Representation of radiation pressure in electrodynamics
James Clerk Maxwell - to whom
the basic equations of electrodynamics
go back - had already shown in 1873 that electromagnetic waves exert pressure on surfaces.
The value of radiation pressure - given here without proof - is equal to the energy density of
electromagnetic waves, which we have already shown in
Supplement 1.4:
In the same supplement, the product of the speed of light and energy density resulted in the
Poynting vector , which
represents the power of the wave per area. The quotient of the magnitude of the Poynting vector
and the speed of light therefore corresponds to the radiation pressure. For absorbing particles
follows again:
Task 1: The radiation pressure of solar radiation
↓
↑
-
Chapter 3, section Climate and
climate change, shows that the irradiance of solar radiation at the outer edge of
the Earth's atmosphere is 1361 W/m²; this is the so-called solar constant.
How much is the radiation pressure of solar radiation?
-
Reduced by absorption and scattering by the air molecules, around 65% of the solar
radiation present at the outer edge of the atmosphere reaches the earth's surface on
a cloud-free day. Compare the radiation pressure here with the air pressure at sea
level, which averages 1013 hPa.
-
The specific radiation of the sun's photosphere is likewise calculated in
Chapter 3, section Climate and
climate change.The photosphere is the area of the solar atmosphere in which the
solar radiation directed into space is generated. It amounts to 62.9 MW/m².
What is the value of the radiation pressure there?
-
Space agencies are investigating how radiation pressure can be used to propel
spacecraft. Although the acceleration achievable with a reflective solar sail is too
low for manned space flights, it should make long-term missions of small unmanned
satellites possible without additional propulsion.
One challenge is to develop materials for lightweight yet robust sails and the
mechanical structures for their deployment in space.
NASA's latest attempt (last access: 16 April 2025) aims to
propel a probe with a total mass of 16 kg using an 80 m² sail.
The Earth's orbit should be synchronised with the sun so that the sun illuminates
the sail vertically at all times with the solar constant of 1361 W/m².
Please calculate the achievable acceleration.
Check your results on this page!
Worksheet 1.2: Comet tails
Please supplement your knowledge of radiation pressure with
worksheet 1.2:
Comet tails.
