Video Transcript
A thermos is the type of cup used to keep hot drinks warmer for longer. To do this, the inside of the cup is coated in a material that prevents infrared radiation from the drink from being transmitted outside of the cup. In order to most effectively insulate hot drinks, should the inside of the thermos be coated in a material that is strongly reflective or weakly reflective?
Let’s say that this is our thermos sealed closed and that this is some hot liquid inside the thermos. The purpose of the thermos, we know, is to keep this liquid as hot as possible for as long as possible. Therefore, the inside of the cup is lined with a material that prevents infrared radiation from escaping. If we think of infrared radiation from our hot liquid being incident on the wall of the thermos, when this radiation reaches the wall, three different things can happen. First, the infrared radiation could be reflected from the inside of the thermos. Second, the radiation could be absorbed by the thermos so that the temperature of the thermos wall increases. And thirdly, the radiation could be transmitted. It could pass through the thermos wall into the outside environment.
In both of these last two cases, transmission and absorption into the thermos, energy is transferred away from the hot liquid. This would tend to make the liquid cool down, which goes against the purpose of the thermos. In designing a thermos then, what we really want infrared radiation to do when it reaches the side of the thermos is to be reflected back. That way, energy is not transferred away from the hot liquid. We choose option (A) strongly reflective.
Let’s now look at part two of this question.
In order to most effectively insulate hot drinks, should the inside of the thermos be coded in a material that has a light color or a dark color?
Our answer to this part of the question actually connects with our answer to the previous part. Say that this is part of the inside surface of the thermos. That means that when infrared radiation is incident on the surface, we want it to reflect back. So the question is, what type of material is most likely to cause that reflection, one with a light color or one with a dark color?
To help us in our answer, let’s recall a little bit about the colors that our eyes perceive. Whenever we look at some object, say this leaf here, the color we say that object is, in this case green, is actually the color of light that reflects off of that object so that it’s able to come back and reach our eyes. In other words, a leaf that we call green is actually reflecting green light rather than absorbing it. In a sense then, the leaf is everything but green.
Let’s recall also that if we combine all the colors of the rainbow, then those colors combine to yield white. That is, whenever we see white light, this is light that actually consists of all wavelengths of visible light.
So, getting back to our thermos, let’s say that this inside surface of the thermos is a white or light color. That means that all wavelengths of visible light and indeed many wavelengths of infrared radiation are reflected from the surface. That’s why to our eyes it looks white. So a lighter-colored object is actually one that reflects more light. Since we want our thermos to reflect infrared radiation, we would prefer its inside surface to be a light color. We choose answer option (B).
