# Question Video: Absorption Physics • 9th Grade

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?

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### Video Transcript

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.