Applied Optics

Dr D. Arun Kumar

Assistant Professor

Department of Physical Sciences

Bannari Amman Institute of Technology Sathyamangalam

LIGHT AS A WAVE

• Light has dual nature. It has both particle nature and

wave nature but it cannot act as a wave and particle

simultaneously.

• Light is also having the properties such as reflection,

refraction, interference, diffraction, polarization.

• Light is also having two most famous effects like

photoelectric effect and Compton Effect.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

• To explain the properties like interference diffraction

and polarization we need to adopt wave nature for light.

• Light is a transverse wave.

• The first person to advance a convincing wave theory for

light was DUTCH PHYSICIST CHRISTIAN HUYGENS, in

1678.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

LIGHT AS A WAVE

Representation of Light wave

• Whose displacement is given b

• y=asinωt

• Where ' a' is amplitude of wave.

• 'ωt' is Phase, Which gives the position and direction of wave at a time 't'

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

PHASE DIFFERENCES:

• The angular separation between two

points of wave is called Phase

difference.

• It is measured in radians or degrees.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

PATH DIFFERENCE:

• The linear separation between two

points of wave is called Path Difference

• It is measured in mm or cm.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

RELATION BETWEEN PHASE DIFFERENCE AND PATH DIFFERENCE:

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

INTERFERENCE:

• Interference is the optical phenomenon

in which brightness and darkness are

produced by the combination of two

similar light waves.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

• When two light waves of same frequency having constant

phase difference coincide in space and time. There is

modification in the intensity of light.

• This modification in the intensity is due to superposition

of two light waves are called Interference.

• And the pattern dark and bright fringes produced are

called Interference pattern.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

INTERFERENCE:

TYPES OF INTERFERENCE:

Interference is of two types.

• Constructive interference / constructive superposition

• Destructive interference / destructive superposition

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

CONSTRUCTIVE INTERFERENCE:

• When crust of one light wave falls on the crust of another

wave then the resultant intensity increases

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

DESTRUCTIVE INTERFERENCE:

• When crust of one light wave falls on the trough of

another wave then the resultant intensity decreases.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

Conditions for sustained or Good interference pattern :

• We require two monochromatic light sources.

• These two sources must be coherent i.e., they constant

phase difference.

• The frequency must be the same.

• The amplitude must be the same.

• They must travel in the same directions.

• The two sources must be the same

• These two sources must be as near as possible and the

screen must be as far from them as possible.

Interference at a wedge shaped films

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

Determination of thickness of a paper or wire

• Two optically plane glass plates are placed one over the other and tied at one end. The given wire is introduced near the other end, so that an air wedge is formed.

• The distance between the wire and the tied end (L) is measured using a scale.

• Light from a sodium vapour lamp is incident on a plane glass plate inclined at 45 to the horizontal.

• The reflected light from the plane glass plate is incident normally on the optically plane glass plates forming the air wedge and reflected back.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

Determination of thickness of a paper or wire

• The reflected light from the air-wedge is viewed through the eye-piece of a microscope. The microscope is moved up and down and adjusted for clear interference fringes of alternate dark and bright.

• The microscope is fixed so that the vertical cross-wire coincides with the dark band (say nth band) and the reading is noted.

• The microscope is moved across the fringes and readings are noted when the vertical cross-wire coincides with the (n+5)th, (n+10)th….. dark bands.

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

Determination of thickness of a paper or wire

• The observed readings are tabulated and the band width () is calculated.

• The thickness of the given wire/thin-sheet is calculated using the formula.

t = L/2 (m)

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam

Order

of

Fringes

Microscope reading Width for 5

fringes

(10-2 m)

Fringe width

(10-2 m)

MSR

(10-2 m)

VC

(div)

TR

(10-2 m)

Determination of thickness of a paper or wire

Testing of flatness of a surface

Department of Physical Sciences, Bannari Amman Institute of Technology, Sathyamangalam