|How much heat does a red, orange, yellow, green,
blue, purple, black, and pink light give off? Thanks!|
|Question Date: 2017-11-20|
Visible light is a portion of the
electromagnetic spectrum , which is a wave
propagating through space and time and is a form
of energy called radiant energy.
Like any form of energy, radiant energy can
be converted either into heat or work. Light
itself is not a form of heat. When light is
absorbed by some object, the electromagnetic
energy is converted into heat and the temperature
of the object increases. Similarly, an object
that is hot (for example the filament in an
incandescent light bulb or the gas molecules in a
star), will emit electromagnetic radiation. The
sun heats the earth by emitting electromagnetic
radiation, which travels through space and is
absorbed by the earth, and it heats up as a
As an object becomes hotter, it will emit
electromagnetic radiation of different
wavelengths. The exact law that describes this
is called Planck’s law. For an ideal object
(which physicists refer to as a black-body) we can
calculate the temperature that an object would
need to be in order to emit light of a certain
color. When the object’s temperature increases the
peak wavelength of the emitted light shifts to
A black-body below about 500 degrees Celsius
only emits electromagnetic energy in the infrared
At around 500 Celsius objects will begin
to emit light of a dim red glow.
Between 1000-1200 Celsius (the
temperature of lava) objects appear to glow
At around 5000 Celsius objects appear
white, and hotter than 6000 Celsius
they appear blue.
However, it is important to realize that the
temperature is not the only reason that flames or
lights could appear various colors. Usually, the
chemistry also plays an important role.
absorbing electromagnetic radiation as heat, the
electrons in the atoms can absorb the energy and
jump to a higher energy level. When the electron
falls back to its normal energy level, it emits
light of a particular wavelength. Each atom has a
certain frequency at which it absorbs and emits
light and that can be more important than the
temperature in determining the color. So different
chemicals can burn at different colors even though
the temperature of the flame is the same. For
example, Barium produces a green flame (seen in
fireworks) and the blue flame on a gas stove is
caused by the combustion of hydrocarbons.
All objects emit radiation, and the
wavelength of the radiation depends on the
object's temperature. The earth, for
example, emits infrared (long wavelength)
radiation because it is fairly cool,
but the sun emits bright radiation that we
can see. Another example is that if you take a
piece of metal and heat it up, it will start
glowing when it heats up. This phenomenon is
described by Wien's displacement law, which
states that the peak wavelength of radiation
emitted by an object is equal to a constant (~2900
K µm) divided by the object's temperature in
So, if you wanted to find out the temperature
of an object that mostly emits red light, for
example, you can solve for temperature:
the wavelength of red light is ~0.7 µm
so you can divide 2900 K µm by 0.7 µm to solve for
the temperature in K (Kelvin).
An object that mostly glows red is therefore
about 4140 K, or almost 7000 degrees Fahrenheit!
You can find the wavelength of orange, yellow,
green, blue, purple, and pink light to calculate
the temperature of an object that emits most of
its radiation in the wavelength of these colors.
A black object would need to be at a low enough
temperature such that it emits almost no radiation
in the visible spectrum. To use the metal analogy
again, a piece of cast iron at room temperature
looks black, and it starts to glow as it is heated
to high enough temperatures.
The heat a color gives off depends on the power
of the light, not the color.
Blue light is more energetic than red
light , so a certain amount of blue light will
make more heat than the same amount of red light,
but a bright red light will still overpower a
faint blue light. Also, most colored lights are
just lights with paint over the bulb. The actual
color of the light is something else (usually
yellow or white) behind the paint. The paint will
absorb light of some colors, which will heat up
the paint and the bulb, but still the same amount
of heat will be produced in the end.
The whole energy carried by a beam of light
depends on both its color, and its "brightness".
The color of light is determined by the
energy carried by each photon (quantum of
light), and the brightness of light is
determined by how many photons that beam of
light carries. If we just look at a single
photon, then we can rank its energy by its color:
purple > blue > green > yellow > orange > red. But
I do not think there is "black light" or "pink
Note from ScienceLine moderator:
The symbol ">" means greater than.
All of the visible light are electromagnetic
waves and they can also be described as
photons with different frequency (or as
The energy of a single photon is
E = hc/lambda,
here h is the Planck constant ,
c is the speed of light, lambda is
the wavelength of light.
Since h and c are known, we end up with
E = 2*10(-19) J/lambda.
Here lambda is still the wavelength but in
the unit of micrometer, J is the unit of
Basically it means a photon with a wavelength
of 1 micrometer carries the energy of about
2*10(-19) J. This is a very small
amount of energy. But remember the sun or other
light source can give a huge amount of photons,
which means non-negligible energy in total.
To talk about the heat that the light gives off,
you need to know about absorption and energy
conversion. Basically when a photon hits an
object, it will either get scattered or absorbed.
Assuming the photon is fulled absorbed, then the
energy is usually turned into heat energy. So a
photon with a wavelength of about 0.7 micrometer
(red light) can give the maximum heat energy of
about 2.9*10(-19) J.
Note: First, the red light wavelength is
not a single number 0.7 micrometer; it has a
range of 0.62 to 0.75 micrometer . This is
also true to light with different colors. Second,
as mentioned before,unless the photon is fully
absorbed, otherwise, the heat energy you get from
the light will be smaller than the energy of the
You can use the formula to calculate the energy
of other photons with different wavelength. It is
good to know that shorter wavelength means
The last thing I want to mention is black
light. There is no such light as black light
An object looks black because it absorbs all of
the visible light and so it appears to be black.
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