CATHODE RAY TUBE – THE PATH FINDER OF CELLULAR AND MOLECULAR BIOLOGY PART1 ELECTRON MICROSCOPY

A cathode ray tube (CRT) is a vacuum tube in which an electron beam is deflected by applied electric or magnetic

fields produce a trace on a fluorescent screen. The function of the CRT is to convert an electrical signal into a visual display.

Cathode Ray Tube (CRT) or Braun’s Tube was invented by the German physicist Karl Ferdinand Braun in 1897 and is today used in computer monitors, TV sets and oscilloscope tubes. The path of electrons in the tube can be observed in a darkened room as a trace of light. The cathode ray tube with modification is used as electron gun in electron microscope to see submicroscopic cells like viruses and used as x-ray tubes to study molecular structure in molecular biology to decipher macromolecule structures like DNA and proteins Karl Braun was awarded

The Nobel Prize 1n 1909 with Guglielmo Marconi for their contribution to the development of wireless telegraphy.

During the 1880s and 90s scientists searched cathode rays for the carrier of electrical properties in matter. Their work culminated in the discovery by English physicist J.J.Thomson of the electron in 1897. Joseph John Thomson was the Cavendish Professor of Experimental Physics at Cambridge University and director of its Cavendish Laboratory. In October 1897 he reported that

“Cathode rays” were actually negatively charged particles in motion which we call today the “electrons” he measured charge of the particle. Thomson was awarded the Nobel prize for physics in 1906. Discovery of electron opened up many areas in science. In the field of biology and microbiology in the form of electron microscope. Electron microscopes use the cathode ray tube with slight modification called “electron gun”

Working of Electron Gun

After exiting from the cathode, the electron passes through the control grid. The control grid is made up of nickel material. It is centrally holed and co-axial with the CRT axis. The intensity of the control beams depends on the number of electrons emitted from the cathode. The grid has negative biasing which controls the flow of electrons.

The electron which passes from the control grid is accelerated by the high positive potential which is applied across the pre-accelerating and accelerating grids. The electron beam is focused by the focusing anode. The beam after passing through the focusing anode hits specimen scatters the electron waves become visible on the fluorescent screen.

Virus particles and sub microscopic cells diffracts the electron waves make their image at the fluorescent screen

The column from the cathode to the specimen holder is evacuated to prevent the scattering of electron by air molecules.

Virus particles are too small to scatter or diffract visible spectrum because of large wavelength and by passes virus particles. So, we have to have short wavelength to get scattered by virus particles. To understand how electrons should be considered as wave we have to invoke some mathematical equations. The most powerful of these equations is the wave particle duality equation developed by Louis de Broglie the French theoretical physicist Type equation here.

𝜆=h/P

This the de Broglie’s equationType equation here.

h = Planck’s constant

p = momentum == mv= mass x velocity

To derive de Broglie equation, we have to in evoke two famous equations

Einstein’s equation E=mc2

Max Planck’s equation E=h𝜐

C=𝜆𝜐 𝜐=𝑐/𝜆

mc2 = h𝑐/𝜆

hc =𝜆 mc2

𝜆=ℎ𝑐/mc 𝜆=ℎ/𝑚𝑐

𝜆 = h/mv

This is the famous de Broglie equation also wave particle duality equation. Louis de Broglie was awarded the Nobel prize for this equation in the year 1929

𝜆= wave length of a particle

h =Planck’s constant- 6.626 x10---34 joules second

mv = momentum of the particle, since momentum is the product of mass and velocity of a particle. This can be expressed as P. debrogile equation can be written as

𝜆 = h/P

The velocity of electron in an election microscope is determined by the accelerating voltage or electron potential which can be expressed as “eV” and also the kinetic energy ½ mv2 with which they are driven

We can write this equation

eV = ½ mv2

e= charge of the electron

V = voltage applied

The velocity of electrons can be calculated

V 2 = eV/1/2m

V = √2eV/m2e

Therefore, wavelength of propagating electrons at given accelerating voltage can be determined. For this we have to invoke de Broglie equation

𝜆 =h/mv

Interposing for v

𝜆 =h/m√2ev/m

Looking at the denominator

m√2ev/√𝑚

m cab written as √𝑚×√𝑚=𝑚 √𝑚×√𝑚×√2𝑒𝑣/√𝑚

The denominator becomes he √𝑚√2𝑒𝑣

So 𝜆 = h/√2𝑚𝑒𝑉

The mass of an electron 9.1x10—31kg

Charge of an electron e= 1.6x10—19

𝜆= 6,62x19—34/√2x9.1x10—31x1.6x10—19xV

= 12.25x10—10/√V

So, when the electron microscope is operated at 100kev

The wave length of electron is 2.74 pm (pica meter) at 200 Kev the wavelength is 2.24 pm (pica meter). The velocity of electron in an electron microscope at 200 Kev

reach 70% of the speed of light. Further increase will lead to Einstein’s relativistic limitation equation which is

m = m0 /√1−𝑣2 / c2

m= relativistic mass

m0 = rest mass

v = velocity of of electron

c = speed of light in vacuum

Ernst Ruska a German electrical engineer is credited with inventing the electron microscope. The earliest electron microscope was developed 1n 1931. Behind a bakery and the first commercial mass-produced instrument became available in 1939. Electron microscopy, considered one of the major scientific advances of the 20th century. It has important application in many fields of science including biology and medicine. It has been used to study metals, viruses, proteins and molecules. For “Fundamental work in electron optics Ruska was awarded the 1986 (after 55 years) Nobel Prize for physics, which he shared with Gerd

Binning and Heinrich Rohrer “for their contribution to the scanning tunneling microscope”.

In electron microscopy the electron waves scattered or diffracted by viruses and other submicroscopic objects are too small to see with human eyes. Hence the scattered and diffracted electron waves are made to impinge on fluorescent screen to see the objects.

K.A.Balasubramanian

M.Sc.Ph.D.(IARI NEWDELHI)Ph.D.(LONDON)D.I.C.(LONDON)