Objective:

To measure the wavelength of helium-neon

light with accurate precision.

Outline: Interference of light Path difference and phase

difference LASER. Metastable state and

population inversion Helium-Neon laser

Interferometer Michelson’s

interferometer Importance of

Michelson’s Interferometer

Observations and calculations

Summary

Interference of light:

When two waves having same amplitude and same wavelength travelling in the same direction both combine together, the phenomenon is called interference of light .

Conditions for interference:

The interfering beams must be monochromatic, that is of a single wavelength.

The interfering beams of light must be coherent.

The interfering beams must be unidirectional.

Types of interference:

Constructive interferenceDestructive interference

When two identical waves are in phase having same amplitude combine together,their amplitude and intensity becomes maximum, known as constructive interference.

Constructive

Interference:

Destructive Interference

: When two

identical waves are out of phase superpose, their amplitude and intensity becomes zero.This is known as destructive interference.

Path difference and Phase difference

path difference:

The wavelength by which one wave leads or lags another wave is called path difference

sphase difference:The time interval or phase angle by which one wave leads or lags another wave is called phase difference

They are both related as :

Path difference/λ = phase difference / 2𝝅

LASERLASER is the acronym for

“Light Amplification by Stimulated Emission of Radiation". As name indicates lasers are used for producing an intense,monochromatic and unidirectional coherent beam of visible light.

Metastable state:

“A metastable state is usually an excited state in which an excited electron is usually stable and from which the electron spontaneously falls to lower state only after relatively longer time.”

The transition from metastable state to the ground state is difficult as compared to other excited states.

Population

inversion: Population inversion is a situation in which the higher energy state has a greater population than the lower energy state.

Helium-Neon Laser:

It is most common type of lasers used in physics laboratories.

Its discharge tube is filled with 85% helium and 15% neon gas.

The neon is the active medium in this tube.

The metastable states of helium and neon are nearly identical,located20.61eV and 20.66eV level.

Excited helium atoms collide with neon atoms and transfer its own energy to an electron in the neon atom.

In this way a population inversion is sustained in the neon gas.

Spontaneous emission from neon atoms initiate laser action and stimulated emission causes electrons in the neon to drop from 20.66eV to 18.70eV and red laser light of wavelength 632.8 nm is generated

It is easy to construct, relatively inexpensive and fairly reliable.

Interferometer: “An interferometer is a device that can be used to

measure length or changes in length with great accuracy by mean of interference fringes”.

We describe the form originally built by Albert A. Michelson.

MICHELSON INTERFEROMETER:

It was built in 1881

CONSTRUCTION:

The apparatus consists of semi silvered glass plate(half silvered mirror) P which is also called beam splitter, another glass plate P′ of equal thickness and two plane mirror M1 and M

2 .The mirror M1 is moveable

because it is mounted on the shaft of micrometer. The two plane mirrors are kept at right angle to each other .

Working: A parallel beam of monochromatic light of wavelength λ is made to fall on the half silvered mirror or plate P at an angle of 45˚ half of the light intensity is reflected back from the plate P toward the plane mirror M1,which is called ray 1.

The other half of incident ray is transmitted through the plate P, and after passing through another glass plate P′ falls on the plane mirror M2 ,which is called ray 2,both of the coherent rays , ray1 and ray 2 are reflected back from the mirrors and they meet each other producing the phenomenon of interference of light.

The observer then sees a series of parallel interference of fringes as the length L1 is changed , the pattern of interference fringes is observed .

Ray1 1

Ray 2

Moveable mirror

Circular Fringes :Circular

fringes are formed when interfering beams(ray 1 and ray 2)are parallel.

Observations:No. of fringes n=100 calibration constant k=0.1 Exact distance L1 =D

Calculations:

Percentage error = Actua

l

Observed-actual 100

632664 100=

= 5%

-632

Mean value λ = 664nm

Importance of Michelson‘s interferometer

It is used to find the wavelength of light with accurate precision .

Its is also used to measure thickness and index of refraction of thin plate or film(transparent).

Michelson measured the length in standard meter in terms of wavelength of red cadmium light and showed that the standard meter as equivalent to 1,553,163.5 wavelengths of thin light.

Non-Existence of ether for the propagation of electromagnetic waves.

SUMMARY The actual vale of the

wavelength of helium neon light is 632 nm.

The calculated value of wavelength of helium neon light is 664 nm.

The percentage error is 5 %

PRESESNTED BY:

1- Masuma Amir 12262- Dania Taqveem 12033- Mehtab Irfan 1233