All objects with a temperature above absolute zero emit electromagnetic radiation, and the amount of radiation emitted at each wavelength depends on the temperature of the object.

  • Hot objects emit more electromagnetic radiation at shorter wavelengths.
  • Cold objects emit more electromagnetic radiation at longer wavelengths.
  • The temperature of an object is related to the wavelength at which the object emits the most radiation, which is known as the peak wavelength.
  • The relationship between the temperature of an object and the peak wavelength of its emitted radiation is described by Wien’s displacement law, which states that the product of the temperature and the peak wavelength of radiation emitted by a black body is a constant.
  • A black body is an idealized object that absorbs all incoming radiation and emits radiation at all wavelengths. Real objects are not perfect black bodies, but they still emit electromagnetic radiation according to their temperature.
About colour & temperature

The surface colour of objects and their thermal temperature can be distinguished as follows.

Surface colour
  • The surface colour of an object seen by an observer is dependent on:
    • The light that falls upon it.
    • The sensitivity of the human eye to the range of wavelengths that correspond to the colours of the visible spectrum.
    • The physical and chemical properties of an object, so its material composition. These determine how it interacts with incident light, including how it absorbs, reflects or scatters light.
  • In terms of the difference between surface colour and thermal radiation, an apple that appears red at 5 degrees Celsius will still appear red at 85 degrees Celsius, but the thermal radiation it emits will be different at the two temperatures.
Thermal radiation
  • Thermal radiation is a measure of the electromagnetic radiation emitted by an object due solely to its temperature, in the absence of incident light.
  • The colour and brightness of most objects that we see in daily life are due to the reflected light such as sunlight or artificial light.
  • Reflected light is typically much brighter than the thermal radiation emitted by the same object at room temperature.
  • The amount of thermal radiation emitted by an object at room temperature is relatively low compared to the amount of radiation it will emit at higher temperatures.
  • However, the amount and distribution of thermal radiation emitted by an object can be affected by factors such as the composition of the object, the properties of its surface, and the ambient temperature and humidity of the surrounding environment.
  • The concept of thermal radiation typically encompasses a broad range of wavelengths across the electromagnetic spectrum, including infrared radiation, visible light, and ultraviolet radiation.
  • At room temperature, most objects emit low levels of thermal radiation in the infrared region of the electromagnetic spectrum.
  • An iron rod would need to be heated to a temperature of around 1000 to 1200 degrees Celsius to emit thermal radiation that is visible to the human eye.
    • At this temperature, the rod would glow red, and the colour of the glow would become brighter and shift towards yellow and then white as the temperature increases further.
    • It’s worth noting that the precise temperature at which an iron rod starts to emit visible thermal radiation can vary depending on  the specific rod and its environment.