Video Transcript
The following graph shows the
intensity of light at different wavelengths for four different radiating
blackbodies. Which of the following quantities
must be different for these radiating objects? Is it (A) temperature or (B)
area?
Here we are given a graph showing
the intensity of light at various wavelengths for four different radiating
blackbodies. And we are asked to figure out
which of the options given must be different for these four radiating objects. In order to figure out this answer,
we need to begin by recalling some important information about what blackbodies are,
how they radiate, and what this graph is telling us.
Blackbodies are materials that
absorb all electromagnetic radiation, or light, incident on it, which causes it to
appear black to us, since it cannot reflect visible light back to our eyes. It is important to note that no
true blackbodies exist, but they are a helpful theoretical tool that can give us
some good approximations to many real-world phenomena.
In addition to absorbing energy,
they can also radiate energy out into their surroundings in the form of light. We’ve likely seen something like
this in our own lives already in a kitchen. Let’s think about a coil on a
cooking stove. When it begins to heat up, the
metal that is black when cool begins to glow red. Just like the coil glowing brighter
as its temperature increases, the light that a blackbody radiates will also be
affected by its temperature.
Now let’s take a look at the graph
we are given and figure out what it is telling us about the four blackbodies.
We can see that the horizontal axis
shows us the wavelength of the radiation from the blackbody, while the vertical axis
shows us the intensity of its radiation. Both of these measures are
representing the light that is being radiated from the blackbody.
Taking a look at the options we are
given, we can rule out option (B), the area of a blackbody. A blackbody’s area would not have
an effect on the intensity of the light or the wavelength, so while the area could
be different for all four blackbodies, they could also all be the same size.
We don’t have enough information
from this graph to know what their areas are. We do know, though, that the
temperatures must be different for all four blackbodies. This is because blackbodies will
radiate at different intensities at different temperatures. And on this graph, we can see that
all four of the blackbodies have different intensities. The red line represents the
blackbody with the strongest intensity, and the orange line represents the blackbody
with the weakest intensity. Therefore, the first option,
temperature, is the correct answer.
