Overview
This lecture introduces electromagnetic radiation (EMR), its nature as oscillating electric and magnetic fields, the electromagnetic spectrum, key EMR equations, and how to relate wavelength, frequency, and energy.
Introduction to Electromagnetic Radiation
- Electromagnetic radiation (EMR) consists of oscillating electric and magnetic fields.
- Phones use EMR, specifically radio waves, to communicate.
- EMR can be abbreviated as EMR; it is a wave created by accelerating charges.
How EMR is Produced
- A charged particle produces an electric field in the space around it.
- If a second charge is placed in this field, it experiences a force.
- Moving or accelerating a charge changes the direction of the electric field, creating a wave.
- Changes in the electric field propagate at the speed of light (c).
- Oscillating charges continuously accelerate, producing oscillating electric and magnetic fields (EMR).
Anatomy of an EM Wave
- An EM wave consists of perpendicular electric (E) and magnetic (B) fields, both at 90° to the direction of propagation.
- Direction of travel: forward; E and B fields oscillate up/down and left/right, respectively.
- Visualize the wave as a sinusoidal pattern where E and B fields are perpendicular to each other and to the direction of motion.
Maxwell’s Five EMR Predictions
- EMR is produced whenever charges accelerate.
- The frequency of EMR matches the frequency of oscillating charges.
- All EMR travels at the speed of light in a vacuum, ( c = 3.00 \times 10^8 ) m/s.
- E and B fields oscillate perpendicular to each other and to the wave’s direction.
- EMR, as a transverse wave, exhibits wave behaviors: interference, reflection, and refraction.
Frequency, Period, Wavelength, and Universal Wave Equation
- Frequency (f) is the number of cycles per second (Hertz, Hz); ( f = 1/T ) where T = period (seconds per cycle).
- The universal wave equation: ( v = f \lambda ), where ( v ) = speed, ( \lambda ) = wavelength (meters), ( f ) = frequency (Hz).
- For EMR, ( v = c ), the speed of light.
The Electromagnetic Spectrum
- Longest wavelength/lowest energy: Radio waves.
- Increasing frequency/energy: radio → microwave → infrared → visible light (red to violet) → ultraviolet → X-rays → gamma rays.
- Visible light spans 750 nm (red) to 400 nm (violet).
- Energy and frequency increase as wavelength decreases.
Calculating Wavelength and Frequency
- Convert visible light wavelength (e.g., 750 nm) to meters for calculations.
- ( f = c/\lambda ); for 750 nm, ( f \approx 4 \times 10^{14} ) Hz.
- Period ( T = 1/f ); for 400 nm, ( T \approx 1.3 \times 10^{-15} ) s.
Key Terms & Definitions
- Electromagnetic Radiation (EMR) — Oscillating electric and magnetic fields propagating through space.
- Wavelength (( \lambda )) — Distance between two identical points on a wave (e.g., crest to crest).
- Frequency (f) — Number of oscillations per second (Hz).
- Period (T) — Time for one complete oscillation (seconds).
- Speed of Light (c) — ( 3.00 \times 10^8 ) meters per second.
- Electromagnetic Spectrum — The full range of EM waves, from radio to gamma rays.
Action Items / Next Steps
- Complete Mr. Reynolds’s questions: section A, numbers 1–8 and 9.
- Memorize wavelength boundaries for visible light: 750 nm (red), 400 nm (violet).
- Be prepared for discussion on measuring the speed of light in the next class.