6. Hallazgos y Discusión
6.2 Capítulo 2. Vereda San José. Impactos y transformaciones en el territorio
Light is one member of the family of electromagnetic radiation which forms a continuous spectrum
beyond both ends of the visible (light) spectrum (Figure 32.1). While each type of radiation has a different source, all result from electrons in atoms undergoing an energy change and all have certain properties in common.
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Properties
1 All types of electromagnetic radiation travel
through a vacuum at 300 000 km/s
(3 × 108 m/s), i.e. with the speed of light.
2 They exhibit interference, diffraction and
polarisation, which suggests they have a transverse
wave nature.
3 They obey the wave equation, v = f λ, where v is the
speed of light, f is the frequency of the waves and λ is the wavelength. Since v is constant for a particular medium, it follows that large f means small λ.
4 They carry energy from one place to another
and can be absorbed by matter to cause heating and other effects. The higher the frequency and
the smaller the wavelength of the radiation, the greater is the energy carried, i.e. gamma rays are more ‘energetic’ than radio waves. This is shown
by the photoelectric effect in which electrons are
ejected from metal surfaces when electromagnetic waves fall on them. As the frequency of the waves increases so too does the speed (and energy) with which electrons are emitted.
Because of its electrical origin, its ability to travel in a vacuum (e.g. from the Sun to the Earth) and its wave-like properties (i.e. point 2 above),
electromagnetic radiation is regarded as a progressive
transverse wave. The wave is a combination of
travelling electric and magnetic fields. The fields vary in value and are directed at right angles to each other and to the direction of travel of the wave, as shown by the representation in Figure 32.2.
electric field
magnetic field
direction of travel Figure 32.2 An electromagnetic wave
l Properties l Light waves l Infrared radiation l Ultraviolet radiation l Radio waves l X-rays
l Practical work: Wave nature of microwaves
gamma
rays X-rays ultra-violet
infra- red radiowaves TV (microwaves light 0.01 nm 1nm = 10–9m 1µm = 10–6m 1 nm 0.1 µm µm0.4 µm0.7 0.01mm 1cm 1m 1km
frequency increases frequency decreases
wavelength increases wavelength decreases
typical wavelength:
source: radioactive
matter X-raytube mercurylamp Sun electricfire microwaveoven transmitting TVand radio aerials radio)
32 electroMagnetic radiation
Figure 32.3 Infrared aerial photograph of Washington DC
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Ultraviolet radiation
Ultraviolet (UV) rays have shorter wavelengths
than light. They cause sun tan and produce vitamins in the skin but can penetrate deeper, causing skin cancer. Dark skin is able to absorb more UV, so reducing the amount reaching deeper tissues. Exposure to the harmful UV rays present in sunlight can be reduced by wearing protective clothing such as a hat or by using sunscreen lotion.
Ultraviolet causes fluorescent paints and clothes washed in some detergents to fluoresce (Figure 32.4). They glow by re-radiating as light the energy they absorb as UV. This effect may be used to verify ‘invisible’ signatures on bank documents.
Figure 32.4 White clothes fluorescing in a club
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Light waves
Red light has the longest wavelength, which is about
0.0007 mm (7 × 10−7 m = 0.7 µm), while violet light
has the shortest wavelength of about 0.0004 mm
(4 × 10−7 m = 0.4 µm). Colours between these in the
spectrum of white light have intermediate values. Light of one colour and so of one wavelength is
called monochromatic light.
Since v = f λ for all waves including light, it follows that red light has a lower frequency, f, than violet light since (i) the wavelength, λ, of red light is greater, and (ii) all colours travel with the same
speed, v, of 3 × 108 m/s in air (strictly, in a vacuum).
It is the frequency of light which decides its colour, rather than its wavelength which is different in different media, as is the speed (Chapter 29).
Different frequencies of light travel at different speeds through a transparent medium and so are refracted by different amounts. This explains dispersion (Chapter 29), in other word why the refractive index of a material depends on the wavelength of the light.
The amplitude of a light (or any other) wave is
greater the higher the intensity of the source; in the
case of light the greater the intensity the brighter it is.
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Infrared radiation
Our bodies detect infrared radiation (IR) by its
heating effect on the skin. It is sometimes called ‘radiant heat’ or ‘heat radiation’.
Anything which is hot but not glowing, i.e. below 500 °C, emits IR alone. At about 500 °C a body becomes red hot and emits red light as well as IR – the heating element of an electric fire, a toaster or a grill are examples. At about 1500 °C, things such as lamp filaments are white hot and radiate IR and white light, i.e. all the colours of the visible spectrum.
Infrared is also detected by special temperature- sensitive photographic films which allow pictures to be taken in the dark. Infrared sensors are used on satellites and aircraft for weather forecasting, monitoring of land use (Figure 32.3), assessing heat loss from buildings, intruder alarms and locating victims of earthquakes.
Infrared lamps are used to dry the paint on cars during manufacture and in the treatment of muscular complaints. The remote control for an electronic device contains a small infrared transmitter to send signals to the device, such as a television or DVD player.
radio waves
b) VHF (very high frequency) and
UHF (ultra high frequency) waves
(wavelengths of 10 m to 10 cm)
These shorter wavelength radio waves need a clear, straight-line path to the receiver. They are not reflected by the ionosphere. They are used for local radio and for television.
c) Microwaves (wavelengths of
a few cm)
These are used for international telecommunications and television relay via geostationary satellites and for mobile phone networks via microwave aerial towers and low-orbit satellites (Chapter 9). The microwave signals are transmitted through the ionosphere by dish aerials, amplified by the satellite and sent back to a dish aerial in another part of the world.
Microwaves are also used for radar detection of
ships and aircraft, and in police speed traps.
Microwaves can be used for cooking since they cause water molecules in the moisture of the food to vibrate vigorously at the frequency of the microwaves. As a result, heating occurs inside the food which cooks itself.
Living cells can be damaged or killed by the heat produced when microwaves are absorbed by water in the cells. There is some debate at present as to whether their use in mobile phones is harmful;
‘hands-free’ mode, where separate earphones are used, may be safer.