Electromagnetic Waves

James Clerk Maxwell

summarized the connection between electricity and magnetism via a set of equations; unified them with the physics of light by discovering that light is an electromagnetic wave

Maxwell’s equations

first one is Gauss’s law, second equation is similar to the first but applies to magnetic fields, third one is Faraday’s law, fourth is Ampere’s law

Faraday’s law

says a changing magnetic field gives rise to an electric field

Ampere’s law

says than a changing electric field gives rise to a magnetic field

Maxwell’s displacement current

final term in the fourth of Maxwell’s equations, refers to changing electric field between the plates of a discharging capacitor

Electromagnetic wave

created by the combined effects of the oscillating electric and magnetic fields

Speed of light

3×10⁸ m/s

Wave

how we describe the transference of energy through a substance without any of the substance itself being transported

Longitudinal wave

wave in which the the components of the material move back and forth in the same direction as the wave

Transverse wave

wave in which the direction of vibration is perpendicular to the direction of the wave

Wavelength

horizontal length between repeating cycles of a wave

Amplitude

maximum height of vibration, intensity of the wave

Wave frequency

how many full wavelengths pass through a given point per second, measured in hertz

Hertz

1 hertz=1 wavelength/second

Velocity of a wave

v=λf

Speed of sound

300 m/s

Sound vs light

sound is slower, cannot travel through the vacuum of space; light is a million times faster than sound, can travel through the vacuum of space

Wavelength of a light wave

λ=c/f

Frequency of a light wave

f=c/λ

Visible Spectrum

400-700 nanometers

Infrared radiation

longer in wavelength and lower in frequency than red, can be felt as heat

Microwave/radio wave

used in long-distance communication

Microwave ovens

has a mini particle accelerator that causes electrons to emit microwave radiation, metal mesh prevents microwaves from escaping

Ultraviolet radiation

higher in frequency and short in wavelength than violet, energetic enough to break apart skin cells

X-rays

can penetrate many materials due to their small size

Wilhem Rontgen

found that X-rays are emitted by electrons accelerated by high voltages, took the first X-ray photograph of his wife’s hand

Gamma rays

have the highest frequency and shortest wavelength, come from atomic nuclei undergoing radioactive decay

Ionizing radiation

UV and beyond, capable of breaking apart atoms in the human body

Wave polarization

direction of oscillation of a wave

Polaroid filters

absorb light polarized in a particular direction, composed of microscopically thin slits

Parallax

difference in the apparent position of an object when seen from different lines of sight

Heinrich Hertz

came up with the idea to send a wireless signal

Guglielmo Marconi

invented the wireless telegraph

Radio telescopes

telescope that focuses radio waves to collect or send signals

Arecibo

one of the largest radio telescopes ever built, located in Puerto Rico

Cosmic Microwave Background

leftover radiation from the birth of the universe that has cooled down to the microwave part of the electromagnetic spectrum

James Webb Space Telescope

largest space telescope ever built, launched in December 2021, detects infrared radiation

Amplitude modulation (AM)

method of converting kHz sound into MHz radio waves; amplitude of the radio wave is varied in proportion to the frequency of the sound wave

RLC circuit

responds to a certain frequency of electromagnetic wave depending on how you tune it

Frequency modulation (FM)

frequency varies with the frequency of the sound

Cathode ray tube (CRT)

used by early televisions to produce images; electrons emitted by cathode are accelerated via high voltages to anode, pass through a magnetic field and are deflected onto a phosphorescent screen

Cathode

collection of negative charge, emits electrons

Anode

collection of positive charge

Liquid crystal display (LCD)

each pixel as two glass plates (polarizers) with a gooey material in between; when voltage is applied, all the liquid crystals line up so light can’t pass through second plate

Cones

eye cells that are sensitive to red, green, and blue

Light-emitting diodes (LEDs)

more efficient alternative to incandescent lightbulbs; use semiconductors

Diode

circuit component that only allows current to flow through in one direction

Semiconductors

mixture of two materials that are normally not good conductors but can act as a conductor when they are combined and exposed to an external voltage

Solar panels

semiconductors absorb sunlight, electrons are freed to the conducting shell, allowing for the flow of current

Light sail

large sheet of reflective material that can catch sunlight, potential way to power spacecrafts; slow acceleration but could reach high speeds after enough time

Ether

theoretical substance that scientists said the speed of light was relative to 

Michaelson-Morley experiment

failed to show that the speed of light can change

Albert Einstein

came up with the theory of relativity

Time dilation

due to the effects of relativity, something moving past you at very fast speeds will appear to experience less time than you do

Length contraction

due to the effects of relativity, moving objects appear shorter in length compared to how they appear when stationary

Lorentz factor

γ=1/√(1-v²/c²)

Speed of causality

c, the ultimate speed of the universe; one event cannot influence another separate event any sooner than information can travel between the two events at the speed of c

On the Electrodynamics of Moving Bodies

Einstein’s paper that introduced the theory of relativity

Electromagnetic force

a fundamental force that can appear as the electric force from certain frames of reference and as the magnetic force for other frames of reference