Which of the following has no effect on radiation?

Analyze the question: We need the factor that does not affect how much heat a body radiates (or absorbs by radiation).

Concept: Thermal radiation is governed by the Stefan-Boltzmann law:

P = εσAT⁴

where:

• ε = emissivity, which depends on the nature of the surface (dull black surfaces radiate and absorb best; shiny ones are poor)
• A = surface area
• T = absolute temperature
• σ = Stefan's constant

Step-by-step check of each option:

• A. Nature of surface: It affects ε, so it affects radiation. ✗
• B. Density: It does not appear anywhere in the formula. Radiation depends on the surface and temperature, not on how much mass is packed into a given volume. ✓ (no effect)
• C. Surface area: The rate of radiation is directly proportional to A. ✗
• D. Temperature: The rate of radiation is proportional to T⁴, so it has a very strong effect. ✗

Answer: The factor with no effect on radiation is density, which is option B.

Exam tip: Remember the three things radiation depends on with the phrase "S-A-T": Surface nature, Area, Temperature. If an option isn't one of these (such as density, mass, or the material's volume), it is likely the "no effect" answer. Don't confuse this with conduction, where the material's properties and the temperature gradient matter, or convection, which needs a fluid.