This is shown by examining the area underneath each line. As temperature increases, the total amount of energy radiated increases.While a body at this temperature would appear red-hot to our eyes, it would truly appear ‘infrared-hot’ if we were able to see the entire spectrum.Ī few other important notes regarding Figure 21.2: This is in the infrared portion of the electromagnetic spectrum. However, the energy is most intensely emitted at a wavelength of approximately 1000 nm. The graph shows that when a blackbody acquires a temperature of 3,000 K, it radiates energy across the electromagnetic spectrum. It may help to just follow the bottom-most red line labeled 3,000 K, red hot. In other words, it shows how the intensity of radiated light changes when a blackbody is heated to a particular temperature. Understanding Blackbody Graphsįigure 21.2 is a plot of radiation intensity against radiated wavelength. Doing this will make the following text more meaningful and will help to remind yourself of some of the key concepts within the section. When encountering a new graph, it is best to try to interpret the graph before you read about it. What relationships exist? What trends can you see? The more time you spend interpreting this figure, the closer you will be to understanding quantum physics! Take a moment to carefully examine Figure 21.2. It is the analysis of blackbodies that led to one of the most consequential discoveries of the twentieth century. This container is a good model of a perfect blackbody. In time, the hole would continue to glow but the light would be invisible to our eyes. Like a burner on your stove, the hole would glow red, then orange, then blue, as the temperature is increased. The hole may even begin to glow in different colors as the temperature is increased. As the temperature of this container gets hotter and hotter, the radiation out of this dark hole would increase as well, re-emitting all energy provided it by the increased temperature. Imagine carving a small hole in an oven that can be heated to very high temperatures. Imagine wearing a tight shirt that did this! This phenomenon is often modeled with quite a different scenario. It is likely best to refer to anything that is not a perfect blackbody as an approximation of a blackbody in order to avoid confusion.Ī perfect blackbody is one that absorbs and re-emits all radiated energy that is incident upon it.

Occasionally, texts refer to blackbody and perfect blackbody as two different concepts. This shirt is a good approximation of what is called a blackbody.

Black shirts, as well as all other black objects, will absorb and re-emit a significantly greater amount of radiation from the sun. You are likely aware that wearing a tight black T-shirt outside on a hot day provides a significantly less comfortable experience than wearing a white shirt. Our first story of curious significance begins with a T-shirt. Is a black piece of paper perfectly black?

Reinforce that the term blackbody is nothing more than its name suggests-that is, a body that is perfectly black.

If a blackbody is perfectly black, what must be happening to all of the energy incident upon it? Discuss how it can be reflected or transmitted.

Discuss what can happen to energy when it strikes a surface.

Why do some objects appear black? Furthermore, why do we see objects that are red as red? It is said that black is the absence of color, but what does that mean in terms of the light reflected into our eyes?

Discuss what could be meant by the term blackbody.

Have students write down a list of equations or statements that relate to the three concepts.

Prior to beginning this section, it would be a good idea to review wave concepts including frequency, wavelength, and amplitude.

Equation of light intensity