Electromagnetic waves carry momentum , exerting pressure on surfaces they encounter. This radiation pressure , stronger on reflective surfaces, plays a crucial role in various phenomena, from shaping comet tails to propelling spacecraft using light sails.
Understanding radiation pressure is key to grasping the interplay between light and matter. It demonstrates how electromagnetic waves can produce physical effects, connecting the abstract concepts of electromagnetism to tangible, real-world applications in space exploration and beyond.
Momentum and Radiation Pressure
Radiation pressure on surfaces
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Radiation pressure exerted by electromagnetic waves on a surface when waves transfer momentum upon absorption or reflection
Absorbing surfaces experience radiation pressure P = I c P = \frac{I}{c} P = c I
P P P radiation pressure
I I I intensity of electromagnetic wave (power per unit area)
c c c speed of light (3 × 10^8 m/s)
Reflecting surfaces experience doubled radiation pressure P = 2 I c P = \frac{2I}{c} P = c 2 I
Change in momentum twice as large compared to absorbing surfaces
Higher intensity electromagnetic waves lead to greater radiation pressure (sunlight vs laser )
Surface properties affect pressure
Absorbing surfaces experience less pressure (black paint )
Reflecting surfaces experience more pressure (mirror )
Radiation effects on comets
Comets affected by radiation pressure from the Sun
Increasing radiation pressure as comet approaches the Sun
Comet's tail points away from the Sun due to radiation pressure
Tail consists of dust and gas particles pushed away by Sun's radiation
Always points away regardless of comet's direction of motion
Radiation pressure can alter comet's orbit over time
Force exerted causes small deviations in trajectory (Halley's Comet )
Deviations accumulate, leading to changes in orbital path (period and shape)
Applications in space exploration
Light sail propulsion method relies on radiation pressure
Large, lightweight sail deployed to catch radiation from Sun or powerful lasers
Radiation pressure exerts force on sail, propelling spacecraft forward
Advantages of light sail propulsion:
No onboard propellant required, reducing spacecraft mass and cost
High speeds achievable over long distances
Suitable for interplanetary or interstellar travel (Breakthrough Starshot )
Sun's radiation or ground-based lasers used as power source
Challenges of light sail propulsion:
Requires large, thin, durable sail materials
Maximize radiation pressure, minimize mass (mylar , graphene )
Navigation and control difficult due to limited maneuverability
Dependence on external radiation sources limits trajectory flexibility
Potential applications:
Interplanetary missions within solar system (Solar Cruiser )
Interstellar probes for exploring nearby star systems (Project Dragonfly )
Deorbiting space debris using radiation pressure to remove defunct satellites (CubeSail )
Fundamental concepts
Photons: particles of light that carry momentum and energy
Conservation of momentum : principle applied when photons interact with surfaces
Force: exerted on surfaces due to change in photon momentum
Pressure: force per unit area, describing the effect of radiation on surfaces
Energy: transferred from electromagnetic waves to surfaces during interactions
Maxwell's equations : describe the behavior of electromagnetic waves, including their momentum-carrying properties