# Question Video: Comparing Radiation Curves for Different Blackbodies Physics • 9th Grade

The 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? [A] Temperature [B] Area

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### 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.