This is a really deep question that requires a
fundamental understanding of electrons, atoms, and
the wave-like nature of light (or electromagnetic
radiation in general). However, let me give you a
glimpse into why this happens.
When you say mirror, let's assume we're talking
about a silver metal coating, and this is also why
other metals are shiny. All atoms have electrons,
and these electrons can interact with light. In
the case of metals, these electrons are only
loosely attached to the metal atoms, so they can
move around (which is related to the fact that
metals conduct electricity). When light hits the
metal, the electrons interact with the light and
cause it to reflect. Specifically, light with a
given frequency causes the electrons to rattle
with that same frequency. In the case of visible
and infrared light, which are low frequency, the
electrons can match the speed and reflect the
light. However, in the case of UV light, the
frequency is too high, and the electrons can't
rattle fast enough to match the frequency of the
light. For this reason, metals are actually
transparent to UV radiation!!! So, if you were
underneath a piece of metal, it could be totally
dark (because no visible light gets through) but
still get sun-burnt! Neat!
Text added to this answer from the author on
The nature of the metal will determine what
wavelengths can pass through. The highest
frequency light that can be scattered (that is to
say, reflected) is referred to as the plasma
frequency, and is different for each metal. For
example, aluminum is transparent below 90 nm,
silver is transparent to UV radiation below 140
nm, platinum is transparent below 240 nm, and
potassium is transparent below 330 nm, which is
almost getting to the visible spectrum of light!
(Visible light ends at about ~400 nm.)
All this said, if you're behind a piece of
steel, you're not going to get a sunburn.
Thanks for the great question! Mirrors reflect
light because of how "difficult" it is for light
to travel through the material that the mirror is
made of. A very common mirror materials is
To understand why mirrors reflect, first think
about light in outer space where there is no
atmosphere. In outer space light can travel
without being reflected, scattered, or impacted by
any surrounding atoms or molecules (like mirror
atoms or gas molecules). Once light enters the
Earth's atmosphere, some of the light begins to be
impacted by the gas molecules in the Earth's
atmosphere, but this impact is very small so the
light can still keep traveling forward very
Once the light gets to the surface of a silver
mirror, the light cannot travel through the
silver, but the silver also cannot absorb the
light. As a result, the light "bounces off" of the
surface of the silver and returns to your eye,
which is why you can see yourself in a mirror.
However, it is important to keep in mind that
light can travel through some solids (window
glass) and be absorbed by other solids (solar
cells), so lots of interesting things can happen
to light at solid surfaces! It is also interesting
to know that light always travels at the same
speed (~671,000,000 miles per hour!) whether in
outer space, the Earth's atmosphere, liquid water,
or solid window glass.
Note from ScienceLine Moderator:
A reader sent the following correction to this
"The speed of light is not constant in all
materials at all.
In the case of liquid water, it would be 3/4 c.
That is directly related to the 4/3 value of the
index of refraction of water."
I hope that this answer helps, and please do
not hesitate to write back to us if you have any
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