it travels in the form of waves that release energy when they are absorbed by an object.
radiant energy that travels in the form of waves that share electrical and magnetic properties
do electro magnetic waves need molecules to propagate from one location to another?
how fast do electro magnetic waves travel in a vacuum?
3 x 10^8 m/s (186,000 miles per second)
this speed is known as the speed of light. nothing can travel faster.
distance between two successive crests in a wave.
wavelength is the greek word lambda
measured in micrometers, 10^-6m
it is 1/100th the width of human hair. it is what we measure waves in
if the wavelength is small, how many wave crests will pass a fixed point in space in 1 second?
what if the wavelength is large?
if small, lots of waves can fit into 300,000 km (the distance light can travel in one second)
if it is large, not many waves will fit into 300,000 km.
number of waves at a fixed point in one second.
greek letter nu.
constant speed of light formula
C = lambda * nu
speed of light = wavelength times frequency
what kind of energy do long wavelength radiation carry?
so low frequency radiation carries low energy
what kind of energy does short wavelength radiation carry?
so high frequency radiation carries high energy.
t/f: all objects with a temperature above 0K emit radiation
what determines an object's temperature?
the wavelengths of electro magnetic radiation that an object omits
objects with very high temperature emits high energy at a greater intensity than objects at a lower temperature
so a small increase in temp leads to a huge increase in emitted radiation
E = constant of proportionality*temperature (kelvin)
the maximum rate of emission of radiation
per square meter of surface area
Wein's displacement law
lambda max = 2897/T which is roughly 300/T microns
the wavelength of maximum emission of an object is related to the object's temperature as the formula above.
when the earth emit's most of its energy near 10 microns
the sun's radiation emits at only .5 microns
wavelengths from .4-.7 microns
the sun emits 44% of its radiation at these wavelengths
shortest visible wavelengths (.4 microns)
has wavelengths shorter than .4 microns.
the longest visible wavelengths (.7 microns)
infra red ligth
wavelengths that are longer than .7 microns
earth emits most of its energy at this wavelength
what happens when an object radiates more energy than it absorbs?
the object will get colder
what happens when an object absorbs more energy than it emits?
the object will warm up
the amount of energy absorbed is exactly equal to the amount of emitted energy. this means the temperature will not change at all.
what determines an object's ability to absorb or omit radiation?
the nature of its surface characteristics.
size, color, texture...
if an object is a good absorber at a certain wavelength, then it will be a good emitter at that wavelength (provided that its temperature allows it to emit at that wavelength)
object that absorbs all of the radiation that strikes it with 100% efficiency
emits the maximum possible radiation
any object that is a perfect absorber and a perfect emitter
earth adn the sun are both examples
radiative equilibrium temperature
temperature of the surface of which radiative equilibrium is reached
radiative eq of the earth is 255 K.
why is the actual surface temperature of the earth's surface 288 K?
because our atmosphere emits and absorbs radiation
does the atmosphere behave as a black body?
it absorbs some wavelengths of electro magnetic radiation, and is transparent to others
any object that absorbs and emtis only some wavelengths of electro magnetic radiation
the atmosphere is an example
reigions in the electro magnetic spectrum in which the atmosphere is transparent
why is low emittance present within the atmosphere?
presence of gasses such as H2o and CO2 that absorb outgoing infra red radiation.
is the atmosphere transparent to visible light?
what are good absorbers of infra red radiation?
H2O, CO2 and Methane are
thus the atmosphere is only transparent to some infra red radiation
all object's whose temperature is above...emit radiation
above absolute zero
the electro-magnetic radiation emitted by an object is determined by the..
temperature of an object
example of a selective absorber
Station A has snow on the ground, and section B doesn?t. which place is going to be colder the next morning? ?
Station A, the station with snow on the ground, because according to kirchkoff's law, an object that can aborb a wavelength well can emit that wavelength equally as well.
since snow is known to be a black body, and is good at aborbing sunlight rapidly, it is equally known as emitting waves just as fast. The rapid emittion of radiation is going to cool station A down faster than Station B.
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