The absorption and
emission of radiation by materials - temperature and surface factors
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physics, IGCSE physics, O level physics, ~US grades 8, 9 and 10
school science courses or equivalent for ~14-16 year old students of
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This page will help you answer questions
such as ... Do all objects continually emit
electromagnetic (EM) radiation? Which surfaces are the best
emitters/absorbers of EM radiation? What is black body radiation? Why does the Sun emit more higher
frequency EM radiation than the Earth? What has the emission and absorption of EM
radiation to do with global warming and the greenhouse effect? EM abbreviation for
electromagnetic (radiation/waves)
Sub-index for this page
(a)
Introduction to absorbing & emitting radiation across the EM spectrum
(b)
Relating temperature, intensity,
frequency and wavelength of emitted radiation
(c)
Surfaces -
reflection & absorption of thermal radiation (infrared) experiments
(d)
The Earth's surface
temperature and 'Global Warming' - the
greenhouse effect
(e)
The super 'greenhouse effect' on the planet Venus
See also
Heat transfer -
including radiation
and
Electromagnetic spectrum,
sources, types, properties, uses, dangers including infrared
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(a)
Introduction to absorbing & emitting radiation across the EM spectrum
All objects are constantly emitting
electromagnetic (EM) radiation over a range of frequencies depending on the
temperature of the material.
At the same time, the same objects are constantly absorbing EM radiation.
At low temperatures, most of this
absorbed or emitted radiation is in the infrared EM waves range.
At higher temperatures objects may:
glow red e.g. the hot elements of an
electric fire (> 550oC), the red glow gets more intense up to
~950oC.
at higher temperatures emit visible light
(orange - violet), e.g. hot blue flame (~1000oC, plus lots of
infrared, you reach what is called 'white heat' at ~1350oC)
and at very high temperatures objects
will emit
ultraviolet light e.g. burning magnesium ribbon flame (~2200oC,
plus lots of infrared and obviously visible light too!).
The EM radiation emitted or absorbed depends on the material and its temperature.
Three possible situations in terms of what
the material is experiencing as regards EM radiation and temperature
When the rate of an object's emitted radiation >
absorbed radiation, it means the object is cooling
(also means: the average power the object
is absorbing < average power object is emitting)
The temperature of the object
is decreasing.
A hot cup of tea on the table will
radiate more infrared than it absorbs, it will give out a net transfer
of heat until, on cooling, it reaches the ambient room temperature. The
heat transfer still involves conduction and convection but the statement
as regards EM radiation is still valid.
When the rate of an object's emitted radiation = absorbed
radiation, it means the object is at the same constant temperature as its
surroundings.
(also means: the average power the object
is absorbing = average power object is emitting)
The input and output radiation
balanced, no increase or decrease in temperature, stays constant.
When the rate of emitted radiation < absorbed
radiation, it means the object is heating up
(also means: the average power the object
is absorbing > average power object is emitting)
The temperature of the object
is increasing.
A piece of bread when placed in a toaster
is cooked as the temperature rises by infrared heat absorption. Other
cases might involve heat transfer by conduction and convection but the
statement as regards EM radiation is still valid.
So the rule is - when an object that is hotter (higher
temperature) than its surroundings, it will emit more radiation than it absorbs,
and, an object that is cooler than its surroundings will absorb more
radiation than it emits.
TOP OF PAGE and
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(b)
Relationship between temperature, intensity,
frequency & wavelength of emitted radiation
The power (P) per unit area is a measure of
the intensity of radiation (units e.g. W/m2).
power units: joules/second (J/s) or Watts (W)
so here the intensity of EM radiation
can be expressed as the
rate at which energy is emitted per unit area.
Reminder - all objects are constantly emitting
electromagnetic (EM) radiation over a range of frequencies depending on the
temperature of the material.
The distribution and intensity of
emitted wavelengths only depends on the temperature.
Black body radiation - absorption
and emission
Absorption
An object that absorbs ALL radiation
falling on it, at all wavelengths (or frequencies) , is called a black body
- the 'perfect' or 'ideal' absorber of EM radiation.
However, most objects reflect light to some
extent.
Graphite powder can absorb 97% of
incoming radiation and I assume it can emit 97% of black body radiation?
There is military interest in
blackbody-like materials for camouflage and radar-absorbent materials for
radar invisibility - the idea is to avoid detection from reflected or
emitted EM radiation.
Graphene nanostructure materials have
been made with almost perfect black body properties.
Artists are interested in these graphene
materials to produce the perfect black surface!
Emission
When a black body is at a specific
uniform temperature, its emission has a characteristic wavelength (or
frequency) distribution that depends ONLY on the temperature.
Its
emission is called black-body radiation - the 'perfect' or 'ideal'
emitter.
Looking at the distribution and intensity of emitted wavelengths/frequencies at
different temperatures
The
intensity of emission for particular wavelengths/frequencies depends on the
temperature of the object.
Intensity is power per unit
area (e.g. units can be W/m2
or J/sm2)
Graph 1
The effect of temperature on the intensity
- wavelength distribution is shown graph 'sketch' 1.
Whatever the temperature, the general
shape of the graph is the same,
and ALL intensities increase in
value for all wavelengths with increase in temperature.
From T1 to T4 represents a temperature
range from ~1000 to 5000 K (~727oC to 4727oC)
Compared to a star surface, T1 is a
relatively cool temperature e.g. glowing coals on a fire.
You would find that an object at room
temperature has a curve lower than T1 and peaking more to the right.
T4 could represent the surface of a very hot star, the
surface of our Sun is ~6000oC, so we get lots of visible light, and lots of
ultraviolet radiation if it wasn't for the ozone layer above us!
As you go from T1 to T4 the object
will shine more and more brightly - increase in overall intensity.
The wavelength with the highest
intensity ('peak') of emitted radiation is called the principal wavelength.
When you heat an object from a low
temperature to a high temperature you observe a sequence of colours.
e.g. when you heat a metal to a high
temperature it changes from red, yellow, blue and then white.
The higher the temperature of an object
the greater the intensity of every emitted wavelength,
AND, the higher the temperature
the smaller the peak wavelength (or the higher the peak frequency).
Looking at graph 1 you can see that the
intensity increases much more for shorter wavelengths (higher frequencies) than
longer wavelengths with increase in temperature.
This is because shorter wavelength EM radiation transfers
more energy.
The energy of EM waves is directly
proportional to frequency
This results in the principal wavelength
being decreased (gets shorter) the higher the temperature.
Therefore as objects get hotter the
principal wavelength gets shorter and the intensity distribution gets wider
and less symmetrical.
Graph 2
The effect of temperature on the intensity
- frequency distribution is shown graph 'sketch' 2.
Note that ALL intensities increase in
value for all frequencies with increase in temperature.
The frequency showing the greatest intensity ('peak') of emission is
called the principal frequency.
The higher the temperature of an object
the greater the intensity of every emitted frequency.
The principal frequency increases with
increase in temperature of the object.
This means, as already stated, the
principal wavelength of greatest emission intensity decreases with increase
in temperature.
Astronomers use spectral data to identify
elements in distant stars BUT can also use the wavelength/frequency distribution
and intensity to work out the temperature of a star.
A hotter star will have a
greater principal frequency (shorter wavelength) than a cooler star.
TOP OF PAGE and
sub-index
(c)
Surfaces - radiation,
reflection & absorption of thermal radiation (infrared) experiments
Although objects are constantly absorbing and emitting
radiation, not all the radiation is absorbed because some of it is reflected
away and absorbed elsewhere.
The nature of the surface of any materials affects the relative
amounts of radiation absorbed or emitted.
See
experiments
further down in this section to investigate this phenomenon.
Heat transfer by electromagnetic
radiation is usually via the infrared part of the spectrum (thermal
radiation).
Dark, matt surfaces
are the best absorbers and best emitters of infrared radiation
eg rough black
surfaces. Black matt surfaces are the nearest
thing to a black body radiator and emitter.
Applications of maximising
absorption of infrared radiation
Solar panels for hot water
comprise of pipes carrying water to be heated, set in a black matt surface to
efficiently absorb the infrared radiation from the Sun.
Applications of maximising
emission of infrared radiation
Hot water radiators should have a
matt surface, preferably black, but rarely so - they don't look very
attractive!
They maximise radiation of
infrared into the room.
The pipes at the back of a
refrigerator should be matt black to maximise thermal energy
transfer by infrared red thermal radiation from the heat pump to the
surrounding air/wall.
The heat pump is a means of
transferring thermal energy from the inside of the refrigerator
to the outside.
Light, shiny surfaces
are poorest absorbers and poorest emitters of infrared radiation
eg white gloss
paint, shiny metal surfaces.
Applications of maximising
reflection of infrared radiation
Light, shiny surfaces are good reflectors
of infrared radiation, this maybe to keep heat in to keep things warm or to
minimise heat radiation in to keep things cool eg the silvered surfaces of
the walls of a
vacuum flask.
Specialised firefighter suits
have shiny surfaces to reflect infrared radiation when going to high
temperature environments.
Applications of minimising
emission of infrared radiation
The shiny surface of 'silver'
teapots reduces heat loss by infrared emission, slowing down the
cooling effect of the surrounding cool air.
Simple experiment to demonstrate the
effect of surface on the rate of emission or absorption of infrared radiation
1. Simple experiment to compare
the absorption of infrared by two different surfaces
Two identical metal plates (same metal in area and thickness - fair test) are set up,
equidistant from a radiant heater - a good source of thermal
radiation (infrared).
The metal plate on the left can be
coated with a matt black material and the other on the right,
with a shiny metal surface or the metal surface painted in gloss
(shiny) white paint.
At the 'shaded' side of the plate you
fix on a brass weight with a drop of molten wax, which holds it
in position on cooling.
The radiant heater is a powerful
source of thermal radiation (infrared) and is absorbed by the surface of
the metal plates.
When the plate is hot enough, the
wax melts, and brass weight slides down!
You can time how long this takes with
different surfaces for the same metal and maybe different metals.
You should observe the blackened
surface plate heats up much faster than the shiny metallic/white
surface, as indicated by the shorter time needed for the brass weight to
fall.
2. Using boiling tubes of hot water
with covered with different surfaces to compare their emission of thermal
radiation (infrared)
You set up four identical pyrex glass boiling tubes in a test tube rack.
Each is covered by wrapping around
the boiling tubes the same area of paper of different textures.
(Two factors to keep constant for
a fair test)
1. black matt paper, 2.
black paper with gloss surface, 3. white matt paper, 4.
white gloss paper
You can try other materials
too, such as aluminium foil.
Each is filled with the same volume
of boiling hot water and lightly seal with an insulating rubber bung.
(Third factor to keep constant
for a 'fair test', so only the external surface is varied)
Allow a minute for the boiling
tube and coating to warm up and the, at regular time 1 minute
intervals, temporarily remove the bung and measure the temperature,
replacing the bung each time.
A graph of the results
(temperature versus time) shows you the cooling curves (idealised
below):
From these you can measure the
initial negative temperature gradients as the boiling tubes of water
cool down.
The black matt surface boiling
tubes cools the fastest - steepest downward temperature gradient - best emitting
surface
The gloss white paper should
cool the slowest - the lowest downward temperature gradient - the poorest emitting surface
This fits in with the
described pattern of behaviour described above.
Doing cooling curve graphs is
a better data analysis than just one set of readings.
The rate of cooling should
be in the surface order
matt black >
shiny black > white matt > shiny white
3. Using the Leslie cube - multisided
box can with different surfaces to compare their rates of infrared emission
The Leslie cube is a hollow aluminium
or steel metal can with four different surface coatings on the four
vertical sides.
e.g. matt and gloss black paint, matt
and gloss white paint, or other surfaces like shiny and dull metal
surfaces.
The cube needs to be made of a good
conductor so the surfaces heat up rapidly.
An infrared (thermal radiation)
detector is positioned in line with the middle of a surface of the cube
and connected to some kind of meter or data logger.
The Leslie cube is filled with
boiling hot water - take care!
Being a cube shape ensures the
same surface area is emitting radiation - fair test factor.
The can is given a few minutes
to warm up all the surfaces - all will come to the same temperature
(fair test).
The thermal radiation reading is
taken for all four faces of the Leslie Cube, making sure there is
an equal
distance between the Leslie cube and detector - must be kept absolutely
constant to make it a 'fair test'.
The radiation spreads out and
intensity decreases by a factor of 1 / distance squared (inverse
square law).
The higher the 'meter' reading, the
greater the intensity of radiation emission and the more efficient the
surface in heat transfer by emitting thermal radiation (infrared).
The value of the infrared
should be in the surface order
matt black paint >
shiny black paint > white matt/shiny paint > shiny metal
For more on heat transfer see
Introduction to heat transfer - conduction,
convection and radiation revision notes
TOP OF PAGE and
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(d)
Global warming - the greenhouse effect
The ideas about surfaces from above in
section (c), and
those introduced about wavelength/frequency in section (a) can now be
applied to considering the Earth's temperature.
The overall temperature of the Earth
depends on three factors relating to EM radiation
Absorption - how much of the
incoming EM radiation from the Sun is absorbed by land and water (seas,
oceans, lakes etc)
Reflection - how much of the
incoming radiation is reflected by the clouds, land or water.
Emission - how much of the absorbed
radiation is re-emitted.
Diagram of some of the possible
absorptions and emissions of the Earth's surface and atmosphere
(the outcomes of the incoming FM
radiation from the Sun, but in no particular order)
1. Reflection of the Sun's EM
radiation from clouds.
2. Absorption of the Sun's EM
radiation by clouds.
3. Re-radiated EM radiation totally
escaping from the Earth's surface.
4. Direct absorption of the Sun's EM
radiation by the Earth's surface - short wavelength as wells visible
light and a little uv radiation.
5. Reflection of incoming sunlight EM
radiation by the atmosphere
6. Re-radiated EM radiation
from the Earth's surface - scattered by the clouds and atmosphere and
eventually re-absorbed by the atmosphere, including greenhouse gases
like carbon dioxide and methane.
7. Direct reflection back into space
of incoming EM radiation from the Sun.
Land coated in 'shiny' ice will
act as a good reflector, so if it melts, more radiation will be
absorbed.
8. Direct absorption by the
atmosphere of the incoming Sun's EM radiation.
During in daytime a huge amount of EM radiation is transferred to the
Earth's surface and atmosphere.
Some radiation is absorbed by the
atmosphere but a lot passes through and absorbed by the Earth's surface.
This warms up the surface and increases the temperature, particularly
areas in bright sunlight.
Overall more EM radiation is absorbed
than is emitted, so the temperature rises in daylight.
The clearer the sky, the less
sunlight energy is reflected back into space, the higher the maximum
temperature reached since more infrared radiation reaches, and is
absorbed by, the Earth's surface.
The reverse is true at night, when
more FM radiation is emitted than is absorbed.
The lack of sunlight causes a decrease in
temperature, and the heat loss increases if the sky is clear, because some of the
re-radiated EM
radiation is absorbed or reflected back off clouds.
It is also absorbed
by greenhouse gases like carbon dioxide and methane, which overall adds
to a reduction in the temperature fall of the earth's surface.
The greatest fall in temperature
occurs when the nighttime sky is clear and the emitted infrared is not
reflected or absorbed by clouds.
Conversely, with cloudy nights, some
of the emitted radiation from the Earth's surface is reflected back off
the clouds or absorbed by them, so the nighttime temperature fall is not
as great..
In terms of the amount of radiation that
the Earth absorbs, emits and reflects, the net result is fairly constant
temperature.
It is neither too hot or too cold
for many forms of life to survive.
Global warming
However, any significant changes in the
Earth's atmosphere may cause the average temperature to change and currently
it is believed (consensus scientific view) that global warming is taking
place due to the increase in carbon dioxide levels due to fossil fuel
burning.
The greenhouse gases, principally,
carbon dioxide (CO2), water vapour (H2O)
and methane (CH4) absorb radiation in the Earth's
atmosphere which allows the Earth to warm up. The relatively small
concentrations of carbon dioxide and methane have a potentially a large
and disproportionate effect on the Earth's average temperature - they
are really good greenhouse gases!
There is a net transfer of heat
energy from the much hotter Sun to the much cooler Earth.
In terms of frequency, the principal
frequency of EM radiation from the Sun is much greater than that of the
Earth.
Although all particles
(atoms/molecules) absorb particular frequencies of radiation from the
Sun, the higher frequency radiation (particularly the infrared, IR) gets
through to the Earth's surface (see diagram above). The higher frequency
IR is not as readily absorbed by most molecules in the atmosphere and
reaches the Earth's surface.
The re-emitted infrared radiation
from the Earth's surface is of lower frequency (longer wavelength) than
the incoming IR radiation. It is the likes of carbon dioxide and methane
(and other human-made molecules) that readily absorb the lower frequency
radiation keeping the Earth warmer than if this radiation escaped.
The more greenhouse gases in the
atmosphere, the greater the absorption of the re-emitted IR radiation
and the warmer the Earth gets and it is believed that human activity is
contributing to this!
This increases the temperature of the Earth
compared to what it would be without the atmosphere - this is one
reason why organic based life exists on Earth - not to cold - Mars
(little atmosphere) and our moon (no atmosphere) are much colder.
Certain gases in the atmosphere are more
effective than others in absorbing the re-radiated energy - water
vapor, carbon dioxide, methane, nitrous oxide and ozone all occur
naturally,
but we have added other greenhouse
gases like chlorofluorocarbons (CFCs) and
hydrofluorocarbons (includes HCFCs and HFCs), albeit in very tiny
concentrations as well as significant extra carbon dioxide in
the atmosphere from fossil fuel burning - both quantities are
still n the increase!
The steady rise in carbon dioxide
concentration means more re-radiated infrared radiation is being
absorbed by the Earth's atmosphere.
The result is that the Earth is warming
up a bit more than might have been expected and the average
temperature is rising.
In particular, it is the rising level
of carbon dioxide from fossil fuel burning that is the most worrying and
major contributor to global warming above what we might expect without
burning fossil fuels.
Graph 1
Graph 1 shows the recent global warming
compared to most of the last 1500 years.
Graph 2
Graph 3
Graphs 2 and 3 show the steady rise in
carbon dioxide concentration as result of the increasing use of fossil
fuels.
You can have cooling effects!
Huge volcanic eruptions transfer
enormous quantities of fie particles into the atmosphere.
These particles scatter sunlight and
decrease the amount of the Sun's infrared radiation that reaches the
Earth's surface.
This causes a cooling effect, and the
Earth's temperature can be significantly lowered.
This is sometimes called a 'volcanic
winter' effect e.g.
The 1815 eruption of Mount Tambora, a
massive volcano in Indonesia caused what came to be known as the "Year
Without a Summer" of 1816. Europe, still recovering from the Napoleonic
Wars, suffered from food shortages. There were large scale crop failures
from the ensuing reduction in both global temperatures and intensity of
sunlight.
I'm not writing any more on this here,
because I've already written a lot in my GCSE chemistry notes on the
evidence and possible consequences of rising carbon dioxide levels and
global warming on ...
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning
I've included everything mentioned in
any GCSE chemistry or physics syllabus relating to the 'Greenhouse
Effect'.
See also Biodiversity, land management,
waste management, maintaining ecosystems - conservation gcse
biology
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(e)
The super 'greenhouse effect' the planet
Venus
Venus is the 2nd planet from the Sun and is named after the Roman goddess of
love and beauty - perhaps because it shone the brightest of the five planets
known to ancient astronomers - but looks can be deceptive, Venus has no love for
the human body!
The surface temperature of Venus is up to
465oC.
The surface atmospheric pressure is 90
atm (90 x that on Earth).
The lower atmosphere is mainly carbon
dioxide (96.5% CO2) and nitrogen (3.5% N2).
[in the lower atmosphere and upper
clouds there may be small amounts of sulfur dioxide (SO2),
oxygen (O2), water (H2O), noble gases - helium
(He), neon (Ne) & argon (Ar), carbon monoxide (CO), sulfuric acid (H2SO4),
basalt rock particles and iron(III) chloride (FeCl3)]
The surface area and volume are about 0.9
x that of Earth.
On Venus it rains sulfuric acid, but this
evaporates before reaching the very hot surface
The surface of Venus is dry because it is
too hot for liquids to condense on
The surface of Venus is relatively flat
but very volcanic with still some active volcanoes.
The dense lower carbon dioxide atmosphere is
encased in 80 km thick clouds of mainly sulfuric acid (H2SO4)
and some sulfur dioxide (SO2) - these clouds reflect 90% of
sunlight - which is why it seems so bright, as well as being closer to the
Sun than the Earth.
This reflection of sunlight would tend to
make the planet cooler - a global dimming effect.
BUT, the hot surface of over 460oC
is a powerful emitter of infrared EM radiation.
This thermal radiation is absorbed by
the lower carbon dioxide rich atmosphere (96.5% CO2).
This produces a super greenhouse
effect compared to planet Earth (0.04% atmospheric carbon dioxide).
Little (if any?) infrared
radiation escapes from the planet's atmosphere.
So the surface temperature is kept at a
much higher temperature than if the infrared could escape and the net effect
completely overrides the dimming effect of the outer reflective clouds.
Note:
(a) Compared to Earth the surface
temperature is much hotter - a much stronger emitter of infrared EM
radiation AND the atmospheric carbon dioxide concentration is 2400 x
that of Earth! - 2400 x the greenhouse effect of carbon dioxide gas
on Earth!
(b) The greenhouse effect on
Venus is different to that on Earth in one respect - it doesn't
actually involve the absorption and remission of infrared radiation
- the infrared source on Venus is only from the hot surface.
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WAVES - electromagnetic radiation, sound, optics-lenses, light and astronomy revision notes index
General
introduction to the types and properties of waves, ripple tank expts, how to do
wave calculations
Illuminated & self-luminous objects, reflection visible light,
ray box experiments, ray diagrams, mirror uses
Refraction and diffraction, the visible light
spectrum, prism investigations, ray diagrams explained
gcse physics
Electromagnetic spectrum,
sources, types, properties, uses (including medical) and dangers gcse physics
The absorption and emission of radiation by
materials - temperature & surface factors including global warming
See also
Global warming, climate change,
reducing our carbon footprint from fossil fuel burning gcse
chemistry
Optics - types of lenses (convex, concave, uses),
experiments and ray
diagrams, correction of eye defects
The visible spectrum of colour, light filters and
explaining the colour of objects gcse physics revision notes
Sound waves, properties explained, speed measure,
uses of sound, ultrasound, infrasound, earthquake waves
The Structure of the Earth, crust, mantle, core and earthquake waves (seismic wave
analysis)
gcse notes
Astronomy - solar system, stars, galaxies and
use of telescopes and satellites gcse physics revision notes
The life cycle of stars - mainly worked out from emitted
electromagnetic radiation gcse physics revision notes
Cosmology - the
Big Bang Theory of the Universe, the red-shift & microwave background radiation gcse
physics
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