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Calderglen High School
WAVES 2
EM SPECTRUM
SUMMARY NOTES
Diffraction
When you watch waves entering a harbour you may see them spreading out after they go
through the gap in the harbour wall
Television signals are much more difficult to receive in hilly areas than radio signals
You may see a halo of colour around street lamps on a foggy night
CDs and DVDs will show beautiful rainbow patterns if held up to the light
If you are in a room with the door open you may be able to hear someone speaking outside
although you cannot see them
If you look at a street lamp through an umbrella you will see an interesting pattern of coloured
dots
All these effects are due to a property of all waves called diffraction.
Diffraction is the bending of waves round an obstacle or through a hole.
You can show diffraction effects with water waves in a ripple tank, microwaves, sound and even light. The
patterns for some water wave diffraction are shown in the diagrams.
They show that:
(a) the diffraction effects are greater for long wavelength waves
(b) the diffraction effects are greater for small holes
Wide gap – small diffraction effect
Narrow gap – large diffraction effect
Large wavelength – large diffraction
effect
The next diagram shows a wave bending round the edge of a barrier. This explains why it is possible to
hear sounds and receive radio signals even if there is something between you and the source of the waves.
If the wavelength of the waves is shorter the spreading, diffraction, effect is much smaller as well. This
explains why television waves are much more difficult to receive in hilly areas than radio waves which have
a longer wavelength and why the diffraction of light is so difficult to observe.
Shorter wave TV signals cannot diffract
around the hills by the same amount.
Radio reception in a hilly area
Refraction
Refraction is the bending of a wave as it passes from one medium to another. Refraction of visible light
occurs when the two medium have different refractive indexes (n).
When light travels from a lower refractive index medium to one of higher refractive index the ray is
refracted towards the normal and vice versa.
ϴi
ϴr
ϴr
Refracted angle
Incident angle
ϴi
Refracted ray
Incident ray
The Law of Refraction.
We consider air to be our reference refractive index so in the following equation refractive index n is the
value for the other material. Perspex, glass, water etc.
n=
Example: A green laser beam strikes a glass block at an angle of 30° to the normal.
Calculate the size of the refracted angle, inside the block, if glass has a refractive
index of 1.4.
ϴi = 30°
ϴr = ?
n = 1.4
n=
1.4 =
=
=
= Sin-1 0.357
= 20.9°
Critical angle
When light moves from a higher refractive index to a lower one the angle of incidence in the denser of two
media reaches a certain (critical) angle ϴc, the light no longer refracts outward, but instead travels along
the surface (see diagram).
90°
ϴc
n=
since Sin 90 = 1
Total internal reflection occurs when the angle of incidence in the higher refractive index of the two media
is greater than the critical angle and light is reflected back into this medium.
≥ϴc
≥ϴc
Applications of Refraction and Total Internal Reflection
Convex lenses
Concave lenses
Using a 90° prism to turn a ray of light through (i) 900 (ii) 1800
Uses include cameras, binoculars,
periscopes etc.
Used in reflective clothing road signs
and car/bike reflectors.
Optical Fibres
1. An optical fibre consists of a glass pipe coated with a second material of lower refractive index.
2. Light enters one end of the fibre and strikes the boundary between the two materials at an angle greater
than the critical angle, resulting in total internal reflection at the interface.
3. This reflected light now strikes the interface on the opposite wall and gets totally reflected again.
4. This process continues all along the glass pipe until the light emerges at the far end.
Note:
 If the second cladding material wasn’t there or had a refractive index greater than that of the central
core total internal reflection would not occur and the light would simply escape out.
 The outer cladding also acts as a protective layer against scratches etc.
Applications
 Telecommunications
 Medicine (endoscopes)
Advantages of optical fibres over copper conductors
Less interference / boosted less often / cheaper raw material / occupy less space / more information
(carried) in the same space / flexible for inaccessible places/ do not corrode
Light Source
Transmitted Light
Low refractive index glass
High refractive index glass
Path of light ray
Total Internal Reflection
Protective coating
Spectra Production by Refraction and Diffraction.
The wave properties of refraction and diffraction are said to be wavelength dependent. This means the
amount of each effect is different for different wavelengths. The effect of refraction on white light is seen
when a rainbow is made and it become separated into its component colours. A rainbow is an example of a
continuous spectrum and the effect can be demonstrated using a triangular prism.
refracted most
A continuous spectrum can also be produced by diffraction using a bit of equipment called a
spectroscope.
spectroscope
Violet
Blue
Green
Yellow
Orange
Red
The order of the
spectrum is reversed
because longer
wavelengths diffract
more than short
wavelengths.
The discovery that white light was in fact made up from individual colours led to the idea that there might be
even more waves like light that extended beyond the red and blue ends of the visible spectrum but which
could not be seen. In fact visible light is only a tiny part of a much bigger group of waves we call the
electromagnetic spectrum. Not only that, but the type of electromagnetic wave given out by an object
related to its temperature on the absolute temperature scale measured in Kelvin. This sort of information is
of particular use to astronomers who can determine the temperature of stars buy analysing the
electromagnetic spectra given out.
Radio waves have the longest wavelength in the electromagnetic spectrum. These
waves carry the news, ball games, and music you listen to on the radio. They also
carry signals to television sets and cellular phones.
Microwaves have shorter wavelengths than radio waves, which heat the food we eat.
They are also used for radar images, like the Doppler radar used in weather forecasts.
Infrared waves have either long wavelengths or short wavelengths. Infrared
waves with long wavelengths are different from infrared waves with short
wavelengths. Infrared waves with long wavelengths can be detected as heat. Your
radiator or heater gives off these long infrared waves. We call these thermal infrared
or far infrared waves. The sun gives off infrared waves with shorter wavelengths.
Plants reflect these waves, also known as near infrared waves.
Visible light waves are the only electromagnetic waves we can see. We see these
waves as the colours of the rainbow. Each colour has a different wavelength. Red has
the longest wavelength and violet has the shortest wavelength. These waves combine
to make white light.
Ultraviolet waves have wavelengths shorter than visible light waves. These waves
are invisible to the human eye, but some insects can see them. Of the sun's light, the
ultraviolet waves are responsible for causing our sunburns.
X-Rays: As wavelengths get smaller, the waves have more energy. X-Rays have
smaller wavelengths and therefore more energy than the ultraviolet waves. X-Rays
are so powerful that they pass easily through the skin allowing doctors to look at our
bones.
Gamma Rays have the smallest wavelength and the most energy of the waves in the
electromagnetic spectrum. These waves are generated by radioactive atoms and in
nuclear explosions. Gamma rays can kill living cells, but doctors can use gamma rays
to kill diseased cells.
Emission Line spectra
When atoms of elements are heated they produce a unique colour spectrum called a
line spectra which is like a finger print for that element, no two elements have the
same line spectra. These spectra are not made from continuous ranges of colours but
specific wavelengths which appear as distinct lines. Astronomers use this to identify
the elements present in stars. Stars A and B have different spectra can you work out
what elements they contain?