Light and Matter: Exploring Planets and Stars

Light and Matter

• The course explores planets, stars, and galaxies, relying on the light they emit, absorb, or reflect since these objects are too far to visit.
• Spectroscopy is a branch of science, that allows astronomers to study the light and determine temperature, chemical composition, and relative motion.

Nature of Light

• Light can be viewed as a particle (like electrons) or a continuous wave of energy (like ripples).
• Isaac Newton described light as a particle in his 1704 optics textbook.
• James C. Maxwell described light as a wave through the electromagnetic field in 1865.
• Heinrich Hertz produced and detected radio waves in 1888, validating Maxwell's theory.

Waves

• A wave involves a disturbance or variation traveling through a medium.
• The medium comprises small parts that oscillate but don't travel with the wave.
• Waves has cycles of excitation and relaxation which makes up its motion through a medium.

Wave Characteristics

• Wavelength (λ) measures the distance between two identical points on a repeating wave, typically in meters.
• Frequency (f) counts the number of full waves passing a point in one second, measured in Hertz (Hz).
  • 1 Hz equals one wave per second.
• The speed of a wave is the product of its frequency and wavelength (speed = f × λ).
• Light waves of all wavelengths and frequencies travel at the same speed in a given medium, with the speed in a vacuum denoted as "c".
  • c = 300,000,000 meters per second, or 3 × 10^8 m/s (approximately 186,000 miles per second).
• Knowing either the frequency or wavelength allows for the calculation of the other value.

Light as Particles: Photons

• Light exhibits both wave and particle properties.
• Light can be thought of as a collection of individual particles called photons.
• A photon represents the smallest packet of energy that light can be divided into.
• Photons are packets of light energy with no mass, traveling at the speed of light (c).
• Sunlight consists of about 100 quintillion photons (1 × 10^20) hitting a hand every second.
• Light radiates in all directions from the Sun uniformly, due to its spherical shape.
• The energy per unit area decreases as the square of the distance from the source, known as The Inverse Square Law.

Photon Energy

• Energy is inversely related to wavelength; Energy is proportional to frequency.
• Short wavelengths have more energy, while long wavelengths have less.
• The electromagnetic spectrum comprises different names for light's various wavelengths.

Electromagnetic Spectrum Types

• Gamma rays: Wavelengths less than 0.01 nm, emitted by objects at temperatures above 10^8 K, produced in nuclear reactions.
• X-rays: Wavelengths of 0.01-20 nm, emitted by objects at 10^6-10^8 K, found in gas clusters and supernova remnants.
• Ultraviolet: Wavelengths of 20-400 nm, emitted by objects at 10^4-10^6 K, detected from supernova remnants.
• Visible: Wavelengths of 400-700 nm, emitted by objects at 10^3-10^4 K, primarily from stars.
• Infrared: Wavelengths of 10^3-10^6 nm, emitted by objects at 10-10^3 K, from cool clouds of dust and gas and planets.
• Microwave: Wavelengths of 10^6-10^9 nm, emitted by objects less than 10 K, includes active galaxies, pulsars, and cosmic background radiation.
• Radio: Wavelengths greater than 10^9 nm, emitted by objects less than 10 K, detected from supernova remnants and pulsars.

Visible Light

• Ranges from violet (about 400 nm) to red (about 700 nm).
• Blue/violet light has more energy than red light.
• ROY G. BV is a helpful acronym.

Radiation and Temperature

• Everything in nature is always in motion at a microscopic level.
• The temperature of an object measures the average motion-energy of its particles.
• Objects with excess energy emit radiation in the form of light, carrying energy away.

Wien's Displacement Law

• As temperature increases, the amount of emitted light increases.
• The wavelengths of the emitted light shift to higher-energy, smaller wavelengths.
• Hotter objects emit blue light, while colder objects emit red light.

Spectra

• White light is a combination of many colors
• White light can be split into constituent colors using a prism.
• Spectral lines are dark lines that appear in the rainbows

Spectra Types

• Continuous Spectra: Emitted by hot objects.
• Emission Spectra: Emitted uniquely by individual atoms.
• Absorption Spectra: Occur when light passes through a gas cloud where atoms absorb specific wavelengths.

Electronic Transitions

• Electrons exist in specific energy levels (orbits) around an atom's nucleus labeled n=1, n=2, n=3, etc.
• Electrons absorb photons with energy matching the difference between energy levels
  • The electron jumps to a higher level (atomic absorption).
• Electrons spontaneously emit photons to shed extra energy and transition to lower energy levels.
  • The energy of the emitted photon matches the energy difference between levels (atomic emission).
• Spectra reveal atomic composition
  • The Orion Nebula is mostly hydrogen.