Great question! We'll start with the most
important part: materials will be colored (to us)
if they absorb visible light. Things that
are transparent (things we can see through) do
not reflect, scatter, or absorb light.
Water, when it is pure water, absorbs no
visible light. So all white light (light from the
sun), will pass right through it, and will look
transparent to us. Glass, when it is pure, is the
same way. All white light will pass right
But think about colored glass, why does it
look blue or green or red, but is still
transparent? Well, it has very small
impurities that will absorb part, not all, of
visible light, making it looked colored. Because
these impurities are so small, the glass is the
majority of the material it is still transparent,
but shows a bit of color.
Why is milk opaque (we cannot see through
it)? Light is hitting the milk, but being
scattered, reflected, and absorbed. Light is not
hitting our eyes from the other side of the milk
because it can't make it through the milk, which
means we cant see through it. This applies to
everything that we cannot see through - light
cannot pass through it.
Hope that helps!
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