The speed of light is a constant - that is true but only when it is traveling through a vacuum. Light travels at slower speeds in a dense medium - like driving on sand. This causes light to be bent at the boundaries. Refraction.



This property of light has useful applications - telescopes




A flat piece of glass will take parallel rays of light, bend then towards the normal as it enters the dense medium and away on exiting. The light then carries on in the same direction - just shifted. If we replace the glass with a lens we can bring the light to a focus




Astronomical objects are so far away that they are essentially parallel rays of light.




Galileo was the first astronomer to use a telescope. He used two lens - the objective lens and the eyepiece lens to view distant objects. Be able to sketch this. The light gathering power depends on the size of the objective lens - propto diameter^2. Magnification is given by the focal length of the objective lens divided by the focal length of the eye piece lens M = fo/fe. In astronomy we do not care about magnification particularly but we do care about light gathering power. Big telescopes are good.




The largest refracting telescope is at Yerkes Observatory. It is a 102cm refractor (the objective lens is 102cm in diameter) but the focal length is 19.5m




This is the lens.




There are problems with refractors: a) chromatic abberation - different wavelength light focuses at different points. Can be corrected at some level but not perfectly. b) glass is opaque to UV light - limited wavelength coverage. c) Glass used to make objective lens must be perfect - hard as lens gets large d) Can only be supported round edge - tend to sag under their own weight.




There is another way we can make a telescope - reflection.




Newton realised a concaved mirror causes light to converge to a focus. Learn how to sketch.




Most modern telescopes use reflection as defects in glass are ok (coated with aluminimum), no chromatic abberation as incident angle independent of wavelength, can be supported from behind. There are four main types of telescope design shown below.




There is one problem - spherical abberation - unless a mirror is perfectly parabolic different paths are focused to different places. This has a narrow field of view. A corrective lens is required.




This is the 10m Keck I telescope in Hawaii. It has a cassegrain design.




There is a limit to how far apart two objects can be distinguished. If two close - the appear to blur. Called angular resolution. The eye can separate things that are 1 arc min apart. The formula is: theta = 2.5 x10^5 lambda/D where D is the telescope diameter and lambda the wavelength. A 5m optical telescope has better angular resolution that a 5m radio telescope. A 10m telescope has better angular resolution than a 5m telescope etc. There are other effects: seeing. The atmosphere blurs the image and makes it appear bigger than it is. This is cos of wind, temperature differences in the atmosphere etc. To minimise this we build telescopes on top of high mountains (or if budget not a problem place them in space). We also build telescopes away from cities (light pollution) where the air is stable, where the weather is good (i.e. dry) in places like Hawaii, Chile, Canery Islands, Australia etc.




Adaptive optics are improving the seeing capabilities on earth though.




The VLT is the first large telescope to use it.




We don't use our eyes to look at images. We used to use photographic plates but they only have 2% quantum efficiency. Use Charge Coupled Devices (CCD's) to count the number of photons that arrive from objects. They are now 80% efficient and very reliable. We use these to do photometry - measure the brightness of objects in various filters.




We also do spectroscopy - get the full spectrum - to reveal clues about chemical composition, temperature, binaries etc




Hydrogen lines in a spectrum




Radio telescopes are large and are easy to construct (as compared to optical ones). Holes are possible due to the long wavelength of radio waves.




Interferometry increases the effective size of the dish even further




Can only do optical and radio astronomy from Earth - emission in other wavebands is blocked by the atmosphere.




UV - the IUE launched in 1978. 45cm reflector. Good for white dwarf detection.




HST - UV to optical.




Light. Galileo and Newton asked about light. In 1676 Romer observed eclipses of Jupiters moon. Look longer for light to reach us at certain times - obtained rough calculation of c.




The Fizeau-Foucault experiment was better in 1850. The mirror rotates - different geometries give different path lengths.




Light is energy but in what form? Newton made an important break through. Split light with a prism - saw spectrum of light. Previously it was thought that the prism added the colours to white light.




Newton isolated part of the spectrum - passed it through another prism and showed no other colours were added. Newton suggested light was made up of tiny particles.




However, like water waves, Young completed an experiment (Young's Slits) that showed light had properties of a wave. See a pattern of constructive and destructive interference.




Light is electromagnetic radiation that travels at 3x10^8 m/s.




We only see part of the whole electromagnetic spectrum (know this!)




Heating something produces electromagnetic radiation. The hotter something gets, it changes colour. Colour => T. More energy, shorter lambda.




Colours show which bits are hot and cool




Radiation from a dense object depends on it's temperature. See radiation at all temperature but there is a peak value. The hotter something is, the shorter the peak wavelength. The curves are for the idealised black body case where all the light absorbed is reradiated.




Sun is almost a black body at 5800K. Human eye evolved to be most sensitive to peak of suns radiation.




Equations here - Wien's Law, Stefan Boltzmann Law, Luminosity, Flux, brightness
























See features in an otherwise continuous spectrum - absorption lines. What are they?




Each element when heated up gives off spectral lines at a certain wavelength - not continuously. Spectral analysis.




What sort of spectrum you see depends on your orientation. Black body, continuous spectrum if viewed directly. Put a cloud of slightly cooler gas in front of it. Some of the energy from the black body is absorbed as it passes through the cloud - absorption line spectrum. If view the fairly hot cloud directly, emission line spectrum.




Detect elements such as iron in the sun this way.




Didn't help with understanding it though. Rutherford showed that when alpha particles are fired at a thin sheet of gold, most get through, some are reflected. Like firing a cannon ball at tissue paper - the returned ones were unexpected.




Suggested most of atom is space with a heavy nucleus at the center of the atom. 99.98% of mass in the center. Like Solar System? Doesn't explain lines though.




Found electrons in certain energy levels only. Takes a certain amount of energy to move an electron from one level to another. If not enough energy - wont move. If too much energy then will still make move to that level unless enough to move it to a higher level. Photoelectric effect.




Move an election up - absorption. When it falls back down, emission.




Whole range of hydrogen features used in astronomy.














One more effect is the doppler effect. If something is moving and emitting light, it's frequency will change. Moving towards you, the frequency of the light appears higher (blueshift) away longer (redshift). Think of sirens.




We see these line shifts in galaxies which are moving away from us.