ASTR 5.1-5.4

How do we experience light?

• Light has energy

  • We measure the flow of energy in units of watts

Colors of Light

Sunlight (which is white light) is composed of numerous wavelengths.

• A glass prism uses refraction to split white light to its constituent wavelengths, revealing a spectrum.

Objects & interaction with light (7)

• Emission

• Transmission

• Reflection

• Refraction

• Absorption

• Scattering

• Diffraction

Light’s properties

Light is composed of perpendicular oscillating electric and magnetic fields

Amplitude

The maximum height of the wave peaks

Wavelength

The distance between two wave peaks

Frequency

Is the number of wave peaks that pass a given location per second

Wavelength and frequency for light waves

For electromagnetic radiation (light), wavelength and frequency obey the relation:

C=wavelength x frequency

• Since the wave speed c cannot change, a greater wavelength must result in a lower frequency and vice versa.

Photons

Particles of Light.

• each photon has a wavelength and a frequency (and a velocity)

  • The energy of a photon depends on its frequency or equivalently, the wavelength.

Light and matters Interaction

• Light interacts with charged particles in materials through the electric and magnetic fields.

• Molecules and atoms absorb light and convert the energy into heat.

• Atoms can absorb photons and reemit them in a different direction.

• Light can simply change direction due to the interaction with electrons.

What is the structure of Matter?

• Atoms contain a nucleus composed of protons and neutrons

  • The nucleus is surronded by an electron cloud where electrons exist.

Electrons

Negatively charged atoms

Protons

Positively charged atoms

Atomic Number

Number of protons in nucleus

Atomic mass number

Number of protons + neutrons

Molecule

Consists of two or more atoms chemically bonded together

Isotope

Same number of protons but different number of neutrons

Ions

Same # of protons but different # of electrons

Phases and Pressure explanation

The phase of a substance (which is tied to its volume) depends on both temperature and pressure

Ionization

The stripping of electrons from atoms, resulting in a plasma composed of ions

Plasma

formed when a gas is superheated or exposed to high-energy electricity, causing electrons to be ripped away from their atoms and creating a hot "soup" of free-moving, positively charged ions and negative electrons

Dissociation

Breaking of molecules into atoms

Evaporation

Breaking of flexible chemical bonds., turning liquid into a gas

Melting

Breaking of rigid chemical bonds, changing a solid into a liquid

how is energy stored in atoms?

Electrons in atoms are restricted to discrete energy levels.

Ground State

The lowest energy state

Energy level transitions

• An atom can gain energy by absorbing a photon whose energy corresponds to a jump between two energy levels

• If the photon energy does not match the energy difference, no transition will occur.

Three basic types of spectra

1. Continuous spectrum

2. Absorption line spectrum

3. Emission line spectrum

Continuous spectrum

• Inside an incandescent light bulb, it contains varying amounts of all visible wavelengths (along with infrared and a bit of ultraviolet)

• There are no gaps in the spectrum.

• But nothing is perfectly continuous

• 
  

Absorption Line Spectrum

• Atoms like monatomic gas can absorb specific wavelengths, resulting in dark absorption lines in the spectrum.

• The positions of the absorption lines reveal which atoms are present in the atomic sample.

• Certain wavelengths bounce out.

Chemical Fingerprints

• Allows us to know what kind of protons are present.

• Each atom has a signature set of energy levels.

• Each transition on an atom’s energy level structure corresponds to a photon with a unique energy and frequency/wavelength

Emission Line Spectrum

Atoms (like monatomic gas) can be stimulated into emitting specific wavelengths such as by an external light source.

• This forms an emission spectrum with bright lines on a dark background.

Thermal (Blackbody) Radiation

• All objects emit thermal radiation, since they have non-zero thermal energy.

  • If an object is hot enough, we can see that thermal radiation as visible light.

The Shape of a blackbody spectrum

Depends solely on the temperature of the emitting object

1. Hotter objects emit more light at all frequencies

2. Hotter objects emit a higher percentage of high-energy photons (blue light)

Blue-Shifted

When a star is moving towards us

Red shift

When a star is moving away from us

Doppler Effect

Describes a shift in the wavelength/frequency of a wave whenever a wave source is moving relative to the observer.

• If the source is moving away from the observer, the wavelength is increased

  • If the source is moving towards the observer the wavelength is decreased.

The Shift and spectral line wavelengths

The speed of an object can be determined by measuring the doppler shift of its spectral line wavelengths.

• The doppler tells us only about the part of an object’s motion toward or away from us.

  • An object perpendicularly to us would not have any doppler shift.

  • Measuring the galaxy’s rotation rate by measuring the Doppler shifts on both sides.