A blackbody is a hypothetical object that… (select all that apply)
A. is completely dark, giving off no light
B. does not reflect any light
C. glows at a wavelength specified by its temperature
D. absorbs all light that hits it
The Correct Answer and Explanation is:
Correct Answers: B, C, and D
A blackbody is a theoretical or idealized physical object that plays a crucial role in understanding thermal radiation and quantum mechanics. Let’s break down each option to identify the correct statements and provide a comprehensive explanation:
Option A: “is completely dark, giving off no light” – ❌ Incorrect
While the name “blackbody” might suggest complete darkness, this is not true. A blackbody emits radiation in the form of electromagnetic waves, and the amount and wavelength of this emission depend on its temperature. For example, as a blackbody is heated, it glows first red, then white, and eventually blue at very high temperatures. The Sun is often approximated as a blackbody with a surface temperature of about 5778 K, and it emits visible light. So, a blackbody is not dark—it radiates energy.
Option B: “does not reflect any light” – ✅ Correct
A perfect blackbody absorbs all incident radiation, regardless of wavelength or angle of incidence. This means none of the light is reflected or transmitted, making it an ideal absorber. That’s why it appears black when cold—because it reflects no light. In practice, materials like Vantablack are engineered to behave similarly, absorbing over 99.9% of light.
Option C: “glows at a wavelength specified by its temperature” – ✅ Correct
This is one of the defining features of a blackbody. According to Planck’s Law, a blackbody emits radiation across a continuous spectrum, with a peak wavelength that shifts based on its temperature. This relationship is described by Wien’s Displacement Law, which states that hotter objects emit radiation at shorter wavelengths. For example, hot metal glows red or white depending on temperature.
Option D: “absorbs all light that hits it” – ✅ Correct
This is another defining characteristic. A blackbody is a perfect absorber, meaning it absorbs 100% of the electromagnetic radiation that falls on it, across all wavelengths. No light is reflected or transmitted.
Summary
A blackbody is not “dark” in the sense of being invisible—it actually glows, especially at higher temperatures. The key features of a blackbody are its perfect absorption (B, D) and its temperature-dependent radiation (C).