Expert: Daniel Mazur - 7/14/2010

Question
QUESTION: Is it true that everything including cells and
atoms is constantly absorbing and emitting
light even if there isn't sunshine always present?
Thanks.

ANSWER: Yes Gena,
it is true. Light does not always come from the Sun or the stars. And also, depending on what exactly they call "light", some light may be invisible to our eyes, yet it still is all around, always.
Take care,
Daniel

---------- FOLLOW-UP ----------

QUESTION: Thanks for answering my question. I had asked
a few different physics experts and some of
the answers seemed to contradict eachother.
One person said that even if there are waves
present that this doesn't mean that it would be
considered a light wave. Another person
said that everything has electromagnetic energy,
but intimated that this wasn't necessarily light.
Could it be that everything has some form of electragnetic
radiation which may or may not be considered
light? I am not sure how light is defined or if it
and radiation always involves light waves. Also,maybe
some science experts interpret things somewhat
differently. I would appreciate your feedback on
this. Thanks.

Answer
Hello Gena,

yes, I anticipated a problem of this sort. But as you haven't indicated at what level the answer should be, I made mine pretty short.

The statement you proposed first, i.e. "everything including cells and atoms is constantly absorbing and emitting light", ought to be corrected by replacing the word "light" either by the term "electromagnetic radiation" or "photons". The word "light" should probably be reserved for VISIBLE electromagnetic (EM) radiation. As this radiation, albeit composed of photon particles, is frequently described in terms of wave properties, one can sort all EM radiation by its wavelength. If we do that, then the visible range is between 250 and 700 nanometers and only radiation from this narrow interval of wavelengths should be called "light".

People are people and tend to be sometimes vague in their use of words, scientists are people as well and "light" is so much shorter word to use than "electromagnetic radiation"... People who use "light" to mean all EM radiation then make a distinction for the part of light visible to human eyes by calling it "visible light". We must be aware of both conventions and be ready to ask questions, wherever understanding to one another may rely on one definition of light or the other.

Keeping that in mind, an absolutely correct statement independent of the definitions is that all atoms (all matter, hence everything living, organic and inorganic) continuously emit and absorb PHOTONS. These photons can have various energies, which (by an inverse proportionality) corresponds to various wavelengths of the associated EM wave. On Earth most of the radiation thus emitted and absorbed is in the microwave and infrared range of wavelengths, so they are invisible with wavelengths longer than 700 nanometers.

A closing note: If you are ever asked the question in a test, whether "everything constantly emits and absorbs light", you need to make sure you understand, what the author meant by "light".

Cheers,
Daniel