Light and telescopes – our window on the universe
What is light?
- The warmth of sunlight reminds us that light is a form of energy.
- Energy can take many forms and can be converted from one form to another
- Some other forms of energy: kinetic energy (energy of motion), heat, sound, chemical energy, etc…
- When light from the Sun warms the ground or your skin, light energy is being converted into heat. Solar power plants
convert light into electrical energy
Light is a form of energy
- Units of energy: joules
- 1 joule of energy is enough to:
o Stat a 1kg object moving at 1m/s
o Heat up 1 gram of water by approx.. 0.25 degrees C.
- Watts are a unit of power: energy per unit time.
o 1 watt = 1 joule/sec. so:
- A 100W light bulb is using 100 joules per second (only part of that energy is actually emitted as light – the rest of it
mostly becomes heat.
Quantifying light intensity
- Intensity of light can be measured in W/m^2
o If the intensity of light from a source is measured to be 1W/m^2, then that means the amount of energy falling
every second on every square meter of surface is 1 joule (if that surface is perpendicular to the direction the light
is coming from)
- The intensity of light is related to its brightness
- To summarize: light involves a flow of energy. Intensity tells you how much energy is passing through each unit of
perpendicular area each second
- But, if a surface is at an angle, then the same energy is spread over a larger area:
Inverse square law (for light intensity)
- As you get farther from the source, the same energy gets spread over a larger area.
- That means the intensity decreases as you get farther away from the source - Stars that are farther away look dimmer than they would up close
- Mathematically speaking, light acts like gravity: if you double the distance, the intensity becomes ¼ as much – triple
the distance, intensity becomes 1/9 as much, etc.
- That’s the same type of mathematical behavior as in Newton’s law of gravity (gravity gets weaker as distance
increases but never really goes away)
Light: Wave or Particle?
- 1670, Hook, Huygens: light behaves like a wave
- 1700, Newton, Light consist of particle (corpuscles)
- 1800, Young, Fresnel: showed wavelike properties of light (interference)
- 1873, Maxwell: identified light as electromagnetic waves
- 1905, Einstein: light consists particles of energy called “photons”
- 1920s, ‘30s: Quantum Mechanics: light behaves as both, particle and wave, which properties are apparent depends on
how you observe it.
- We can say that light travels through space as a wave, but some of its interactions with matter (absorption, emission)
are best understood in terms of particles.
What are waves?
- A wave is a periodic (repeated) motion that can carry energy without carrying matter along with it.
Properties of waves
- Wavelength: distance between two wave crests
- Frequency: number of times per second that a wave vibrates up and down
- Wave speed = wavelength x frequency (frequency tells you how many waves go past you each second)
- In general, the wave speed depends on the medium (what type of wave, that it’s moving through)
- The frequency depends on the source (something is vibrating at a certain frequency)
Light as a wave
- Maxwell understood light as an electromagnetic wave: a vibration of electric and magnetic fields together
- Light interacts with matter (electrons, etc.) through these electric and magnetic fields
- Visible light and radio waves are two forms of the same thing – both are electromagnetic waves
Light: Wavelength, Frequency, and Color
- Wavelengths/frequencies are related to colors
- Longer wavelengths/lower frequencies: redder light
- Shorter wavelengths/higher frequencies: bluer light - Newton showed that white light consists of many different colors (wavelengths) mixed together
- Human eyes cannot see most forms of electromagnetic radiation
- Electromagnetic spectrum: the full range of possible frequencies (wavelengths). There is really no upper or lower limit
- Visible light is only a small part of the electromagnetic spectrum: it consists of electromagnetic waves within a certain
(narrow) range of frequencies.
Light: Our Messenger from the Universe
- Light is our window on the universe: almost everything we know about any objects outside earth comes from
observing either visible light or other forms of light
- Some objects (esp. stars, including the sun) emit light through processes that convert other forms of energy into light
- Others, such as planets, do not emit their own light but are visible through reflected light
- Light gives us a lot of information about its source – temperature and chemical composition of distant objects, and
about their motion
- Not all forms of light can get through Earth’s atmosphere: on earth, we can only see astronomical objects through
visible light and radio waves.
How do telescopes help?
- They magnify the apparent angular size of distance objects, allowing you to see more detail.
Limitations of the human eye: how telescopes help
- Light intensity (brightness) describes the amount of energy falling on a surface per unit area.
- Some objects (e.g. faraway stars) are hard to see because they are too dim. Not enough energy from then is reaching
- But you can increase the amount of energy collected by increasing the surface area.
- Pupils of eyes have a small area: one thing telescopes do is concentrate light from a larger area.
Lightgathering power: how telescopes help
- “light bucket”, “light funnel”
- Ability of a telescope to collect more light is called “lightgathering power” Light and telescope II
Photons: particles of light
- When it interacts with matter, light energy is emitted or absorbed in discrete chunks or packets: photons
- The size (energy) of a photon depends on its frequency: E=hf (h = Planck’s constant = 6.62 x 1^34 J s)
- In other words, photons of higher frequency light have more energy than for lower frequency light.
Photons: higher frequency = higher energy
- Photons of higher frequency light have more energy than for lower frequency light
- Medical (and other) consequences of this: higherenergy photons can do more damage to living cells when absorbed.
So, visible light is pretty harmless, but ultraviolet (with its higher frequency/more energetic photons) can cause
sunburn and skin cancer.
- Xray and gamma ray photons have even higher energies, so too much exposure to them can be more dangerous
Telescopes: why bigger is better?
- Review: one of the most important things telescope do is act as a “light funnel” to concentrate more energy from faint
- A wider telescope means more light gathering power. (for the same reason, eye pupils get wider in low light)
- Another limitation of human eyes: diffraction
o The bending of the edges of waves as they pass through an opening (or around an obstacle)
- Diffraction blurs images, so if two stars are too close together in angular distance, their images blur into one
- This puts a limit on the smallest angular sizes we can observe with a telescope (or eye, camera, etc)
- Notice that this depends on both wavelength and aperture size: for bigger wavelengths, you need a bigger telescope to
get the same resolution
Other factors affecting angular resolution
- Aside from diffraction, two other things that limit angular resolution include optical quality and atmospheric distortion
- Telescopes are based on either reflection or refraction of light
- Both reflection and refraction are basic wave behaviors that also happen with sound, water waves, etc. as well as light.
- The bending of waves when they cross boundary between two different media where they travel at different speeds
- Ray Tracing: it is often helpful to describe refraction and other wave phenomena by drawing rays which represent the
direction a wave is moving
- A wave moving from a faster to a slower medium bends so that its direction is closer to the normal (i.e., perpendicular
to the surface)
- From slower to faster is the opposite
Lenses focus light by refraction
Real and virtual images - Image appears at the place where light rays appear to be coming from
- You can figure out where that is by ray tracing using some simple rules
o A ray that enters the lens parallel to the axis goes out through the focus
o A ray that goes through the focus comes out parallel to the axis
- If the object is farther away than the focal length, then the image is smaller, upside down, and in front of the lens: “real
- If the object is inside the focal length, then the image is larger, and behind the lens: “virtual image”
- For objects extremely far away (like stars), the image is very small and almost at the focus
- Refracting telescopes use two lenses:
o objective lens: creates a very small real image at its focus
o eyepiece lens: then magnifies the real image, forming a larger virtual image that you can see
- Angular magnification is the ratio of the two lenses’ focal lengths.
Power of a Telescope
- Lightgathering power
o Ability to collect and concentrate light
o The wider the objective lens, the greater the lightgathering power
- Resolving power
o Ability to see fine detail clearly. This is partly limited by diffraction (so, wider is better) partly by the quality of
the lens, and partly by the atmosphere
- Magnifying power
o The increase of apparent angular size. This is easily changed by changing the eyepiece, and in some ways is the
least important, since magnifying a faint or blurry image doesn’t help.
Limitation of Lenses
- Spherical aberration or other imperfections prevent a sharp focus - Chromatic aberration: different colors focus at different points, because different wavelengths of light refract at
o This is actually what is used to separate colors in a prism, but it is a problem for telescopes or cameras.
o Coatings on lenses or sets of multiple lenses can reduce chromatic aberration
- Large highquality lenses (for the objective lens) are difficult and expensive to make
- Large lenses are also very heavy, so they tend to sag under their own weight when supported only at the edges
- Chromatic aberration is never completely eliminated
- To get around these problems, Newton built the first reflecting telescope
- Reflecting telescopes use a curved mirror instead of a lens to focus light
o Light reflects off a reflective coating, and does not have to pass through glass. That means no
o Only the surface needs to be polished, while a lens needs to be free of defects all the way through
to work well
o Mirrors can be supported easily from behind
- Better mirrors that tarnish less have become available since the 19 century
- For these reasons, large optical research telescopes built since the early 20 century have almost all been reflecting
Another challenge for astronomers
- Distortion caused by Earth’s atmosphere is called “seeing”
- Uneven temperature and moisture cause refraction as light travels through the air, blurring and distorting images.
- As atmospheric conditions change, image is unsteady and stars appear to twinkle
- Why don’t planets twinkle? They have bigger angular sizes, so light from different parts of the planet’s image is
affected differently. Changes cancel each other out.
What to do about seeing: several solutions to the problem
- Minimize distortion by placing telescopes in dry climates and on
- Put telescopes in space above the atmosphere
- Adaptive optics – technology (developed in the 1990’s)uses a computer and
small motors to compensate for atmospheric distortion by continuously
changing the shape of the secondary mirror o Requires a bright source in a known location for the computer to focus on and calculate the correction
o Can use a known star: Guide star
o Or, create an artificial guide star by bouncing a laser beam off of the upper layers of the atmosphere
Observing nonvisible light
- Earth’s atmosphere is transparent to visible light and radio waves
- With adaptive optics, we may soon be able to get visiblelight images from the ground that are as clear as we get from
the Hubble Space telescope
- But, to observe other types of light that don’t get through the atmosphere, we still need telescopes in space.
- Power of a telescope:
o Lightgathering power (depends on size)
o Resolving power: ability to distinguish fine details: depends on optical quality, diffraction limit, and the
o Magnifying power: actually the least important. Depends on focal length of eyepiece compared to objective
o Diffraction limit: a basic limit on resolving power – depends on wavelength compared to the size of the
- Refracting telescopes use an objective lens to focus light and produce an image, and an eyepiece lens to magnify the
image and make it visible.
- Reflecting telescopes have a curved primary mirror instead of an objective lens
- The basic ideas are the same for both – both have a part that does the main job of focusing light, and an eyepiece to
magnify the image.
- Modern research telescopes are reflecting telescopes
- Largest visiblelight telescopes currently in use have a diameter of about 10m
- Most capture images with Charge coupled devices (CCDs): essentially a digital camera.
Why look at other types of light?
- Infrared and radio waves can pass through clouds of gas and dust that are opaque to visible light. This can allow us to
see more of our own galaxy.
- Different types of sources emit different ranges of wavelengths
o Object that are cooler than stars (like planets, or warm dust clouds) emit mostly infrared
o Cosmic microwave background shows us the last remaining heat from the Big Bang and gives us clues to the
early history of the Universe. o Some radio sources include active galactic nuclei, pulsars (neutron stars)
o Many stars emit at least some Xrays (including the Sun). matter falling into black holes emits a lot of Xrays.
o Short gammaray bursts from distance galaxies my come from the explosions of very large dying stars
o In2010, the Fermi gamma ray telescope discovered two giant “gammaray bubbles” near the centre of the Milky
How do we do astronomy with nonvisible light?
- Radio telescope
o Reflecting telescopes for radio waves
o Since radio waves have large wavelengths, diffraction is a problem. We need large mirrors to get good angular
o The good news: an optical mirror needs to be smooth compared to the wavelengths of visible light, but a radio
mirror just needs to be smooth compared to radio waves.
o Another way to improve angular resolution: combine signals from several telescopes
o Easiest with radio waves, but it can now also be dome with visible light
o As far as angular resolution is concerned, it’s like having a much bigger telescope
o In most ways, infrared optics are not that different from visible light. The problem is that earth’s atmosphere
blocks a lot of IR
o IR telescopes also need to be insulated from heat
- Shorter wavelengths: Xray and gammaray astronomy
o Xray and gammaray telescopes need to be above the atmosphere
o Xray and gamma rays are very difficult to focus: an ordinary lens or mirror won’t work
o To focus Xrays, we use grazingincidence mirrors
- Spectroscopy: getting more information from light
o Spectroscopy: breaking light from an object into its different wavelengths, and comparing the amounts of light
emitted at different wavelengths
o Three basic types of spectra
Absorption (line) o Most sources show some combination of these three basic types
o Continuous spectrum (thermal radiation)
A hot, dense object emits light at all wavelengths at once in a continuous spectrum
Ex) a standard incandescent light bulb, the sun, a glowing heating coil on the stove, a human body
This emission is also called blackbody radiation
The spectrum depends on the temperature
StefanBoltzman Law: hotter objects emit more radiation than cooler ones
Wien’s Law: hotter objects emit photons with a higher average energy. The wavelength of peak
intensity shifts toward lower wavelengths as temperature increases
This means that we can figure out the temperature of a star by looking at its thermal spectrum.
Betelgeuse is redder in color than Rigel: it is a cooler star
Objects at temperatures around room temperature or body temperature emit mostly infrared light.
Continuous spectra are produced by dense objects with many atoms packed together.
o Line spectra
Each type of atom has particular energies of photons that it is able to absorbe or emit. These photon
energies correspond to particular transitions between energy levels of that atom
The set of all these energies (and corresponding light frequencies) form a kind of “chemical
fingerprint” for that atom
Molecules often have more complicated spectral fingerprints than single atoms
What are the main objects in the solar system? – Sun, planets, meteors, asteroids, comets and Dwarf planets
Solar system objects: The Sun
- The sun includes most of the matter in the solar system
- Mostly hydrogen and helium
Solar system objects: Planets
- Terrestrial planets (Mercury, Venus, Earth, Mars) are small, rocky, and closer to the sun
- Jovian planets (Jupiter, Saturn, Uranus, Neptune) are large, gassy (contain a lot more hydrogen, helium, methane, etc)
and farther from the Sun
- Jovian planets also have more moons than the terrestrial planets
- Jovian planets all have rings, but Saturn’s are the most noticeable
- Order of planets by distance from the Sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune The Asteroid Belt
- Asteroid belt is a region at distance approximately 24 au from the Sun, between the orbits of Mars and Jupiter
- Total mass adds up to about 3x10^21 kg, or 4% of the Moon’s mass
- Includes between 700,0001,700,000 objects with diameters of 1km or more
- Rocky and metallic,