Evan's Space

Wonders of Physics

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Black is a good emitter of infrared radiation

In theory, we have learned that

Dull (matt/rough) and black surface is a good emitter and good absorber of infrared radiation.

In this simple demonstration, the container sides are painted white matt, silver smooth and black matt.

Hot water is poured into the container and the thermal energy is conducted to the whole container. The temperature of the container is more or less uniform.

Using a infrared thermometer, we measure the temperature of the different surfaces.

From this simple demonstration, we can conclude that dull black surface is the best emitter of infrared radiation and smooth silver surface is the worst emitter of infrared radiation.


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Radiometer – Thermal Transfer

Radiometer is a device for measuring the amount of infrared radiation. It consists of freely pivoted rotor with four vanes perpendicular to one another. The vane surface is painted alternate black and silver. The rotor is enclosed in a partial vacuum glass bulb.

In general, when exposed to infrared radiation, the rotor will spin. The greater the amount of radiation, the faster the rotor spins.

The actual working principle is actually much more complicated. Refer to the videos below for detailed explanation.

For our context in O-level, we can briefly explained based on what we learned. As the vanes of the rotor are exposed to infrared radiation, the black side of the vane absorbs more radiation as it is a good absorber of radiation and hence its at higher temperature. The silver side reflects the radiation. The air molecules at the black side will get heated up and gain more kinetic energy. Hence rate of collision is higher and the air molecules collide on the black side with more force than the silver side. This results in a net force on the black surface and the rotor spins in a specific direction as shown in the video.

The more complicated theory how radiometer works


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Processes for Transfer of Thermal Energy

Three processes of thermal transfer: conduction, convection and radiation.

Conduction vs Convection:


  • both requires a medium for thermal transfer to take place.


  • convection cannot take place in solid but conduction can.
  • conduction is due to vibration and collision of molecules while convection is due to fluid density changes.

Radiation vs Conduction & Convection

  • Radiation can take place in vacuum but both conduction and convection require a medium.