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Final

AS101 Study Guide - Final Guide: Light Curve, Runaway Greenhouse Effect, Subduction


Department
Astronomy
Course Code
AS101
Professor
Shohini Ghose
Study Guide
Final

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Nature of Light
The warmth of sunlight tells us that light is a form of energy: radiative energy
• We can measure the flow of energy in lightin units of watts: 1 watt = 1 joule/s
Properties of Waves
Wavelength is the distance between two wave peaks
Frequency is the number of times per second that a wave vibrates up and down
wave speed = wavelength x frequency
Wavelength, Frequency, and Energy
x f = c
= wavelength , f = frequency
c = 3.00 x 108 m/s = speed of light
E = h x f = photon energy
h = 6.626 x 10-34 joule x s = Planck’s constant
The electromagnetic spectrum
Interactions of Light With Matter
Emission: Energy in matter can be converted into light that is emitted
Absorption: Matter can absorb energy in the form of light and convert it to another form or re-emit it
Transmission
o Transparent objects transmit light
o Opaque objects block (absorb) light
Reflection or Scattering: Light can bounce off objects inone direction (reflection) or random directions
(scattering)
We see objects that emit light directly
We see others by light reflecting off these objects.

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Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
The structure of matter
Atoms are the building blocks of matter.
Every element is made up of a different type of atom
All atoms are made of
- protons with positive charge of +1
- neutrons with 0 charge
- electrons with negative charge of -1
Atoms of different elements have a different number of protons
Chemical properties depend on number of electrons in the atom
Atomic Terminology
• Atomic Number = # of protons in nucleus
• Atomic Mass Number (weight) = # of protons + neutrons
Neutral atoms have no net charge: number of protons = number ofelectrons
Ions are either positively charged (number of electrons < number of protons) or negatively charged (number of
electrons > number of protons).
Atomic Terminology
• Isotope: same # of protons but different # of neutrons. (4He, 3He)
• Molecules: consist of two or more atoms (H2O, CO2)
Summary
What is light?
Light can behave like either a wave or a particle.
A light wave is a vibration of electric and magnetic fields. It does not need a medium to propagate.
The wavelength/frequency of light determines colour.
Speed of light is constant.
Photons are particles of light.
• What is the electromagnetic spectrum?
Human eyes cannot see most forms of light.
The entire range of wavelengths of light is knownmas the electromagnetic spectrum.
• How does light interact with matter?
Matter can emit light, absorb light, transmit light, and reflect (or scatter) light.
Interactions between light and matterdetermine the appearance of everything we see.
Light is the cosmic messenger
• Light travels to us from all parts of the universe
Matter in the universe interacting with light leaves its fingerprints in the light
Spectroscopy is the process of dispersing light into its spectrum (different wavelengths) and deciphering the
information in each part of the spectrum

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• A spectral graph shows the intensity or energy (number of photons) at each wavelength/frequency.
• Spectroscopy can tell us about the composition, temperature and motion of objects in the universe
Three basic types of spectra
Spectra of astrophysical objects are usually combinations of these three basic types
Continuous (Thermal) Spectrum
• The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption
• Continuous spectra are observed from hot, dense objects.
Properties of Thermal Radiation
1. Hotter objects emit more light at all frequencies per unit area.
2. Hotter objects emit photons with a higher average energy.
Emission Line Spectrum
• A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and
temperature, producing a spectrum with bright emission lines
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