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Theme B — The Particulate Nature of Matter

B.2.1 Albedo and Emissivity — IB Physics HL Revision Notes & Topical Questions

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B.2.1 Albedo and Emissivity — IB PHYSICS SL notes: Define emissivity and albedo and use them to describe how real surfaces radiate and reflect.Emissivity e = the ratio of the power radiated per unit area by a surface to that of a black body at the same temperature.e = power radia…

  • Assumptions
  • Perfect black body
  • Dependence
  • Stars and planets
  • Fourth power
  • Absorbed power

Section 1 of 10Emissivity

Emissivity e
Emissivity e the ratio of the power radiated per unit area by a surface to that of a black body at the same temperature.
  • e = power radiated by the object / power emitted by a black body.
Assumptions
  • Assumptions the black body is at the same temperature and has the same dimensions as the object.
Perfect black body
  • Perfect black body e = 1; this must be remembered because it is not in the data booklet.
Range
  • Range between 0 and 1; a matt black surface is close to 1 and polished silver close to 0.02.
Dependence
  • Dependence it depends on the surface finish and, in general, on the wavelength.
Stars and planets
  • Stars and planets stars are good approximations to a black body, whereas planets are not.

Section 2 of 10Modified Stefan–Boltzmann law

Equation
  • P = eσAT4 for an object that is not a black body.
  • P is the total power emitted in W.
  • e is the emissivity.
  • σ is the Stefan–Boltzmann constant.
  • A is the total surface area in m2.
  • T is the absolute temperature in K.
Fourth power
  • Fourth power doubling the kelvin temperature increases the power sixteen-fold.
Units
  • T must be in kelvin.

Section 3 of 10Albedo

Albedo a
Albedo a the ratio of the total scattered power to the total incident power of radiation reflected by a surface.
  • a = total scattered power / total incident power.
Planet
  • Planet the albedo of a planet is the ratio between the total scattered radiation and the total incident radiation.
No unit
  • No unit it is a ratio between 0 and 1 (perfect scatterer).
    • (perfect absorber)
Earth
  • Earth the average is generally taken as 0.3, so 30% of incident sunlight is scattered back.
Absorbed power
  • (1 − a) multiplied by the incident power.

Section 4 of 10What affects the albedo

Cloud and season
  • Cloud and season thicker cloud cover gives a higher reflection.
Latitude
  • Latitude it changes with the position on the planet.
Terrain
  • Terrain different materials reflect different amounts.
Angle of incidence
  • Angle of incidence the incident angle of the radiation matters.
Daily variation
  • Earth's albedo varies every day.

Section 5 of 10Albedo of common surfaces

SurfaceAlbedo
Fresh asphalt0.04
Bare soil0.17
Green grass0.25
Desert sand0.40
New concrete0.55
Ocean ice0.50 to 0.70
Fresh snow0.85
Open ocean
  • Open ocean about 0.06.
Fresh snow
  • 15% of the radiation is absorbed, so absorbed/reflected = 0.15/0.85 ≈ 0.18.

Section 6 of 10Albedo compared with emissivity

FeatureAlbedoEmissivity
ConcernsReflection of incoming short-wavelength sunlightEmission of outgoing long-wavelength thermal radiation
Range0 to 10 to 1
High value meansReflects most of the lightEmits close to a black body
White roof
  • White roof it has a high albedo and a high infrared emissivity, so it reflects sunlight and radiates its own heat efficiently.
Shiny metal foil
  • Shiny metal foil it reflects both, so it also emits poorly.

Section 7 of 10Feedback and links

Ice loss
  • Ice loss a surface that loses ice becomes darker, absorbs more energy and warms further, a positive feedback.
Climate models
  • Climate models albedo is one of the key quantities.
Links
  • Links albedo and emissivity connect to the solar constant, the greenhouse effect and the energy balance.

Section 8 of 10More from the specification

Atmospheric greenhouse gases
  • Atmospheric greenhouse gases atmospheric carbon dioxide and methane absorb outgoing thermal radiation, intensifying global natural and human-enhanced greenhouse effects.
Solar radiation intensity and distance
  • Solar radiation intensity and distance solar radiant flux intensity incident upon Earth's upper atmosphere decreases inversely with orbital distance squared from the Sun.
Stratospheric ozone layer absorption
  • Stratospheric ozone layer absorption the natural stratospheric ozone layer absorbs high-energy ultraviolet radiation before solar rays reach planetary surface ecosystems.
Albedo and Solar Radiation Intensity
  • Planetary albedo α is the ratio of total scattered or reflected radiation power to the total incident solar intensity from the Sun, varying with distance, cloud cover,
  • and surface terrain.
    • (average Earth albedo ≈ 0.30)
Atmospheric Absorption and Greenhouse Gases
  • The natural greenhouse effect maintains Earth's global mean temperature;
    • natural and human-derived gases in the atmosphere (principally carbon dioxide, water vapour,
  • and methane) absorb outgoing terrestrial infrared radiation and re-radiate it in all directions.
Stratospheric Ozone vs Greenhouse Effect
  • Stratospheric ozone absorbs harmful ultraviolet radiation in the upper atmosphere,
  • whereas greenhouse gases trap infrared thermal radiation in the troposphere;
    • human emissions of carbon dioxide drive anthropogenic global climate change.
Intensity
  • Power per unit area perpendicular to the wave direction.
Distance
  • Total length of path travelled, a scalar.
Activity
  • Number of decays per second of a sample.

Section 9 of 10Key terms

TermMeaning
EmissivityRatio of the power radiated per unit area to that of a black body at the same temperature
AlbedoRatio of total scattered power to total incident power
Black bodyPerfect absorber and emitter with emissivity 1
Stefan–Boltzmann constantσ = 5.67 × 10−8 W m−2 K−4
Scattered radiationRadiation reflected or scattered back by a surface
Positive feedbackA change that reinforces itself

Section 10 of 10Exam technique

1
Confusing albedo with emissivity — albedo is reflection and emissivity is emission.
2
Giving albedo a unit — it is a ratio.
3
Forgetting that a black body has e = 1 — the value is not in the data booklet.
4
Using Celsius in P = eσAT4 — use kelvin.
5
Assuming a reflective surface emits well — a shiny surface emits poorly.

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