Describe the effect of surface colour (black or white) and texture (dull or shiny) on the emission, absorption and reflection of infrared radiation

2.3.3 Radiation – Effect of Surface Colour and Texture

Learning Objective

Describe how the colour (black or white) and texture (dull or shiny) of a surface influence its:

• Emission of infrared radiation
• Absorption of infrared radiation
• Reflection of infrared radiation

Key Concepts

All objects above absolute zero emit electromagnetic radiation. The amount and wavelength distribution depend on temperature (Stefan‑Boltzmann law) and on the surface properties.

Infrared radiation (IR) is the part of the spectrum most relevant to heat transfer in everyday situations.

Colour Effects

Colour is a visual perception of the wavelengths of visible light that are reflected. It also gives an indication of how a surface interacts with infrared radiation.

• Black surfaces:

  • High absorption of both visible and infrared radiation.
  • High emission of infrared radiation (they are good “radiators”).
  • Low reflection of infrared radiation.

• White surfaces:

  • Low absorption of visible light; they reflect most of it.
  • Similarly, they reflect a large proportion of infrared radiation.
  • Consequently, they emit relatively little infrared radiation.

Texture Effects

Texture determines how smooth or rough a surface is at a microscopic level, influencing the direction and amount of reflected radiation.

• Dull (matte) surfaces:

  • Scatter reflected radiation in many directions (diffuse reflection).
  • Effective absorption and emission are not strongly altered by texture alone, but the scattered reflection reduces the amount of radiation that can be reflected directly back.

• Shiny (polished) surfaces:

  • Reflect radiation in a single, well‑defined direction (specular reflection).
  • For a given colour, a shiny surface will reflect more infrared radiation than a dull surface of the same colour.
  • Consequently, a shiny black surface still absorbs well, but a portion of incident IR is reflected away, reducing net absorption compared with a dull black surface.

Combined Influence of Colour and Texture

The table below summarises the qualitative behaviour of four common surface types.

Physical Reasoning

When infrared radiation strikes a surface, three processes compete:

1. Absorption – energy is taken up, raising the temperature of the material.
2. Emission – the heated material radiates energy away according to \$P = \varepsilon \sigma A T^{4}\$, where \$\varepsilon\$ is the emissivity (≈1 for black, ≈0 for white).
3. Reflection – part of the incident energy is sent away without being absorbed.

Colour mainly changes the emissivity \$\varepsilon\$, while texture modifies the reflectivity \$R\$ (specular vs diffuse). The relationship \$A + R + \varepsilon = 1\$ (where \$A\$ is absorptivity) holds for a given wavelength.

Practical Implications

• Thermal insulation: Dull white paints are used on roofs to reflect solar IR and keep buildings cool.
• Heat sinks: Black, dull surfaces are favoured for radiators because they emit heat efficiently.
• Solar collectors: Black, shiny surfaces concentrate and absorb solar IR while reflecting visible light to reduce glare.
• Clothing: Dark, matte fabrics feel warmer in sunlight than light, shiny fabrics.

Sample Examination Question

Q: A black matte metal plate and a white shiny metal plate are both placed in a room at 20 °C. They are heated to 60 °C and then left to cool. Which plate will cool faster and why?

Answer outline:

• The black matte plate has a high emissivity (\$\varepsilon \approx 0.95\$) and low reflectivity, so it radiates heat efficiently.
• The white shiny plate has low emissivity (\$\varepsilon \approx 0.10\$) and high reflectivity, so it radiates much less.
• Therefore, the black matte plate will lose heat more rapidly and cool faster.