Download Optics - Haiku

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What is optics?
Optics is the study of the nature
and behaviour of visible light,
along with its interactions with
matter.
These interactions include
reflection, refraction, total
internal reflection and
diffraction.
This definition can be extended
to encompass the family of
waves to which light belongs:
the electromagnetic spectrum.
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Properties of EM waves
Electromagnetic (EM) waves travel as oscillations in electrical
and magnetic fields, and can transmit energy through a
vacuum. They are always transverse waves.
electric field
magnetic field
In a vacuum, EM waves travel at the speed of light
(299,792,458 ms-1) but they slow down in different
substances.
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The EM spectrum
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Lasers as light sources
Lasers are often used as light sources because their light is:

monchromatic – emitted
with only one wavelength

coherent – all waves
are either exactly in
phase or display a
constant phase
difference

collimated – a narrow, approximately parallel, beam
Other light sources, such as the Sun and light bulbs, are
polychromatic, incoherent and uncollimated.
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Electromagnetic spectrum: a summary
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What is refraction?
Refraction is the change of direction a light ray undergoes
when it enters a medium with a different optical density.
Light travels at different
speeds in materials with
different optical densities,
and the change in
direction occurs due to the
change in the speed of the
light.
In a vacuum, light travels at
299,792,458 ms-1.
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Why does refraction occur?
As a light ray enters a
medium that is more
optically dense, it slows
down and bends
towards the normal.
As a light ray enters a
medium that is less optically
dense, it speeds up and
bends away the normal.
In this diagram:


i1
normal
air
glass
r1
normal
i2
r2
i1 > r1

i1 = r2

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incident
ray
i2 < r2
r1 = i2
refracted
ray
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Investigating refraction
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Refractive index
The speed of light in a particular substance is expressed as
the refractive index (n) of that medium.
refractive index
=
of substance (ns)
medium
speed of light in vacuum (c)
speed of light in substance (cs)
speed of light (ms-1)
refractive index (n)
vacuum
air
299,792,458
299,702,547
1
1.0003
water
224,900,569
1.333
glass
198,538,051
1.51
diamond
123,933,393
2.419
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Snells’ law of refraction
Refractive indices can be used to make predictions about
refraction.
θ1
normal
medium 1: refractive index = n1
θ2
medium 2: refractive index = n2
Snell’s law of refraction states:
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n1sinθ1 = n2sinθ2
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Refraction calculations
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Total internal reflection
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Finding the critical angle
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Optical fibres and TIR
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Structure of optical fibres
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TIR and the critical angle: a summary
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What is diffraction?
Diffraction is the spreading out
or bending of waves as they
pass through a gap or around
an obstacle.
All types of waves can be
diffracted, but the amount
depends on the ratio of the
wavelength to the size of
the opening or obstacle.
Diffraction is greatest when the
wavelength is approximately the
same as the width of the gap.
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When light passes
through a narrow slit,
a pattern of alternate
bright and dark
fringes is produced.
This is a single slit
diffraction pattern.
The intensity of the
fringes against
distance from the
centre can be
plotted on a graph.
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intensity
Single slit diffraction
distance from centre
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Single slit diffraction pattern intensity

The central maximum
is twice as wide as
the other fringes.

The central maximum
is much brighter than
the other fringes.

intensity
The intensity of a single slit diffraction pattern displays some
important features:
distance from centre
The pattern becomes more spread out if a narrower slit
and larger wavelength of light are used.
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Diffraction gratings
A diffraction grating
is a plate with many
closely spaced parallel
slits. It produces
widely spaced
interference patterns
as a result of the
superposition of waves
from the many slits.
The pattern from a diffraction grating consists of brighter and
sharper fringes than the pattern from a double slit arrangement.
The pattern is much clearer, which makes it possible to
calculate the wavelength of the light more accurately.
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The diffraction grating equation
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The diffraction grating equation
The diffraction grating equation:
nλ = dsinθ
Where d = slit spacing (equivalent to 1 / number of slits per m).
Fractions of a degree are measured in minutes ('),
where 1° = 60'.
The maximum number of orders, n, can be found by
substituting θ = 90° into the equation. Given that sin90° = 1:
n = d / λ, rounded down the nearest whole number.
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The spectrometer
A spectrometer is an important application of diffraction
gratings. It used to measure wavelengths of light very
accurately.
This has many applications, such as determining the
chemical compositions of stars and hence showing the ‘red
shift’ evidence for the expanding universe
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Investigating diffraction gratings
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Diffraction grating calculations
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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