A light ray that strikes an object
is partly reflected and partly transmitted, as shown in the diagram. What percentage of the light ray’s
energy is absorbed by the object?
Taking a look at our diagram, we
see our object, this box here, and then light is coming in to the object from the
right. We see that some of the light 15
percent makes it all the way through and it’s transmitted and 35 percent is
reflected back. The question asks us how much of
the light ray’s energy is absorbed by the object. To start figuring this out, let’s
recall the ways that light can interact with an object that it’s incident on.
When light hits an object, there’re
only one of three things that can happen: the light can be transmitted, reflected,
or it’s absorbed by the object. This means that for any interaction
between light and an object, if we add up all the light that’s transmitted,
reflected, and absorbed, then we’ll get the total light that was incident on the
object in the first place. We could write it this way as an
equation. We could say that the total light
in incident on an object is equal to the light reflected plus the amount transmitted
plus the amount absorbed.
Now, in our scenario, we have some
amount of light in; we don’t know what that is. But we do know what percentage of
that light is transmitted and what percentage is reflected. Going to our equation then, we can
replace Trans, the amount transmitted, with 15 percent. And then we can replace reflected
with 35 percent of the amount of light that comes in. So we have that the total amount of
light incident on our object is equal to 35 percent of that amount plus 15 percent
of that amount plus the amount of light absorbed. If we add together 35 percent and
15 percent, we get 50 percent.
So 50 percent of the incident light
plus the amount that’s absorbed by the object is equal to the total incident
light. This tells us that 50 percent or
half of the light incident is being absorbed by this object. If we add that percentage to the
percentage that are reflected and transmitted, we get 100 percent. So we know that 50 percent of the
incident light ray’s energy is absorbed by this object.