Chapter 6: Optics and Telescopes

• the bending of light when it passes from one substance into another
• your eyes use refraction to focus on light
• can cause parallel light rays to converge to a focus

ex. sun appears distorted at sunset because of how light bends in Earth's atmosphere

• the focal plane is where light from different directions comes into focus
• the image behind a single (convex) lens is actually upside-down
• we initially see things upside down but our brain changes them to being upside right

• a camera focuses light like an eye and captures the image with a detector
• the CCD detectors (charge-coupled devices) in digital cameras are similar to those used in modern telescopes

• light collecting area
• angular resolution

• telescopes with a larger collecting area can gather a greater amount of light in a shorter time
• a telescope's diameter tells us its light-collecting area. Area = pi x radius²
• the largest telescopes currently in use have a diameter of about 10 meters

• the minimum angular separation that the telescope can distinguish
• ultimate limit to resolution comes from interference of light waves within a telescope
• larger telescopes are capable of greater resolution because there's less interference

the angular resolution that a telescope could achieve if it were limited only by the interference of light waves: it is smaller for larger telescopes (better angular resolution

• focus light with lenses
• need to be very long, with large heavy lenses

• focus light with mirrors
• can have much greater diameters
• most modern type of telescopes

• imaging
• spectroscopy
• timing

• taking pictures of the sky
• astronomical detectors can record forms of light our eyes can't see and they generally record only one colour of light at a times
• several images must be combined to make full-colour pictures
• colour is sometimes used to represent different energies of non visible light

• breaking light into spectra
• a spectrograph or spectrometer separates the different wavelengths of light before they hit the detector
• graphing relative brightness of light at each wavelength shows the details in a spectrum

• measuring how light output varies with time
• a light curve represents a series of brightness measurements made over a period of time

• Calm (not too windy)
• High (less atmosphere to see through)
• Dark (far from city lights)
• Dry (few cloudy nights)

the Moon would be an ideal observing site

• bad
• scattering of human-made light in the atmosphere is a growing problem for astronomy
• turbulent air flow in Earth's atmosphere distorts our view causing stars to appear to twinkle

• good
• rapidly changing the shape of a telescope's mirror compensates for some of the effects of turbulence

• only radio and visible light pass easily through Earth's atmosphere
• we need telescopes in space to observe other forms of light
• sharper images are possible because there is no turbulence

a standard satellite dish is essentially a telescope for observing radio waves

it's like a giant mirror that reflects radio waves to a focus

infrared and ultraviolet-light telescopes operate like visible-light telescopes but need to be above atmosphere (in space) to see all IR and UV wavelengths

• need to be above the atmosphere (in space)
• focusing of x-rays require special mirrors
• mirrors are arranged to focus x-ray photons through grazing bounces off the surface

• need to be in space
• focusing gamma rays is extremely difficult

• a technique for linking two or more telescopes so they have the angular resolution of a single large one
• easiest to do with radio telescopes
• now becoming possible with infrared and visible-light telescopes