Physics · Class 11

Compare absorption and emission

A surface that absorbs strongly also emits strongly under matching conditions. In an enclosure of the same temperature, absorbed and emitted power balance for every absorptivity. Unequal temperatures produce a net exchange.

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Compare absorption and emission

வெப்பச் சமநிலையில் நல்ல உட்கவரும் பரப்பு நல்ல உமிழும் பரப்பாகவும் இருக்கும். ஒரே வெப்பநிலையில் உட்கவர்ந்ததும் உமிழ்ந்ததும் சமம்.

Key notes, relationships and model limits

Key relationship

α=ε at thermal equilibrium (same wavelength/direction); net greybody flux=ασ(Tenclosure⁴−Tsurface⁴).

Remember

Common trap

A shiny weak absorber is also a weak emitter under the same conditions.

Model limits

Opaque grey surface in a uniform black enclosure. Wavelength-independent α is an idealization of Kirchhoff law; positive net flux heats the surface.

What you will learn

Link absorptive and emissive powers and identify equilibrium.

Try the experiment

Try both scenarios, change one control, and predict the result before checking the graph and readouts.

Think of it this way

Change one input while keeping the others fixed, as you would in a controlled experiment.

Worked example

At α=0.8, enclosure 600 K and surface 500 K, net flux=0.8σ(600⁴−500⁴)≈3044 W/m² into the surface.

Why this matters

The model connects compare absorption and emission with an observable reading. Check the equation and assumptions before applying it.

Before you start

SI units · algebra · graph reading

Check your understanding

At equal surface and enclosure temperatures, what is net radiation?

The higher-absorptivity surface always gains heat
Zero for every absorptivity
Every surface emits without absorbing

Read the explanation

α multiplies both equal blackbody fluxes, so their difference is zero.

Course connection: Class XI Chapter 10. Consult the roadmap for CBSE, NEET, JEE Main and Advanced scope.