420 likes | 431 Views
Telescopes: Portals of Discovery. How do light and matter interact?. • Emission • Absorption • Transmission – Transparent objects transmit light – Opaque objects block (absorb) light • Reflection or Scattering. Reflection and Scattering. Mirror reflects light in a particular
E N D
How do light and matter interact? • Emission • Absorption • Transmission – Transparent objects transmit light – Opaque objects block (absorb) light • Reflection or Scattering
Reflection and Scattering Mirror reflects light in a particular direction Movie screen scatters light in all directions
Reflection of light with mirrors The flat surface enables an incoming light beam to perfectly bounce Angle of Incidence = Angle of Reflection
Reflection and Scattering with Matter Interactions between light and matter determine the appearance of everything around us
Thought 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.
Thought 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.
Refraction (occurs during transmission) • The universal speed of light is measured in a vacuum. • The speed of light is slower in different substances.
Refraction • E.g. Light travels slower through glass or water than through air. • Refraction is the bending of light when it passes from one substance into another
Focusing Light • The speed of light through the liquid in your eye is slower than the speed of light in air. • Refraction can cause parallel light rays to converge to a focus
Image Formation • The focal plane is where light from different directions comes into focus • The image behind a single (convex) lens is actually upside-down!
What have we learned? • How do light and matter ineract? – 4 forms of interaction, including reflection and transmission – Refraction is when light slows and bends when travelling through a substance • How does your eye form an image? – It uses refraction to bend parallel light rays so that they form an image. – Cameras focus light like your eye and record the image with a detector (CCDs in digital cameras).
Telescopes: Giant Eyes • Our goals for learning • What are the two most important properties of a telescope? • What are the two basic designs of telescopes? • What do astronomers do with telescopes?
What are the two most important properties of a telescope? 1. Light-collecting area: Telescopes with a larger collecting area can gather a greater amount of light in a shorter time. 2. Angular resolution: Telescopes that are larger are capable of taking images with greater detail.
Light Collecting Area • A telescope’s diameter tells us its light- collecting area: Area = π(diameter/2)2 • i.e The larger the aperture size, the better the telescope • The largest telescopes currently in use have a aperture diameter of about 10 meters
Thought Question How does the collecting area of a 10-meter telescope compare with that of a 2-meter telescope? a) It’s 5 times greater. b) It’s 10 times greater. c) It’s 25 times greater.
Angular Resolution • Distant objects look very close together; i.e. a small 'angular separation' • Angular resolution is the minimum gap between objects that the telescope can distinguish. • Eventually the car is so far away that you see the headlights as one light. -you have reached the limit of your eye to resolve the angular separation.
Angular Resolution • 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 for the amount of light
What are the basic designs of telescopes? • Refracting telescope: Focuses light with lenses • Reflecting telescope: Focuses light with mirrors • Catadioptric telescope: Focuses light with both lenses and mirrors
Refracting Telescope • Refracting telescopes need to be very long, with large, heavy lenses
Chromatic Aberration • Light focuses imperfectly in a lens because the light must transmit though glass • All lenses suffer from chromatic aberration (slight blurring caused by the short wavelength light focusing before the long wavelengths)
Reflecting Telescope • Reflecting telescopes do not suffer chromatic aberration • Reflecting telescopes can have much greater diameters. • Most modern telescopes are reflectors
Mirrors in Reflecting Telescopes The largest mirrors in the world are in the Keck telescope on Hawaii Twin Keck telescopes on Mauna Kea Segmented 10-meter mirror of a Keck telescope
Newtonian Telescope 1672 One of the most common amateur telescopes deep sky observing. for Early Newtonian used by Herschel Modern version with a Dobsonian mount
Cassegrain Reflector Telescope, 1672 More efficient, but requires a more complex secondary mirror. (hyperbolic rather than flat)
Some Cassegrain Correctors -Catadioptrics Schmidt-Cassegrain Telescope 1930 One of most common amateur telescopes Maksutov-Cassegrain Telescope 1941
Catadioptric Telescopes Corrector lens Schmidt-Cassegrain telescope on an equatorial mount
What do astronomers do with telescopes? • Imaging: Taking pictures of the sky • Spectroscopy: Breaking light into spectra • Timing: Measuring how light output varies with time
Imaging • Early astronomers had no choice, they observed by eye and recorded with descriptions and drawings. – Visible light only. • Camera - permanent record, non-subjective, reproducible world-wide in publications. – Can be built to detect any wavelength.
Imaging • Astronomical detectors generally record only one color of light at a time • Several images must be combined to make full-color pictures
Imaging • Astronomical detectors can record forms of light our eyes can’t see • Color is sometimes used to represent different energies of nonvisible light
Imaging • True color image – The colors are the same as if you were viewing with your own eyes • False color image – The arbitrarily assist image interpretation colors have assigned been to
The famous image of the Eagle nebula is false color! (Nebula in true color)
Spectroscopy • A spectrograph separates the different wavelengths of light before they hit the detector Diffraction grating breaks light into spectrum Light from only one star enters Detector records spectrum
Spectroscopy • Graphing relative brightness of light at each wavelength shows the details in a spectrum
Timing • A light curve represents a series of brightness measurements made over a period of time
What have we learned? • What are the two most important properties of a telescope? – Collecting area determines how much light a telescope can gather – Angular resolution is the minimum angular separation a telescope can distinguish • What are the basic designs of telescopes? – Refracting telescopes focus light with lenses – Reflecting telescopes focus light with mirrors – Catadioptric telescopes focus light with both. – The vast majority of professional telescopes are reflectors
What have we learned? • What do astronomers do with telescopes? – Imaging – Spectroscopy – Timing