The Black-Body Ideal Absorber and Emitter of Electromagnetic Radiation

The term absorption goes to the heart of the definition of a black-body. A black-body is defined by its perfect absorption of all wavelengths of radiation. This makes it an ideal absorber, but it does not necessarily mean it will appear black to the human eye, as we will discuss further.

Absorption and Emission by Black-Bodies

In absorption, a black-body absorbs all wavelengths of radiation incident upon it. This perfect absorption makes it an ideal absorber. According to emission, the behavior of a black-body also depends on its temperature, as described by Planck's law. The temperature of the black-body determines the spectrum of wavelengths it emits.

How Temperature Affects Emission

At different temperatures, a black-body emits radiation in a spectrum of wavelengths. Let's consider some practical examples:

Lower temperatures: At lower temperatures, the black-body primarily emits infrared radiation, which is not visible to the human eye. Visual transition: As the temperature increases, the black-body begins to emit visible light, which transitions from red to orange to yellow to white as the temperature gets hotter.

Appearance of a Black-Body

While a black-body is defined by its ideal absorption properties, the appearance of the object depends on the temperature. At low temperatures, the emitted light is minimal in the visible spectrum, making the black-body appear very dark or almost black. However, as the temperature rises, the visible component of the radiation increases, revealing the characteristic colors of glowing hot objects.

The colors range from the warm orange of glowing charcoal embers in the ashes of a fire to the white hot of the surface of the Sun.

Real Physical Objects vs. Ideal Black-Body

In practice, a real physical object cannot be perfectly black because the presence of electrical charges makes some reflection unavoidable. However, if a true black-body did exist, it would appear truly black to our eyes as long as it is not too hot.

Visible Emission and Temperature

When such an object gets sufficiently hot, the light it emits by glowing becomes visible. This critical temperature is approximately 460°C (900°F). However, the exact boundary depends on background lighting conditions. In total darkness, with dark-adapted eyes, it might be possible to see at a slightly lower temperature.

The Most Perfect Black Body: A Hole

Because a black-body will emit all wavelengths of light, the most perfect black-body is a hole. A hole in a perfect material with no reflection would absorb all incoming radiation and emit all emitted radiation without reflection, making it an ideal black-body.