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Nicol Prism
It is calcite (CaCO3) crystal whose length is times its breadth or thickness.
Its working is based on a phenomenon of double refraction. In double refraction
an incident ray splits into two component rays called as ordinary ray (O- ray) and
extra ordinary ray (e-ray).
O-ray obeys ordinary laws of physics and is plane polarised E-ray does not obey
ordinary laws of physics. It is also polarised.
The two rays are plane polarised.in mutual perpendicular planes.
• Optic Axis
It is a direction inside the crystal such that the single ray of light does not split into
O-ray and E-ray as their velocities are equal.
Double refraction or birefringence is not observed along optic axis.
• Principal plane and Principal section
A plane containing the optic axis and perpendicular to the opposite faces of the
crystal called principal plane.
Section of a crystal along principal plane is called principal section of the crystal.
• Construction of Nicol Prism
A calcite crystal of length 3 times its breadth is cut along the proper direction. Its
principal section is a parallelogram of 71° and 109° angles.
The two pieces are ground so that the angles of principal section are changed to
68° and 112°. As shown in the figure.
The two pieces are joined together along diagonal AD by sticky liquid known as
Canada balsm. The side faces are blackened.
Working : Nicol as polariser.
A ray of light is split into O-ray and E-ray which are both polarised.
O-ray has vibrations perpendicular to principal section. RI of calcite for O-ray is
1.658 and RI of Canada balsm is 1.55. Thus for O-ray the calcite crystal is denser
medium. It is totally reflected by Canada balsam and absorbed by blackened
surfaces.
E ray has vibration in the plane of principal section
RI of the crystal for E-ray is 1.486. Calcite behaves as rarer medium. So it is
transmitted by Canada balsam and finally emerges out of the crystal as polarised
beam of light.
• Nicol as Analyser
Two nicols N1 and N2 are arranged in the line of incident ray. N1 is polariser and
gives out a plane polarised light.
(i) Two nicols are parallel, i.e. their diagonals of principal sections are parallel.
(ii) N1 and N2 are crossed. The principal section of N1 and N2 are perpendicular. Eray
becomes O-ray as its vibration are perpendicular to principal plane of N2. So the
ray is totally reflected and absorbed by side faces.
Thus intensity is zero for emerging light. Rotation of N2 can detect plane polarised
and unpolarised light. Hence N2 is called analyser.
It is calcite (CaCO3) crystal whose length is times its breadth or thickness.
Its working is based on a phenomenon of double refraction. In double refraction
an incident ray splits into two component rays called as ordinary ray (O- ray) and
extra ordinary ray (e-ray).
O-ray obeys ordinary laws of physics and is plane polarised E-ray does not obey
ordinary laws of physics. It is also polarised.
The two rays are plane polarised.in mutual perpendicular planes.
• Optic Axis
It is a direction inside the crystal such that the single ray of light does not split into
O-ray and E-ray as their velocities are equal.
Double refraction or birefringence is not observed along optic axis.
• Principal plane and Principal section
A plane containing the optic axis and perpendicular to the opposite faces of the
crystal called principal plane.
Section of a crystal along principal plane is called principal section of the crystal.
• Construction of Nicol Prism
A calcite crystal of length 3 times its breadth is cut along the proper direction. Its
principal section is a parallelogram of 71° and 109° angles.
The two pieces are ground so that the angles of principal section are changed to
68° and 112°. As shown in the figure.
The two pieces are joined together along diagonal AD by sticky liquid known as
Canada balsm. The side faces are blackened.
Working : Nicol as polariser.
A ray of light is split into O-ray and E-ray which are both polarised.
O-ray has vibrations perpendicular to principal section. RI of calcite for O-ray is
1.658 and RI of Canada balsm is 1.55. Thus for O-ray the calcite crystal is denser
medium. It is totally reflected by Canada balsam and absorbed by blackened
surfaces.
E ray has vibration in the plane of principal section
RI of the crystal for E-ray is 1.486. Calcite behaves as rarer medium. So it is
transmitted by Canada balsam and finally emerges out of the crystal as polarised
beam of light.
• Nicol as Analyser
Two nicols N1 and N2 are arranged in the line of incident ray. N1 is polariser and
gives out a plane polarised light.
(i) Two nicols are parallel, i.e. their diagonals of principal sections are parallel.
(ii) N1 and N2 are crossed. The principal section of N1 and N2 are perpendicular. Eray
becomes O-ray as its vibration are perpendicular to principal plane of N2. So the
ray is totally reflected and absorbed by side faces.
Thus intensity is zero for emerging light. Rotation of N2 can detect plane polarised
and unpolarised light. Hence N2 is called analyser.
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