Albedo & Emissivity (DP IB Physics): Revision Note

Emissivity

• Stars are good approximations to a black body, whereas planets are not

  • This can be quantified using the emissivity

• Emissivity is defined as

The ratio of the power radiated per unit area by a surface compared to that of a black body at the same temperature

• It can be calculated using the equation:

• Calculations of the emissivity assume that the black body:

  • is at the same temperature as the object

  • has the same dimensions as the object

• For a perfect black body, emissivity is equal to 1

• When using the Stefan-Boltzmann law for an object which is not a black body, the equation becomes:

• Where:

  • P = total power emitted by the object (W)

  • e = emissivity of the object

  • σ = the Stefan-Boltzmann constant

  • A = total surface area of the object black body (m²)

  • T = absolute temperature of the body (K)

Albedo

• Albedo is defined as

The ratio of the total scattered power to the total incident power of radiation that is reflected by a given surface

• It can be calculated using the equation:

• More specifically, the albedo of a planet is defined as

The ratio between the total scattered, or reflected, radiation and the total incident radiation of that planet

• Earth’s albedo is generally taken to be 0.3, which means 30% of the Sun’s rays that reach the ground are reflected, or scattered, back into the atmosphere

• An albedo of 1 represents a surface that scatters all the incident radiation

• Earth’s albedo varies daily and depends on:

  • cloud formations and season – the thicker the cloud cover, the higher the degree of reflection

  • latitude

  • terrain – different materials reflect light to different degrees

  • incident angle of radiation

• It is useful to know the albedo of common materials: 

  • Fresh asphalt = 0.04

  • Bare soil = 0.17

  • Green grass = 0.25

  • Desert sand = 0.40

  • New concrete = 0.55

  • Ocean ice = 0.50 - 0.70 

  • Fresh snow = 0.85

• Albedo has no units because it is a ratio (or fraction) of power