# AST201H1 Lecture Notes - Lecture 2: Thermostat, Radiography, Photosphere

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25 Aug 2016
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AST201 Week 2
Light – Our Cosmic Messenger
What is Light?
- White Light: A combination of all visible colors of light. Ex: light from the sun, light
bulb. Black is what we perceive when there is no light/color
- Isaac Newton – light is a stream of tiny particles. Depending on how you observe it, it
may act as both
Light can interact with matter through:
- Emission: Matter emits energy in the form of light. Ex: light coming from a light bulb or
computer screen.
- Absorption: Matter absorbing energy. Ex: visible light does not pass through a black
wall, blue light hits a red sweatshirt
- Transmission: Light passing through matter. Ex: cell phone signals passing through
walls, visible light meets clear glass
- Reflection/ Scattering: Matter changes the direction of light, which can be randomly
scattered. Ex: red light hits red shirt, white light hits white paper (light bouncing off
matter
Properties
oAll light has a wavelength: the distance between one peak to the next of the electric and
magnetic field.
oFrequency: the number of times each second that the electric and magnetic field vibrates
up and down/side to side at any point/ number of peaks passing by any point each second
oSpeed: how fast the peaks and the energy travels from one place to another
oAll light travels at the same speed (in empty space) The speed of light: 300,000,000 m/s
oOur eyes perceive different wavelengths as different colors/ we assign a color to each
wavelength. Our eyes are sensitive to only visible light
Electromagnetic Spectrum p.148
- Different wavelengths show different phenomena
oInfrared: light with wavelengths somewhat longer than red light, lies beyond the
red end of the rainbow
oVisible light: light our eyes can see
oUltraviolet: light with wavelengths somewhat than blue light, lies beyond
blue/violet end of rainbow
oX-rays: in between ultraviolet and gamma
oGamma rays: shortest wavelength
- When you pass sunlight through a spectrograph, it appear to separate into a perfectly
smooth rainbow
- We can display a spectrum using either an image (continuous stream of color/rainbow) or
a graph (shows you if light intensity changes)  decreasing wavelength/ bluer, increasing
wavelength/redder
- Orange/Red has the longest wavelength out of all the colours
- The sun appears to have a continuous spectrum: one with some of every colour of light
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AST201 Week 2
- Dense objects, solids, liquids, or even dense gases, emit a particular kind of continuous
spectrum, called a blackbody spectrum: intensity of light goes up, peaks, and slowly
decreases (graph).
- Stars and other dense objects are blackbodies with a continuous spectrum.
- As you heat a dense object (blackbody) up, it emits more light/ hotter over wider range of
energies. The peak is further to the left for hotter objects, showing that hotter objects emit
more light at shorter wavelengths, which means they have high energy.
- If a star is red, it is relatively cool (3000K), blue stars are relatively hot (15000K), white
stars have an intermediate temperature (9000 K)
- If we spread the light from the sun out a lot, we see that the solar spectrum (spectrum of
the sun) is not strictly continuous  breaks.
oThe lines/breaks (absorption lines) tell us what the object is made of/ spectral
lines that tell us what the sun is made of. The lines/breaks come from atoms
oProtons: have a positive electrical charge
oNeutrons: have no electrical charge
oElectrons: have a negative electric charge, distribution of electrons in an atom
gives it its size
oNucleus: the centre of an atom made up of protons and neutrons
- Niels Bohr – electrons can only orbit in the orbitals allowed for that atom
- Orbitals (rings) have different energies – closer to the nucleus (low energy), further away
(higher energy)
- Electrons can drop to a lower energy orbital by emitting light, called emission
- Electrons can jump to a higher energy orbital by absorbing light, called absorption.
- Ionization: when an electron gains more than enough energy to jump to a higher energy
orbital that it leaves the atom, causing it to ionize
Three Types of Spectra p.156-158
1. Continuous spectrum – the spectrum of an ordinary light bulb is a rainbow of color
without interruptions
2. Emission line spectrum – a low density cloud of gas emits light only at specific
wavelengths that depend on its composition and temperature (bright emission lines
against black background)
3. Absorption line spectrum – a cloud of gas in front of a light bulb produces most of
the continuous light emitted by the bulb. But the cloud absorbs light of specific
wavelengths, showing dark absorption lines over a rainbow background.
oThe Sun’s spectrum is an absorption line spectrum: hot interior produces
continuous rainbow of color but cooler gas at the surface absorbs light at
particular wavelengths, causing the spectral/absorption lines
- To transition between two orbitals, an electron has to absorb or emit exactly the right
energy/wavelength/colour of light.
- Different chemical elements have different sets of orbitals (hydrogen, helium etc.). They
also differ in the energies of light they can absorb and emit, and also the colour of light
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