Video Transcript
What colour is a beam of originally
white light after the beam is both transmitted through object B and then reflected
from object A?
Looking at our diagram for this
problem, we see these two objects, object B and object A. Both of these objects have all
these different colours red, orange, yellow, green, blue, indigo, and violet
incident on them. The fact that those specific seven
colours are being used tells us that white light is being shine on these two
objects; that is light that contains all the colours of the visible spectrum. But we see as we look at the
diagram that object A and object B respond differently to these colours.
For example, looking at object A,
we see that it reflects three of these seven colours. It reflects red light, yellow
light, and blue light. For object B on the other hand, we
don’t see what colours it reflects, but what colours it transmits. Object B transmits green light,
blue light, indigo light, and violet light. Based on the diagram, we can say
that all the other colours of light that reach object B are absorbed by it. And that brings us back to our
question. If white light, that is light with
these seven colours red, orange, yellow, green, blue, indigo, and violet is
transmitted through object B and then reflected from object A, what colour or
colours will remain?
Here’s how we could think of
it. Say that we have object B in front
of object A and we shine white light at an angle on object B. Some light will make it through
that has be transmitted through object B and then afterwards running to object A and
be reflected off of it. It’s that light in particular the
colour of that light that we want to solve for. To answer that question, we’ll need
to know what are the colours of light that are transmitted through object B and then
what are the colours of light reflected from object A.
Our diagram helps us answer this
question. We know that object B transmits
four colours of the original seven. It transmits green light, blue
light, indigo light, and violet light. So at this point in the light’s
journey after it’s made it through object B, there are four remaining colours, these
four we’ve just named. Then, these four colours run into
object A. And we want to see which colours if
any are reflected. Again, our diagram will help
us. We saw that object A reflects light
of three different colours. It reflects red light, yellow
light, and blue light. Now, in this case, the red and
yellow don’t matter much because no red or yellow light is actually reaching object
A in the first place. Those colours have been filtered
out by object B. But blue light is among the list of
colours that reach object A.
So of the green, blue, indigo, and
violet light that reach object A, only blue is gonna be reflected. Green, indigo, and violet will be
absorbed as we see in the diagram. Because blue light is the only
colour of light that’s both transmitted by object B and reflected by object A, we
know that that is the answer to our question. It’s blue light that will be
reflected from object A after this white light has been transmitted through object B.
