iGCSE Physics (0625) 2.3.3 Radiation-Exam Style Questions- New Syllabus
▶️ Answer/Explanation
Detailed solution:
Thermal energy transfer via radiation depends heavily on the surface characteristics of an object.
Darker colours are better emitters than lighter colours; thus, black surfaces emit more than white ones.
Texture also plays a role: dull or matt surfaces are more efficient at emitting infrared radiation than shiny, reflective surfaces.
Since the containers are at 80 °C and the room is at 20 °C, they will lose energy to the surroundings.
A dull black surface is the most effective configuration for both absorbing and emitting thermal radiation.
Consequently, the container with the dull black surface will emit energy at the greatest rate.
▶️ Answer/Explanation
Detailed solution:
Thermal energy can be transferred via infrared radiation, which does not require a medium.
Shiny, silvered surfaces are excellent reflectors of infrared radiation and very poor emitters and absorbers.
In a vacuum flask, the silvered internal walls act as a mirror for thermal energy, reflecting it back toward the hot liquid.
This minimizes the rate of energy transfer to the external environment via radiation.
Conduction and convection are already restricted by the vacuum layer, so the silvering specifically targets radiation.
Therefore, the primary purpose of the silvered walls is to reflect thermal radiation back into the flask.
▶️ Answer/Explanation
Detailed solution:
The ability of a surface to reflect infrared radiation depends on its color and texture.
Light-colored surfaces, such as white, are better reflectors than dark-colored surfaces like black, which tend to absorb radiation.
Smooth, shiny surfaces reflect significantly more radiation than rough, dull surfaces.
Therefore, a surface that is both shiny and white provides the maximum reflective efficiency.
In contrast, a dull black surface would be the best absorber and the worst reflector.
Consequently, option D is the correct choice as it combines the two best reflective characteristics.