2.1 Introduction
2.2 Generation of Electromagnetic Waves
2.2.1 Speed of Electromagnetic Waves
2.2.2 Direction of Propagation of Electromagnetic Waves
2.2.3 Doppler Effect of Electromagnetic Waves
2.3 The Electromagnetic Spectrum
2.4.1 Energy Carried by Electromagnetic Waves
2.4.2 Radiation Pressure
2. Summary
Unit 2 - Multiple Choice - Electromagnetic Waves Questions
Unit 2 - Multiple Choice - Electromagnetic Waves Answers
Unit 2.1 Multiple Choice Extended Questions - Waves
Unit 2.1 Multiple Choice Extended Answers-Waves

Unit 2 Electromagnetic Waves

2.4 Energy and Momentum in Electromagnetic Waves

2.4.2 Radiation pressure

Electromagnetic waves carry energy, therefore they possess linear momentum. In a 'collision' between an electromagnetic wave and a surface, momentum wiil be transferred in a similar manner to an moving object colliding with a surface.The force The force per unit area exerted on the surface as a result of the collision by the EMR is called radiation pressure. The existence of radiation pressure was predicted by Maxwell and it is evidence of the wave-particle duality.

He showed that if a beam of EMR is completely absorbed, then

From Newton's second law Force = ΔP/Δt

Work done or energy expended in stopping the EMR (ΔU) in unit time = F X distance moved in unit time,

ΔU = Fs where s = vΔt

ΔU = FvΔt

ΔU = F.vΔt but F = ΔP/Δt and v = c (speed of light)

ΔU = [ΔP/Δt].cΔt

ΔU = ΔP.c

ΔP = ΔU/c

If the beam is reflected then the force required to change the velocity from c to -c is doubled (Force required to change c to 0 + Force required to change 0 to -c)

ΔP = 2ΔU/c

Radiation pressure Prad = Force/Area

Prad =[ΔP/Δt]/A

Prad =[ΔU/c.Δt]/A


Prad =[ΔU]/[c.AΔt]


If we define Iavg as as energy per unit time per unit area, then

Prad = [Iavg]/c for total absorption and

Prad = [2Iavg]/c for total reflection

So we can generalize into Prad = [kIavg]/c where 1 >= k <=2. k is 1 for perfect absorption and 2 for perfect reflection.

Example 1:

Radiation from a bright lamp falls on your face at a rate of about 1000 Wm-2. Estimate the radiation pressure and force exerted by the the lamp on your face.

Prad = [kIavg]/c where 1 >= k <=2.

3 X 10-6 Wm-6 >= [Prad] <=6 X 10-6 Wm-6

If the face area is about 0.08m-2 then

Force = Prad X Area

3 X 10-7 N >= Force <=6 X 10-7 N



Concept by Kishore Lal. Programmed by Kishore Lal... Copyright © 2015 Kishore Lal. All rights reserved.