The picture shows a jar of grape jelly. if you were to shine a bright light through the jar, what would happen to most of the light?

Some light gets through grape jelly, but not much.
If you were to shine a bright light at the jar . . .

-- A little bit of the light would reflect off of the jar, and never
get as far as the jelly.  The rest of the light would pass through
the glass and hit the jelly. 

-- Some of the light that hit the jelly would reflect off of it, and
never actually enter the jelly.  The part that reflected off of it
would be mostly the purple wavelengths. They would go back
out through the jar, into the kitchen.
The rest of the light would enter the jelly.   

-- Of the light that entered the jelly, MOST of it would be absorbed
by the jelly and never seen again.  This would happen to all of the
wavelengths of light EXCEPT purple, although SOME purple light
would also be absorbed.

-- At the end of the jelly, when the light reached the other side of the
jar, there would be very little of it left, and that little bit would be almost
all purple wavelengths.  That little bit of light would come out the other
side of the jar into the kitchen.  If anybody was standing there and some
of the light happened to go into his eyes, he would say "Oh look !  I can
SEE that the stuff in the jar is dark purple!".

It’s a) absolute dating