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Optics
Microscopes, contact lenses, holograms, telescopes, optical fibres, sunglasses ... all rely on
understanding the nature of light and making use of the ways we can change its path.
The three experiments in the Optics section of the course will introduce you to three main ideas.

Prism spectrometers are used to measure the wavelengths of light emitted by a sample.
The key to its operation is a glass prism, which disperses light into a spectrum. Experiment
1 develops your understanding of how the prism spectrometer works, as well as the skills
necessary to using it - adjustment of the components which make up the spectrometer and
reading its vernier scales. You may use the spectrometer to measure the wavelength of
light emitted by glowing hydrogen gas - in doing this you will be reproducing the
experiment which provided one of the first glimpses of the quantisation of energy in the
atom.

Many optical instruments exploit the change in the path of light that occurs when it passes
from one medium to another. The Thin Lenses and Microscope exercise will provide
you with an understanding of how lenses achieve magnification, either singly or in pairs.
After a two year gap in your study of optical properties this experiment also provides an
opportunity for you to consolidate your understanding of what an image is.

The wave properties of light show up when you place polarising filters in its path. You will
explore what polarisation means and some of its intriguing uses in the experiment
Polarisation of Light. Here’s where you begin to understand how some sunglasses work
and why Polaroid sunglasses are particularly useful at the beach.
The physics you encounter in this segment is not covered in lectures. This is a deliberate decision,
taken because the ideas and skills are learnt most effectively in the lab. Understanding how a
spectrometer works needs time working with the real object. Ray tracing diagrams and lens
formula calculations are no substitute for the understanding of magnification that comes from
observing and measuring the effect of lenses on the appearance of an object. For this reason this
material in your physics course is taught in the laboratory rather than in lectures.
The introductory preparation section that begins on the next page is intended to remind you
about the properties of light. It should be reviewed before you arrive for your first optics
experiment. In addition there are preliminary exercises within the first exercise that should be
completed before the first optics lab session.
Physics 121/2 and 141/2 Laboratory Manual
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Introductory Preparation
It’s probably been a while since you thought much about optics. The following background
information is intended to help you revise your understanding of the properties of light before
embarking on the Optics experiments. Make sure that you discuss any points you do not
understand with your demonstrator.
Properties of Light
Waves, with speed, frequency and wavelength
"Light" is a term for those electromagnetic waves that can be detected by the human eye. As with
all waves, their speed, v, frequency, f, and wavelength, , are related by
v = f ,
where the speed of light in a vacuum
-1
c = 3.00 x 108 ms .
The frequency range of visible light is from about 4 x 1014 to approximately 7.8 x 1014 Hz, and the
colours you see depends on the frequency:
red
:
(4.0  4.7) x 1014 Hz
orange
:
(4.7  5.0) x 1014 Hz
yellow
:
(5.0  5.4) x 1014 Hz
green
:
(5.4  6.0) x 1014 Hz
blue
:
(6.0  6.6) x 1014 Hz
violet
:
(6.6  7.8) x 1014 Hz
White light is made up of all these frequencies.
When light travels through a medium other than just a vacuum, it slows down. In air we can
neglect the difference and still take v = c. (The speed of 589.3 nm light in air at standard
temperature and pressure is 0.9997 c. ) For other materials, eg. glass and water, v < c. The speed
of light in the medium depends on:
1.
the frequency of the light (ie. the colour) and;
2.
the electrical and structural properties of a material.
The frequency, f, is just the number of "wave crests" passing any point in each second, so the
frequency stays constant as the wave travels from one medium to another. Hence in order for the
equation v = f to remain true, the wavelength must vary within the new medium.
As light travels from air, medium 1, into medium 2 (eg. glass), then
and
O-2
v1
=
c
=
v2
=
f 2,
f 1
Physics 121/2 and 141/2 Laboratory Manual
so
1
2
=
v1
=
v2
c
=
v2
n
>
1.
c/v is called the refractive index, n, of a material. As you can see, n = 1 for a vacuum (or air, to a
good approximation) and n > 1 for anything else. Of course, if the light travels out again, ie. from
the glass (now medium 1) to air (now medium 2), then
2
1
=
v2
=
v1
v2
=
c
1
n
<
1
and so the wavelength increases again.
Some typical refractive indices are:
Crown Glass
1.52
Ice
1.31
Diamond
2.419
Water
1.333
Flint Glass
1.62
Carbon Dioxide
1.00045
(solids and liquids at 20C, gases at 0C and 1 atm)
The refractive index allows you to calculate the change in wavelength as light travels from one
medium to another. But it is more useful than that...
Light changes direction
Imagine light waves spreading out from a point source (a small light globe will do) towards a
parallel plane of glass (eg. a window). We could sketch representative wave crests :
From the diagram it can be seen that the curvature of the wavefronts decreases as the light
enters the medium. The centre of the wavefront enters the glass and is slowed down. The wings
of the wavefront, however, are still propagating through air at a greater speed. This enables the
wings of the wavefront to catch up with the centre, flattening the wavefront. The reverse process
occurs upon exiting the glass.
Physics 121/2 and 141/2 Laboratory Manual
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Instead of drawing wave crests, we could construct lines perpendicular to the crests, which would
indicate the direction in which the light was travelling at any point.
normal to
glass surface
air
glass
air
Note that rays entering the glass are bent towards a normal to the surface. Rays travelling from
glass to air are bent away from a normal to the surface. This is an example of refraction. If we
look at one of these cases in more detail:
normal
1
air
glass
(refractive index, n)
2
For the ray entering the glass 1 is called the angle of incidence and 2 is called the angle of
refraction.
It is possible to show that
sine of angle in medium 2
sine of angle in medium 1
=
refractive index of medium 1
refractive index of medium 2
n glass
sin  1

sin  2
n air
Applying this to the rays shown in the diagram:
This is known as Snell's law.
Since the refractive index of air is sufficiently close to one for the experiments we perform we can
write:
sin  1
n
=
1
sin  2
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Physics 121/2 and 141/2 Laboratory Manual
Dispersion of light
The velocity of light (and hence refractive index) of a material varies depending on the
wavelength of the incident light. For example, double extra dense flint has vred=0.522c and
vblue=0.511c. The consequence of a medium having different refractive indices for different
wavelengths is that multicoloured light passing through the material will be separated into its
component colours.
Two simple cases are drawn below (only rays for the red and blue components of white light have
been shown, and the differences between the red and blue light paths have been greatly
exaggerated).
As you can see, in the case of a
prism, the light diverges into a
spectrum of colours. Red light is
deviated the least.
blue
red
The lens has focused the light to
a point; the position of this point
depends on the colour.
parallel beam
of white light
red
simple converging
lens
blue
If you have an optical bench in front of you (west end), start the Thin Lenses and Microscopes
experiment; if there is a spectrometer on your bench (east end) begin the Prism Spectrometer
experiment which follows.
Physics 121/2 and 141/2 Laboratory Manual
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Physics 121/2 and 141/2 Laboratory Manual