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The Question
(Submitted May 29, 1997)
Can light only be seen when it is reflected off particles? Is space dark
because it is a vacuum and there are no particles for a light wave to reflect
off of? How is it possible for light to travel through a vacuum? I am also
puzzled whether light is emitted by photons, waves, or both?
The Answer
What an interesting set of questions. I'm going to explain light in
terms of the visible spectrum and our eyes (our sensors of light), but
it holds true for the entire electro-magnetic (em) spectrum.
Your eye has specialized cells (rods and cones) that detect the
intensity (brightness) and color of visible light photons. When one of
these photons enters your eye, these cells convert its energy into a
nerve signal that registers in your brain.
So to see an object it must either:
1) Emit photons towards your eye;
(the Sun, a candle flame, a light bulb, a TV).
2) Deflect photons towards your eye;
(the Moon, a dog, a plant, a telephone).
As to the reason space is dark, you're right! It's because there is a
vacuum in space, and no particles to reflect the Sun's light from
space and into our eyes.
In respect to how light travels in a vacuum, I recommend that you
check out Imagine the Universe! at:
http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html,
which explains how light moves, and has a link to a definition of
the particle/wave duality of light:
http://imagine.gsfc.nasa.gov/docs/dict_qz.html#wave_particle_duality
Speaking of which, light is neither a wave nor a particle, but has aspects
of both. It can be
considered to consist of particle-like packets of wave-energy called photons.
The particle and wave interpretations are not in conflict. Rather they are
useful when considering different properties of light. For instance, the
scattering of X-rays as they pass through a metal foil is easy to
understand using a particle model, while the diffraction patterns
produced when light is passed through narrow slits are easier to understand
in terms of overlapping waves. Wave-particle duality is an outcome of the
quantum mechanical nature of matter, under which the universe on the sub-
atomic scale is not made up of hard objects with precise positions, but rather
of entities with some spread of possible locations.
For more details, you might want to look for an elementary textbook on
quantum mechanics.
Keep wondering about things,
Michael Arida and Paul Butterworth
for Ask a High-Energy Astronomer
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