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Theme C — Wave Behaviour

C.2.4 Electromagnetic Waves — IB Physics HL Revision Notes & Topical Questions

5 min read 10 parts Lesson 15 of 37 1 quiz

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Curriculum Equation Key Wave Speed & Doppler Effect FormulasFast Exam Solution
v = f × λ | f_obs = f × (v ± v_o) / (v ∓ v_s) | T = 1 / f
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Lesson summary

C.2.4 Electromagnetic Waves — IB PHYSICS SL notes: Describe the electromagnetic spectrum, the common properties of EM waves and their uses, and compare them with mechanical waves.Electromagnetic (EM) wave — oscillating electric and magnetic fields at right angles to each other an…

  • Wave equation
  • Continuous range
  • Visible region
  • Wave behaviour
  • What the symbols mean
  • Interaction

Part 1 of 8Nature of electromagnetic waves

Nature of electromagnetic waves
Electromagnetic (EM) wave
  • Electromagnetic (EM) wave oscillating electric and magnetic fields at right angles to each other and to the direction of energy transfer
  • It is a transverse wave and can be polarized.
Source
  • EM waves are generated by accelerating or oscillating charges, such as electrons in an aerial
Vacuum
  • Vacuum they need no medium and travel through a vacuum
Speed
  • Speed all travel at c = 3.00 × 108 m s−1 in a vacuum, whatever their frequency
  • In a material the speed is lower.
  • In a medium of refractive index n the speed is c/n, the frequency is unchanged and the wavelength is λ/n.
Wave equation
  • Wave equation c = fλ, so frequency and wavelength are inversely proportional

Part 2 of 8The spectrum

Continuous range
  • EM waves form a continuous spectrum by frequency or wavelength
Energy
  • Energy the shorter the wavelength, or the higher the frequency, the greater the energy of the wave
Visible region
  • Visible region humans sense only about 400 nm to 700 nm, with red the longest wavelength and violet the shortest
  • Longer and shorter wavelengths are invisible to the eye.
Overlap
  • Overlap the boundaries between regions overlap and are only approximate
RegionApproximate wavelengthTypical use or source
Radiogreater than 10−1 mBroadcasting, communications
Microwave10−3 to 10−1 mOvens, mobile phones, radar
Infrared7 × 10−7 to 10−3 mThermal imaging, remote controls
Visibleabout 400 to 700 nmVision
Ultraviolet10−8 to 4 × 10−7 mSterilizing, fluorescence
X-ray10−12 to 10−8 mMedical imaging
Gammaless than 10−11 mRadioactive decay, cancer treatment

Part 3 of 8Common properties of all EM waves

Common properties of all EM waves
Speed
  • Speed all travel at c in a vacuum
Wave behaviour
  • Wave behaviour all are transverse and can be polarized, reflected, refracted, diffracted and made to interfere
Energy
  • Energy it is delivered in packets called photons whose energy increases with frequency, E = hf
    What the symbols mean: E = energy of one photon (J) · h = Planck constant (6.63 × 10−34 J s) · f = frequency (Hz)
Interaction
  • Interaction all are emitted and absorbed by matter, with effects that depend on frequency
Frequency from wavelength
  • Frequency from wavelength use c = fλ with wavelengths converted to metres

Part 4 of 8Differences between regions

Long wavelength
  • Long wavelength radio waves diffract round hills, so they reach receivers that cannot see the transmitter
Short wavelength
  • X-rays and gamma rays have high photon energies and can damage living cells
Ionizing radiation
  • Ionizing radiation only the high-frequency end can remove electrons from atoms
    • (ultraviolet and above)
Microwaves
  • Microwaves they heat water molecules

Part 5 of 8Uses matched to interaction

Microwaves
  • Microwaves they are strongly absorbed by water,
    • so they heat food, yet pass through the atmosphere with little loss,
    • so they link satellites to ground stations
Infrared
  • Infrared warm objects emit it, so thermal cameras see in the dark
Ultraviolet
  • Ultraviolet it causes fluorescence and sunburn
X-rays
  • X-rays they pass through soft tissue but are absorbed more by bone
Radio waves
  • Radio waves they are weakly absorbed and diffract widely, so they carry signals over long distances
Atmosphere
  • Atmosphere it blocks most ultraviolet, X-ray and gamma radiation but transmits visible light and radio
    • (ozone absorbs UV)
  • Telescopes for the blocked radiation must be placed in orbit.

Part 6 of 8Inverse square law

Point source
  • Point source the radiation spreads over a sphere, so intensity falls as 1/r2
  • A lamp twice as far away delivers one quarter of the light.
  • The Sun's radiation at Mars is less than half that at Earth.
Source power
  • Source power doubling the power doubles the intensity at every distance
Use
  • Use the same law is used to compare the brightness of stars

Part 7 of 8Mechanical and electromagnetic waves compared

FeatureMechanical wavesElectromagnetic waves
MediumRequired (fluid or solid)Not required
TypeTransverse or longitudinalOnly transverse
VacuumCannot travel through oneCan travel through one
OriginOscillation of particles in a mediumOscillating charged particles
ExamplesSound, ocean waves, seismic wavesRadio waves, UV, X-rays
SpeedMuch slower than cc in a vacuum
  • Misconception
    • mechanical waves are not only longitudinal, since seismic S-waves and water waves are transverse

Part 8 of 8More from the specification

More from the specification
Electromagnetic waves
  • Transverse waves of oscillating electric and magnetic fields travelling at c = 3.00 × 108 m/s in a vacuum.
Wave quantities
  • Period T = 1/f, amplitude is maximum displacement and wavelength is measured between successive crests.
Spectrum
  • Radio, microwave, infrared, visible, ultraviolet, X-ray and gamma in order of decreasing wavelength.
No medium needed
  • Unlike sounds, they propagate through a vacuum.
Polarisation
  • Transverse nature is shown because the oscillation can be polarised in a single plane.
Period
  • Time for one revolution.
Amplitude
  • Maximum displacement from equilibrium.
Displacement
  • Change in position.
Position
  • Location relative to a reference point.
Equilibrium
  • State with zero net force.
Equilibrium position
  • Position where the net force is zero.

Key terms

TermMeaning
Electromagnetic waveTransverse wave of oscillating electric and magnetic fields travelling at c in a vacuum.
Speed of light, c3.00 × 10⁸ m s⁻¹ in a vacuum.
SpectrumOrdered range of EM radiation by frequency or wavelength.
PhotonPacket of EM energy, E = hf.
Ionizing radiationRadiation with enough photon energy to remove electrons from atoms.

Avoid these mistakes

1
Thinking different EM waves travel at different speeds in a vacuum — all travel at c.
2
Believing EM waves need a medium — they do not.
3
Putting the spectrum in the wrong order, for example infrared at a higher frequency than visible light.
4
Calling microwaves or radio waves ionizing — only UV and above is ionizing.
5
Mixing up wavelength and frequency order — higher frequency means shorter wavelength.

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