic   energy   of   the   photo electrons,   and   their   dependence   on   the   wave   length   of   incident radiation used.

Photoelectric effect

Photoelectric effect

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  in  1887  while  working  with resonance electrical circuits. Hallwachs,Elster and Geitel investigated the phenomenon with a simple experimental arrangement after one year.

What is Holography and Maser

Holography

When an object is photographed by a camera, a two dimensional
image  of  three  dimensional  object  is  obtained.  A  three  dimensional
image   of   an   object   can   be   formed   by   holography.   In   ordinary
photography,  the  amplitude  of  the  light  wave  is  recorded  on  the
photographic film. In holography, both the phase and amplitude of the
light waves are recorded on the film. The resulting photograph is called Hologram

MASER
The   term   Maser   stands   for   Microwave   Amplification   by
Stimulated  Emission  of  Radiation.  The  working  of  maser  is  similar  to
that of laser. The maser action is based on the principle of population
inversion  followed  by  stimulated  emission.  In  maser,  the  emitted
photon, during the transition from the metastable state belongs to the
microwave  frequencies.  The  paramagnetic  ions  are  used  as  maser
materials. Practical maser materials are often chromium or gadolinium
ions doped as impurities in ionic crystals. Ammonia gas is also a maser
material.  Maser  provides  a  very  strong  tool  for  analysis  in  molecular spectography

Different application of X-ray

Applications of X–rays
X–rays  have  a  number  of  applications.  Some  of  them  are  listed-
Scientific research

1. X–rays are used for studying the structure of crystalline solids and alloy
2. X–rays  are  used  for  the  identification  of  chemical  elements including  determination  of  their  atomic  numbers
3. X–rays  can  be  used  for  analyzing  the  structure  of  complex molecules  by  examining  their  X–ray  diffraction  pattern.

Medical applications

1. X–rays  are  being  widely  used  for  detecting  fractures,  toumers, the  presence  of  foreign  matter  like  bullet  etc.,  in  the  human  body.
2. X–rays  are  also  used  for  the  diagnosis  of  tuberculosis,  stones in  kidneys,  gall  bladder  etc.
3. Many  types  of  skin  diseases,  malignant  sores,  cancer  and toumers  have  been  cured  by  controlled  exposure  of  X-rays  of  suitable quality.
4. Hard X–rays are used to destroy toumers very deep inside the body.

Industrial applications

1.  X–rays are used to detect the defects or flaws within a material
2. X–rays  can  be  used  for  testing  the  homogeneity  of  welded joints,  insulating  materials  etc.
3. X-rays  are  used  to  analyse  the  structure  of  alloys  and  the other  composite  bodies. 
4. X–rays  are  also  used  to  study  the  structure  of  materials  like rubber,  cellulose,  plastic  fibre etc.

Soft X–rays and Hard X–rays

Soft X–rays and Hard X–rays
X–rays  are  of  two  types-

•  Soft  X–rays  
• Hard  X–rays

Soft  X–rays
X–rays  having  wavelength  of  4Å  or  above,  have  lesser  frequency
and  hence  lesser  energy.  They  are  called  soft  X  –  rays  due  to  their  low
penetrating  power.  They  are  produced  at  comparatively  low  potential
difference.

 Hard  X–rays
X–rays  having  low  wavelength  of  the  order  of  1Å have  high
frequency  and  hence  high  energy.  Their  penetrating  power  is  high,
therefore   they   are   called   hard   X–rays.   They   are   produced   at
comparatively  high  potential  difference.
The wavelength of X–rays depends upon the kinetic energy of the
electrons  producing  them  and  this  kinetic  energy  depends  upon  the
potential  difference  between  the  filament  and  the  target.

Determination of specific charge (e/m) of an electron – Thomson’s method.

Determination  of  specific  charge  (e/m)  of  an  electron  –
Thomson’s method.

In  1887, J.J.  Thomson,  measured  the  specific  charge  (e/m)  of
the  cathode  ray  particles.  The  specific  charge  is  defined  as  the  charge
per  unit  mass  of  the  particle.  Thomson  discovered  that  the  value  of
(e/m)  was  independent  of  the  gas  used  and  also  independent  of  the
nature  of  the  electrodes.

Principle
The  fact  that  the  cathode  rays  (electrons)  are  deflected  by
electric  and  magnetic  fields  is  made  use  of  in  this  method.
Experimental arrangement

A  highly  evacuated  discharge  tube  used  in  this  experiment.
Cathode rays are produced by the discharge between the cathode and the anodes.
A thin pencil of cathode
ray comes out through fine pin
holes  in  the  anode  discs.  The
cathode   rays   then   pass
between  two  parallel  metal
plates as shown in figure
 and strike the flat  face  of  the  tube.  This  face
is   coated   with   suitable
fluorescent material. A spot of
light  is  produced   But  when  a  potential  difference  V  is  applied
between tow plates, the beam is deflected By the use of a pair
of  coils,  uniform  magnetic  field  is  produced  perpendicular  to  the  plane
of  the  paper  and  outwards  through  out  the  region  between vertical deflection plates.

Impulsive force and Impulse of a force

Impulsive  force  and  Impulse  of  a  force

(i)  Impulsive  Force
An impulsive force is a very great force acting for a very short time
on a body, so that the change in the position of the body during the time
the  force  acts  on  it  may  be  neglected.
(e.g.) The blow of a hammer, the collision of two billiard balls etc.

(ii)  Impulse  of  a  force
The  impulse  J  of  a  constant  force  F
acting for a time t is defined as the product
of  the  force  and  time.
(i.e)  Impulse  =  Force×time
               J=F×t
The impulse of force F acting over a time interval t
is  defined  by  the  integral,

J=F∫dt  (0 to t)

When a variable force acting
for  a  short  interval  of  time,  then  the  impulse  can  be  measured  as,
J  =  F average×dt

Type of Inertia

The  inertia  is  of  three  types

(i)Inertia  of  rest

(ii)    Inertia  of  motion

(iii)   Inertia  of    direction.

(i)     Inertia  of  rest

It  is  the  inability  of  the  body  to  change  its  state  of  rest  by  itself.

Examples

(i)  A  person  standing  in  a  bus  falls  backward  when  the  bus

suddenly starts moving. This is because, the person who is initially at

rest  continues  to  be  at  rest  even  after  the  bus  has  started  moving.

(ii) A book lying on the table will remain at rest, until it is moved

by  some  external  agencies.

(iii) When a carpet is beaten by a stick, the dust particles fall off

vertically downwards once they are released and do not move along the

carpet  and  fall  off.

(ii)  Inertia  of  motion

Inertia  of  motion  is  the  inability  of  the  body  to  change  its  state  of

motion  by  itself.

Examples

(a) When a passenger gets down from a moving bus, he falls down

in  the  direction  of  the  motion  of  the  bus.

(b) A passenger sitting in a moving car falls forward, when the car

stops  suddenly.

(c)  An  athlete  running  in  a  race  will  continue  to  run  even  after

reaching  the  finishing  point.

(iii)  Inertia  of  direction

It  is  the  inability  of  the  body  to  change  its  direction  of  motion  by

itself.

Examples

When a bus moving along a straight line takes a turn to the right,

the  passengers  are  thrown  towards  left.  This  is  due  to  inertia  which

makes the passengers travel along the same straight line, even though

the  bus  has  turned  towards  the  right.

Assumption with kenetic theory of gases

1-The molecule of gas moves all direction with all possible velocity during motion,the molecules colloid with one another but the collision do not effect the molecular density of gases

2-The motion of molecules is random

(the center of mass f gas remain at rest)

3-Between two collisiona molecules move in straight line with uniform velocity this is because no force act on the particle b/w the collision.The distance covered by molecules is  called mean free path

4-The diamension of the molecules may be neglected as compared to the dimension of free path.

5-No approciable force of attraction or repulsion by molecule on in another accept during collision

6-Collision between  melecules and with the wall of the container or perfectlly elastic and the time of impact is of neglagible duration

(elastic collision means no change of linear momentum and kenetic energy )

elastic collision means no change of momentum but energy change

7-Molecules obey newtons law of motion