Have you ever gazed up at the night sky and marveled at the glow of distant stars? But did you know that everything in the world is a source of light?

Indeed, just like every star in the universe, every object emits light in a unique way—radiating electromagnetic waves.

This is not a scene from a science fiction novel but a real scientific phenomenon known as thermal radiation. To study the laws of thermal radiation, physicists have defined an ideal object called a black body, which serves as the standard for studying thermal radiation. Therefore, thermal radiation is also known as "black body radiation."

What is Black Body Radiation?

Black body radiation is a physical phenomenon in which a black body at any temperature completely absorbs all incident radiation of all wavelengths and re-emits energy with maximum efficiency.

The theory of black body radiation helps scientists understand the temperature and energy output of stars, planets, and other celestial bodies. Many real-world objects, such as the Sun and light bulbs, can be approximated as black bodies, and studying their radiation characteristics helps us understand their physical properties.

The Relationship Between Kelvin and Celsius

The Kelvin scale and Celsius scale are two commonly used temperature measurement methods, and there is a simple conversion between them. The Celsius scale is based on the freezing point (0°C) and boiling point (100°C) of water and is suitable for everyday life and general scientific applications. The Kelvin scale is based on absolute zero (0K = -273.15°C), suitable for scientific research and engineering applications because it avoids negative temperature values, making it more convenient for describing thermodynamic processes.

Radiation Spectrum of Objects at Different Temperatures

Through animation, we can more intuitively understand how spectral intensity changes with wavelength and temperature. At lower temperatures, objects mainly emit infrared radiation; as the temperature increases, the spectrum gradually enters the visible range.

The human body temperature is about 310K, and our naked eyes cannot see the electromagnetic waves radiated by the human body. However, with infrared imaging equipment, we can see that the human body continuously emits far-infrared radiation.

Illustration: Human Body Emitting Far-Infrared Radiation

Temperature and Color

Red Light (around 1000K)

Starting from about 1000 Kelvin (K), objects emit a faint red glow, similar to the red glow of heated metal, such as the heating element of an oven operating at 800°C.

Red Glow of a Black Body at 1000 K

Red Glow of an Oven Heating Element

Yellow Light (2000-3000K)

At 2000 K, the proportion of yellow wavelengths in the spectrum increases, and objects begin to emit some yellow light. At around 3000 K, they emit more yellow wavelengths.

Yellow Glow of a Black Body at 2000 K

Pale Yellow Glow of a Black Body at 3000 K

Incandescent lamps, which are common lighting tools, have a temperature of about 2700K and emit visible light. However, most of the thermal radiation is actually in the infrared band. The following diagram shows how the emission spectrum changes with the temperature of the object. At low current, the filament glows slightly red; at high current, it emits a strong yellow light.

White Light (around 6000K)

When the temperature reaches about 6000 K, the intensity of almost all visible wavelengths in the radiation spectrum is the same, and this radiation appears white. This explains why the Sun appears white, as it is almost a perfect black body with a surface temperature of 5778 K.

White Glow of a Black Body at 6000 K

Sunlight at a Surface Temperature of 5778 K

In the vast universe, every beam of light emits its unique charm and power. They illuminate our world in different ways, giving our lives color, warmth, and infinite possibilities. GMY, as an explorer and innovator in the field of light technology, is dedicated to create better life with light. Here, we sincerely invite you to join us in exploring the mysteries of light, discovering the potential of light, and creating more innovative applications of light technology together.