Basic Properties of Light and Matter & Telescopes
- Light that comes from the Sun: it's white light because it's a blend of each light spectrum (colors).
- Electromagnetic Energy Spectrum, spectrum, energy
- Shorter wavelengths = higher energy, longer wavelengths = lower energy
- Visible surface of the Sun: the Photosphere
- The Earth is a cool body emitter: infrared heat
- Collecting light with telescopes
- Basic telescope design:
- Refraction: prism telescope
- Telescopes
- Refractors: lenses bend light
- Galileo
- Reflector: mirrors
- Most research telescopes today are reflecting
- Hubble: UV and visible
- X-Ray telescopes
- To study black holes: Chandra
- Refractors: lenses bend light
- Basic telescope design:
How do we experience light?
- The warmth of sunlight tells us that light is a form of energy.
- Measuring the flower of energy in light: 1 watt = 1 joule/s
- Colors of Light
- White light is made up of many different colors.
- Emission (fusion)
- Absorption (heat up matter)
- Transmission (movement of electromagnetic matter through a medium)
- Transparent objects transmit light.
- Opaque objects block (absorb) light.
- Reflection (changes its direction without changing its wavelength or giving up energy)
- Albedo: the reflective quality of an object
- Scattering (light reflection: changes direction without using up energy in any direction and allows us to see it)
- Raleigh Scattering: shorter wavelengths experience greater scattering. (Longer wavelengths experience less scattering.)
- Reflection and Scatter Example:
- Mirrors reflect lights in a particular direction.
- Movie screens scatter lights in all directions.
- Interactions of light with matter
- Interactions between light and matter determine the appearance of everything around us.
- Question: Why is a rose red?
- A. The rose absorbs red light.
- B. The rose transmits red light.
- C. The rose emits red light.
- D. The rose reflects red light.
5.2 Properties of Light
What is light?
- Light can act either like a wave or like a particle (photons).
- Waves: A wave is a pattern of motion that can carry energy without carrying matter along with it.
- 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
- Light: Electromagnetic Waves
- A light wave is a vibration of electric and magnetic fields.
- Light is an electromagnetic wave.
- Energy: anything that performs work
- Electromagnetic
- Gravity
- Weak (subatomic)
- Strong (subatomic)
- Light interacts with charged particles through these electric and magnetic fields.
- Wavelength and Frequency
- wavelength X frequency = speed of light = constant
- If the wavelength shortens, the frequency goes up = more energy
- Speed of light: 300,000 km/sec
- wavelength X frequency = speed of light = constant
- Particles of Light
- Particles of light are called photons.
- Each photon has a wavelength and a frequency.
- The energy of a photon depends on its frequency.
- Special Topic: Polarized Sunglasses
- Polarization describes the direction in which a light wave is vibrating.
- Reflection can change the polarization of light.
- Polarized sunglasses block light that reflects off of horizontal surfaces.
- Question: The higher the photon energy...
- A. The longer its wavelength.
- B. The shorter its wavelength.
- C. Energy is independent of wavelength.
5.3 Properties of Matter
What is the structure of matter?
- Molecules ← Elements ← Atoms ← Subatomic Particles: protons, neutrons, electrons ← Quarks
- Conservation of Momentum
- The total momentum of interacting objects cannot change unless an external force is acting on them.
- Interacting objects exchange momentum through equal and opposite forces.
- Atomic Terminology
- Atomic number = # of protons in nucleus
- Atomic mass number = # of protons + neutrons
- Example: Carbon
- Atomic number = 6
- Atomic mass number = 12
- 6 electrons
- Molecules: consist of two or more atoms
- Isotope: the same # of protons but different # of neutrons
- Familiar phases:
- Solid
- Liquid
- Gas
- Phases of same material behave different because of differences in chemical bonds.
- Thermal energy is related to temperature but is not the same. Temperature is the average kinetic energy of the many particles in a substance.
- Thermal energy is a measure of the total kinetic energy of all the particles in a substance. It therefore depends on both temperature and density.
- Phase Changes
- Ionization: stripping of electron, changing atoms into plasma
- Dissociation: breaking of molecules into atoms
- Evaporation: breaking of flexible chemical bonds, changing liquid into solid
- Melting: breaking of rigid chemical bonds, changing solid into liquid
- Phases and Pressure
- Phase of a substance depends on both temperature and pressure
- Often more than one phase is present
- Conservation of Energy
- Energy can be neither created nor destroyed.
- It can change form or be exchanged between objects.
- Energy Level Transitions
- The only allowed changes in energy are those corresponding to a transition between energy levels.
5.4 Learning from Light
What are the three basic types of spectra?
- Three Types of Spectra
- Continuous Spectrum
- The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.
- 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.
- Absorption Line Spectrum
- A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.
- Continuous Spectrum
- Chemical Fingerprints
- Each type of atom has a unique set of energy levels.
- Each transition corresponds to a unique photon energy, frequency, and wavelength.
- Downward transitions produce a unique patter of emission lines.
- Because atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.
- Each type of atom has a unique spectral fingerprint.
- Observing the fingerprints in a spectrum tells us which kinds of atoms are present.
- Energy Levels of Molecules
- Molecules have additional energy levels because they can vibrate and rotate.
- The large number of virbrational and rotational energy levels can make the spectra of molecules very complicated.
- Many of these molecular transitions are in the infrared part of the spectrum.
- Question: Which letter(s) label(s) absorption lines?
- Answer: C & D
- Question: Which letter(s) label(s) the peak (greatest intensity) of infrared light?
- Answer: E
- Question: Which letter(s) label(s) emission lines?
- Answer: A
- Thermal Radiation
- Nearly all large or dense objects emit thermal radiation, including stars, planets, and people.
- Properties of Thermal Radiation (Wien's Law)
- Hotter objects emit more light at all frequencies per unit area.
- Hotter objects emit photons with a higher average energy.
- Question:Which is hottest?
- A. A blue star
- B. A red star
- C. A planet that emits only infrared light
- Why don't we glow in the dark?
- A. People do not emit any kind of light.
- B. People only emit light that is invisible to our eyes.
- C. People are too small to emit enough light for us to see.
- D. People do not contain enough radioactive material.
- By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.
5.5 The Doppler Shift
How does light tell us the speed of a distant object?
- The Doppler Effect
- We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.
- The amount of blueshift or redshift tells us an object's speed toward or away from us.
- Doppler shift tells us only about the part of an object's motion toward or away from us.
- Question: I measure a line in a lab at 500.7 nm. The same line in a star has a wavelength 502.8 nm. What can I say about this star?
- It is moving away from me.
- It is moving toward me.
- It has unusually long spectral lines.
- Measuring Blueshift (objects moving toward us)
- Measuring Redshift (objects moving away from us)
- Measuring Velocity
- Different Doppler shifts from sides of a rotating object spread out its spectral lines.
- Spectrum of a Rotating Object
- Spectral lines are wider when an object rotates faster.
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