AQA A Physics AS - Chapter 13 Optics Revision Notes


These are the notes for AS Physics AQA A specification chapter 13 - optics

you can use them with my chapter 12 - waves notes (they have also been uploaded)

hope you find them useful...

i'm sorry if i've made any mistakes (please feel free to correct me! :D)

good luck :)

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  • Created by: ADB :)
  • Created on: 31-03-11 11:21
Preview of AQA A Physics AS - Chapter 13 Optics Revision Notes

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Refraction of Light
The wave theory of light can be used to explain reflection and refraction of light. However,
when we consider the effect of lenses or mirrors on the path of light, we usually prefer to
draw diagrams using light rays. Light rays represent the direction of travel of wavefronts.
REFRACTION is the change of direction that occurs when light passes non-normally across a
boundary between two transparent substances.
The diagram to the left shows the change of direction of
a light ray when it enters and exits a glass block in air.
At a boundary between two transparent substances,
the light ray bends:
Towards the normal if it passes into a more
refractive substance (air glass) light ray slows down
Away from the normal if it passes into a less
refractive substance (glass air) light ray speeds up
No refraction takes place if the incident light ray is along the
Any part of the electromagnetic spectrum has a
frequency that decides what type of wave it is. This
frequency does not change when the wave is refracted.
If the speed of the wave is reduced the wavelength of
the wave must therefore also be reduced as:
Speed = Frequency x Wavelength
So the wavelength of blue light in air will be slightly
longer than the wavelength of blue light in glass.
Snell's Law
So we know that waves slow down when they enter optically denser materials, and bend
towards the normal line.
If we label the angle of incidence as (i) and the angle of refraction as (r), then it can be
shown that when travelling from a vacuum into a material, the ratio:

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This is Snell's Law.
We call the constant from Snell's law the refractive index (n)
The refractive index always has a value greater than 1 (Refractive index of air is 1)
All transparent materials have a refractive index. It shows how well the material refracts
light. The index is always given for the case of light crossing from a vacuum into the
material.…read more

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Can be rearranged to give
c1 sini = c12 sinr
Multiplying both sides of this equation by (c)
c c
c1 sini = c2 sinr
Substituting n1 for c1
where n1 is the refractive index of substance 1
And n2 for c2 where n2 is the refractive index of substance 2 gives
n1sin(i) = n2sin(r)
The speed of light is 3.00x108 ms-1 in a vacuum. A certain type of glass has a
refractive index of 1.62.…read more

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The glass prism refracts light by different amounts, depending on its wavelength.
The shorter the wavelength in air, the greater the amount of diffraction.
Total Internal Reflection
Remember: Partial
Internal Reflection always occurs at a boundary when the angle
of incidence is less than or equal to the critical angle.
CRITICAL ANGLE: the angle of incidence of a light ray must exceed the critical angle for
total internal reflection to occur.…read more

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OPTICAL FIBRE is a thin flexible transparent fibre used to carry light pulses from one end
to the other.
Communications Optical Fibre:
A communications optical fibre allows pulses of light that enter at end, from a
transmitter, to reach a receiver at the other end. Such fibres need to be highly transparent
to minimise absorption of light.
Each fibre consists of a core surrounded by a layer of cladding of a lower refractive index.…read more

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Spectral Dispersion: The speed of light in glass depends on its wavelength.
Therefore, if white light is used, violet light travels more slowly than red light. The
difference in speed would cause white light pulses to become longer and merge
into each other.
Spectral Dispersion is prevented by using a monochromatic source of light
Medical Endoscope
Medical endoscope contains two bundles of fibres:
The endoscope is inserted into a body cavity,
which is then illuminated using light sent through one
of the bundle fibres.…read more

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Double Slit Interference
The single slit causes diffraction of the light so that the same wavefront illuminates both of
the double slits. Therefore, the double slits act as coherent sources because the light
emerging from them is from the same wavefront.
Diffraction at the double slits causes the two sets of waves to overlap. Interference takes
place where they overlap.
A series of equally spaced, parallel, light & dark (called FRINGES) are formed which can
be viewed through a travelling microscope.…read more

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Constructive Interference: occurs when the path difference is zero or a whole number of
wavelengths so that the signals arrive in phase. (At a bright fringe)
P ath Difference = n
Destructive Interference: occurs when the path difference is an odd number or half
wavelengths. (At a dark fringe)
P ath Difference = (2n + 1)2
At the centre of the pattern the path difference is zero.…read more

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At the first fringe either side of the pattern, the path difference is
one wavelength. For the second fringe either side of the pattern,
the path difference is two wavelengths, etc.
To measure the fringe separation (w) measure across several fringes from the centre of a dark fringe to the
centre of another dark fringe, because the centres of dark fringes are easier to locate that the centres of
bright fringes. Obtain (w) by dividing your measurement by the number of fringes you measure across.…read more

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Laser is a convenient source of COHERENT light: When used during the double
slits investigation, we can illuminate both slits directly using the laser. Unlike
non-laser sources where we have to pass it through a narrow single slit first.
White Light Fringes
White light has all the wavelengths present in it. Each wavelength (colour) will
form its own fringe system. The fringe systems for different colours will have different
fringe widths.…read more


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