The Wave Nature of Light
The English Physicist Thomas
proved that light was made of
.
To Demonstrate the Wave nature of Light (Young’s
Experiment)
A laser is used as it is a monochromatic light source (light that is not a mixture of different
).
To show the Wave nature of Light
Shine light from the laser on to Young’s Slits .
It undergoes
at each of the slits and both act as two
sources.
On the screen (where light from each overlaps)
and
occurs.
The result is that a series of bright lines are seen either on the screen.
Conclusion: The fact that light undergoes
and Interference tells us that light travels as a wave.
Interference Colours can be seen on petrol films and soap bubbles, due to the
of light waves which have been
from the different interfaces.
The Diffraction Grating
A diffraction grating consists of a piece of
material on which a very large number of opaque (
) parallel lines are engraved.
The distance between two
slits is referred to as ‘the slit width’ or ‘the
constant’. its symbol is
.
In general, if a grating has n lines per mm therefore d = 1/ n
or
if a grating has n lines per m therefore d = 1/ n metres
In the exam you will often be told in a question that the grating has n lines per mm,
so multiply by
to get the number of lines per metre, then just invert to get d, the grating constant.
Formula for a diffraction grating
n λ = d Sin θ
n =
(first order, second order etc.)
λ =
d = distance between
(slit width)
θ =
between
through position and the order in question.
Polarisation
A Polarised wave is a wave which
in one
only.
To Demonstration Polarisation using two polaroids
• Light from an incandescent source (something which emits light when heated) is unpolarised, i.e. the
and
fields are
in many different planes.
•
If light from such a source is passed through a substance called a
Polaroid the emerging rays are now polarised, i.e. oscillate in
plane only.
• If this light is then passed through a second polaroid, it only gets through if the second polaroid is
to that of the first.
• If the second polaroid is then rotated through
degrees, no light gets through.
Note: only
waves can be polarised so the fact that light can be polarised shows that
is a transverse wave.
Applications:
, Stress Polarisation (used to detect faults or stresses in materials)
Dispersion
Dispersion is the
out of the different colours present in
light.
Dispersion can be brought about by either a
or a
grating.
Dispersion is the principle behind the array of colours seen in
, polished gemstones and on surface of CDs.
Dispersion due to a Diffraction Grating
A Diffraction Grating causes
because from the formula n λ = d Sin θ if
is different, θ will be different,
i.e. different colours are
by different amounts.
From this we can see that the colour with the largest wavelength (
) gets deviated the most.
Dispersion due to a Prism
A Prism causes dispersion because the
Index of the medium is slightly different for different wavelengths, therefore each
gets refracted (bent) by a different amount.
In this case Blue gets deviated the most.
Recombination
If a given prism is used to disperse white light, a second identical –
- prism can be used to recombine the components back into
light
.
Primary and Secondary Colours
Primary Colours
The primary colours are three colours such that when
in equal intensity produce
light.
The three primary colours are
, Green and
.
Secondary Colours
When two
colours are mixed in equal intensity, the colour formed is a
colour.
, Cyan and Magenta are the three secondary colours.
Complementary Colours
Complementary colours are pairs of colours consisting of a
and a
colour, such that when combined they give
light
.
The
fact that any given colour can be produced from a combination of the
three primary colours means that only these three coloured-bulbs are
needed in televisions or in Stage Lighting kits.
The Electromagnetic Spectrum
Frequency
→ going from left to right
Radio Micro Infrared Visible Ultraviolet X-ray Gamma ( Different to book )
Going left to left ← Wavelength
All Electromagnetic waves travel at the speed of
and undergo typical wave properties such as
and diffraction.
The colours of visible light (in order of increasing frequency) are
, Orange, Yellow,
, Blue, Indigo,
.
Red has the lowest frequency and Blue one of the highest
Ultra-Violet Radiation
Characteristics
1. Is an electromagnetic wave
2. Causes objects to fluoresce
3. Can be detected by
plating
Infra-Red Radiation
Characteristics
1. Is an electromagnetic wave
2. Can be detected with a
-sensitive camera e.g. ‘night-vision’ cameras.
Applications of Infra-Red technology
Infra-red camera (used in
goggles)
The Greenhouse Effect
The
Earth receives energy from the Sun in the form of radiation, most of
which is in the visible wavelength region and which passes through the
Earth’s atmosphere on the way to the planet’s surface. Some of this
radiation then gets
(re-radiated) off the surface of the Earth as infra-red radiation. Much
of this radiation, which would otherwise radiate back out to space,
gets absorbed by the
(by carbon dioxide, water vapour and methane) and as a result the
heats up.
Mandatory Experiment:
To Measure the wavelength of Light