Slide 1
Thermal radiation can cross a vacuum
Thermal radiation transfers energy through electromagnetic waves. Unlike conduction, it can cross a vacuum without matter carrying the disturbance.
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That is how energy from the Sun reaches Earth through interplanetary space. The same mechanism operates across a small evacuated gap, although conduction through supports may still provide another heat-transfer path.
Slide 2
Objects both emit and absorb radiation
Objects at ordinary temperatures emit mainly infrared radiation. They also absorb radiation arriving from their surroundings.
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A room-temperature object does not need to glow visibly to radiate. Visible incandescence requires much higher temperatures, while infrared detectors can measure radiation that human eyes cannot see.
Slide 3
Net transfer is the difference between two-way exchanges
A hotter object can radiate toward a colder one while receiving radiation back. Net thermal transfer reflects the difference between those exchanges.
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At equal temperatures, objects still emit and absorb radiation. Equilibrium concerns a balance in the net exchange, not the disappearance of electromagnetic activity at their surfaces.
Slide 4
Visible colour does not determine infrared behavior
A surface’s ability to emit and absorb radiation depends on wavelength. Its visible colour alone does not fully describe its infrared behaviour.
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A shiny metal surface can have low thermal emissivity even when it looks bright. Surface treatments and coatings change radiative properties, so thermal design needs spectral information beyond ordinary appearance.
Slide 5
Geometry and shields change the radiation exchanged
Radiation travels between surfaces that can exchange rays directly or through reflections. Shields can reduce transfer even when a vacuum is already present.
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An intermediate reflective shield can intercept radiation and alter the balance of absorption and re-emission. Practical insulation therefore combines control of radiation with control of conduction and any remaining gas movement.