AH 3.3.7 Polarisation Flashcards
Copy and complete the following paragraph.
Electromagnetic waves are so called because they consist of two component waves: an …………………… field component and a ………………….. field component. The wave direction is at …………….. angles to both components which are at ……………….. angles to each other i.e. all three are orthogonal.
The ……………………. field component is stronger than the …………………….. and so we regard this as the dominant wave. The direction of the …………………….. field oscillation is called the ………………………………….. (direction) of the EM wave.
Electromagnetic waves are so called because they consist of two component waves: an electric field component and a magnetic field component. The wave direction is at right angles to both components which are at right angles to each other i.e. all three are orthogonal.
The electric field component is stronger than the magnetic and so we regard this as the dominant wave. The direction of the electric field oscillation is called the polarisation (direction) of the EM wave.
a) Sketch an electromagnetic wave showing the oscillation of the Electric and Magnetic fields as well as the direction of energy transfer.
b) Indicate the polarisation direction.
See the diagram below.
The diagram shows a wave plane (linearly) polarised in the x-direction in the plane of the page.
a) Another wave is found to be unpolarised. What does this mean?
b) Sketch a diagram to represent an unpolarised wave.
a) An unpolarised wave’s electric field oscillates in all possible directions within a plane.
b) See the diagram below.
a) Can longitudinal waves be polarised?
b) Explain your answer to (a).
a) No - longitudinal waves cannot be polarised.
b) Since, for a polarised wave, the wave direction is perpendicular to the plane of polarisation, it follows that only transverse waves can be polarised.
The diagram shows two pieces of polaroid film used as a polariser and an analyser.
Explain why
a) No light is transmitted through the analyser in arrangement A.
a) Light is transmitted through the analyser in arrangement B.
a) In arrangement A the polariser selects one polarisation direction. This light is polarised in the x-direction as shown.
This light has no component at all aligned with the transmission axis of the analyser so no light is transmitted.
b) In arrangement B the polariser selects one polarisation direction. This light is polarised in the x-direction as before.
But now the transmission axis of the analyser is parallel to the polarisation direction so this light is transmitted.
The diagram shows two pieces of polaroid film. In the left-hand arrangement the transmission axes of each film are parallel. Light polarised in this direction is therefore transmitted. In the right-hand arrangement the transmission axes of each film are perpendicular. No light is therefore transmitted.
Describe and explain how light transmission varies as the analyser is rotated through 180o, beginning with the arrangement on the left.
Starting position
The transmission axes of polariser and analyser are parallel. So light polarised in this direction is transmitted. There is maximum light transmssion.
Analyser is rotated 0-90o
The component of the analyser’s transmission axis parallel to that of the polariser decreases so the irradiance of transmitted light decreases. After a 90o rotation there is zero light transmission as there is now no component of the analyser transmission axis parallel to that of the polariser.
Analyser is rotated 90-180o
The component of the analyser’s transmission axis parallel to that of the polariser now increases so the irradiance of transmitted light increases. After a 180o rotation there is maximum light transmission again as the transmission axes of polariser and analyser are again parallel.
Look at the diagram below. Comment on the polarisation of
a) The incident light
b) The reflected light
c) The refracted light
a) Unpolarised (i.e. polarised in all directions)
b) 100% polarised (i.e. polarised in one single direction)
c) Partial polarisation (i.e. Electric Field components stronger in some directions compared to others)
a) What is meant by the polarising angle ip?
b) State another name for this angle ip
A beam of white light is incident on a flat glass surface at an angle of 56o. The reflected beam is plane polarised.
c) Calculate the angle of refraction in the glass
d) Calculate the refractive index of the glass.
a) When light is incident upon a boundary between air and an electrical insulator, the polarising angle ip, is the angle of incidence in air which causes the reflected light to be linearly polarised.
b) The Brewster angle.
c) r = 90-56 = 34o
d) ng = sin i / sin r = sin 56 / sin 34 = 1.48
Look at the diagram below then copy ands complete the text.
Unpolarised Incident Light reflected from the surface of an
electrical ………………….. is ………………………….. and may be fully …………………………. depending on the …………………………………. index of the material and the incident ………………….
In the diagram the shorter arrows indicate ……………………… polarisation.
Unpolarised Incident Light reflected from the surface of an electrical insulator is partially and may be fully polarised depending on the refractive index of the material and the incident angle.
In the diagram the shorter arrows indicate partial polarisation.
NOTE THIS THIS IS A FORMAL SQA DERIVATION AND SHOUDL BE MEMORISED
Derive the relationship between the polarising angle and the refractive index.
See the diagram below.
