Slideshow on Polarisation for IB HL Waves Phenomena; also SL option on Sight and Waves. Flippingphysics by Nothingnerdy

1. presents
a production
Polarisation
1

2. EM wave
An electromagnetic wave consists of sinusoidally
varying electric and magnetic fields which
oscillate at right angles to each other. The wave
energy travels perpendicularly to these fields.
Electric
field
Wave
energy
Magnetic
field

3. Plane polarisation
Light from most sources eg
the Sun has its electric and
magnetic fields in all
showing only E-field
directions perpendicular to
the direction of travel.
If the electric field only
vibrates in one direction (with Polarised
the magnetic field at right
angles to it), the wave is
polarised.

4. Polaroid
A plastic which polarises light as it passes through it.
If the plane of the
If the plane of the If the plane of the
polaroid is parallel
polaroid is polaroid is at an
to the electric
perpendicular to angle to the
field, all of the
the electric field, electric field, only
energy of a
none of the wave some of the wave
polarised wave
energy passes energy passes
passes through.
through. through.

5. Reflected light
When light is reflected from a surface it is reflected
and refracted and both waves are partly polarised
Unpolarised light
Incident Reflected part
Plane-polarised light unpolarised polarised
Light polarised in other plane Refracted
part polarised

6. Polaroid

7. Brewster angle
Light reflected from a
surface is partly polarised.
At a certain angle, known
as the Brewster angle,
when the refracted and
reflected rays are at 90º,
the reflected ray is totally
polarised.
eg Brewster angle of
water (n = 1.33),
where ø is the Brewster angle
ø = tan-1(1.33) = 53.1º
and n is the refractive index

8. Polarisation filters The unpolarised light
Polarising passes through a polarising
filter filter and the intensity is
reduced to 50% whatever
the angle of the filter.
Light
direction
2 parallel
filters
If the second filter is
aligned with the first one,
all of the light polarised by Light
the first filter is transmitted direction
by the second one.
Images: lectureonline

9. Filters at an angle
2 perpendicular 2 angled
filters filters
Light Light
direction direction
If the second filter is at 90º If the two filters are at an
to the first one, none of the angle to each other, only
light polarised by the first some of the light polarised by
filter is transmitted by the the first filter is transmitted
second one. by the second one.
Images: lectureonline

10. Malus’ law
Calculates intensity of polarised light
incident on a filterI = intensity of emerging
light;
I0 = intensity of light
incident on second filter;
ϑ = angle between
filters
If 2 filters are at 45º to
each other, and I0 = 400
Wm-2, I = 400*cos245 =
Image: fsu 400*0.7071 2 = 200 Wm-2
First filter is called polariser; second filter is analyser

11. Optical activity
Optically active polariser analyser
substances (eg strong
no light
sugar solution) rotate the unpolarised vertically polarised
plane of polarisation. This
Perpendicular
can be demonstrated by
filters block all light
placing sugar solution
between a polariser and
an analyser which will Light with rotated plane
change the intensity of of polarisation emerges
the emerging light.
unpolarised vertically polarised rotated plane some light emerges
sugar solution

12. Stress analysis
Some plastics are optically active. When they
experience forces, they rotate the plane of
polarisation of different colours by different amounts.
We can analyse the forces from the colours.

13. Liquid Crystal
A liquid crystal display such as the one in a laptop screen, rotates
the plane of polarisation by 90º. When it is under a voltage,
however, it does not. The screen contains polarisers at right angles.
No voltage: backlight
transmits through
first polariser, liquid
crystal (turns light),
and perpendicular
polariser.
Voltage: no rotation
Image: bestgamingmonitor
at crystal, so the light
does not pass the By applying voltages across different
second polariser. pixels, dark areas construct the images.

14. Practice problems
What is the Brewster angle of a diamond
with a refractive index of 2.42? [67.5º]
Two polarisers are oriented at 50º to
each other. What proportion of the light
intensity is transmitted? [0.41]
Two polaroids are aligned so the light passing
though them is a maximum. At what angle
should one be placed so that the intensity is
reduced to a) half? b) 10%? [a)45º;b)72º]

15. a production
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