Radiation
Infra-red radiation transfers heat between all objects.
Infra-red radiation is an electromagnetic wave and can
travel through a vacuum.
Heat from the Sun reaches us through the vacuum of
space by travelling as infra-red radiation.
These heaters emit radiant heat
energy.
They are useful for
heating large areas from
above, especially outdoors
where hot air would be
blown away easily.
An object can absorb (take in), emit (give out) and reflect
radiation.
A hot object will emit more infra-red radiation than it absorbs.
A cold object will absorb more infra-red radiation than it emits.
In this way heat is transferred from hotter to colder objects.
An object whose temperature does not change
will emit infra-red radiation at the same rate as it is absorbed.
Objects which are at the same temperature as each other
will absorb, emit and reflect infra-red radiation at different rates
depending on the type of surface which the object has.
Why is it hotter inside a
black car in sunshine,
than inside a white car?
Radiation - Surfaces - Absorption
and Emission.
An object with a matt (dull) surface will absorb and emit
infra-red radiation at a faster rate than an object with a
shiny surface.
An object with a dark surface will absorb and emit
infra-red radiation at a faster rate than an object with a light
surface.
Radiation is also reflected at the surface.
Matt, black objects ABSORB and EMIT more heat
than light-coloured, shiny ones.
Imagine a hot piece of metal which has a matt black
surface and a shiny white surface.
A thermometer is placed at the same distance from both
surfaces.
The thermometer next to the matt black surface
shows a higher temperature because it emits radiation at a
faster rate.
Heat leaves the metal more quickly through the matt black
surface than the shiny white surface.
Part of the space shuttle is covered in matt black tiles.
These help the craft to lose heat from its surface
when it re-enters the Earth's atmosphere on returning from
space.
The space shuttle gets very hot in the Earth's atmosphere
in the same way that meteors do.
The pipes of solar heaters are painted black to
absorb most heat.
…and “solar
cookers” have
reflective shiny
parabolic dishes
to concentrate the
sun’s heat onto a
small spot.
Infra-red imaging cameras show us a very different
picture of things – “Seeing by Heat”
IR imaging can be
used in diagnosing
illness – and also for
surveillance at night.
IR imaging from space shows details of storms
and other weather patterns that can’t be seen in
visible light
Pulses of IR can be used
in remote control
mechanisms
NOW ANSWER THE QUESTIONS
1. What form of radiation is given out by a warm
object?
2. What “spectrum” does this radiation belong to?
3. What is it easy for heat radiation to travel through?
(Give an example of this)
4. What kind of places are suitable for radiant heaters
to be used in?
5. Why does a black car get hotter than a silver car
on a sunny day?
6. Why does a satellite have “radiator”, and what is it
“getting rid” of? What colour is the radiator
Copy and complete:
A h___ object will e______ more infra-r___radiation
than it absorbs.
A cold o_______ will absorb m______ infra-red
r________ than it emits.
In this way h_______ is transferred from hotter to
c__________ objects.
An object whose t_________ does n____ change
will e_____ infra-red r_______ at the same rate as it
is a_____________.