Presentation on theme: "What we call “light” is only one type of … Electromagnetic Radiation – a way in which energy moves through space. Do not confuse EM radiation with Particle."— Presentation transcript:
What we call “light” is only one type of … Electromagnetic Radiation – a way in which energy moves through space. Do not confuse EM radiation with Particle radiation. We’ll start by discussing the visible light spectrum. When white light is passed through a prism, a “rainbow”-like band of color is seen. ROY G. BIV … Ever met him? Red Orange Yellow Green Blue Indigo Violet This is the order from longest to shortest wavelength. This order of colors never changes!
What is wavelength?… Wavelength (λ) – length over which the wave repeats. Or… Wavelength (λ) – Distance between two crests of a wave. Light wavelengths are very short! λ measured in Angstroms (Å). Named after 19th century Swedish physicist A.J. Angstrom. 1 Å = 1x10 -10 m Approximate sensitivity limit of the human eye. 4000 Å - 7000 Å (or… 400 nm - 700 nm) λ > (longer than) 7000 Å- Infrared λ < (shorter than) 4000 Å- Ultraviolet
Electromagnetic Radiation – a way in which energy moves through space. Speed of light in a vacuum c = 3 x 10 8 m/s = 186,000 miles/sec 7 times around the Earth in 1 second!! “Electromagnetic Radiation” gets its name from the fact that it is a manifestation of rapidly varying Electric & Magnetic fields.
There are seven different regions of the E-M Spectrum They are, in order of increasing wavelength; Gamma Rays X-Rays Ultraviolet Visible Infrared Microwave Radio wave (Be able to describe applications of each region, or where we encounter them in life.)
All of these regions can be used in astronomy to measure objects in the universe. Different methods are needed to detect different wavelengths of radiation. Only certain parts of the E-M spectrum can penetrate the Earth’s atmosphere.
The parts that get through are called windows of transparency. Earth’s atmosphere has two main windows in visible and radio.
For this discussion, we will use the word telescope to mean Optical Telescope These are the types of telescopes you (the student) are most likely to use. What are the most important things telescopes do in astronomy? 1.Gathering light (Our eye’s pupil is only about 8 mm wide) 2.Magnification 3.Increasing light-gathering time Using CCD’s or film Our brain “refreshes” about 30 times/sec. No such thing as a “long-exposure” brain images! 4.Increase resolution or resolving power. Resolution [1/(diameter of optical element)] (see Dawes Limit) Resolution inversely proportional to diameter – The larger the optical element, the smaller the features that can be resolved.
Gathers light – it’s most important job! –Allows us to see faint, distant objects. –Doubling the diameter quadruples the light gathered. Since A = r² – (1)² / (2)² = ¼ –Thus, the bigger one gathers 4x the light! Radius = 1 Radius = 2 So What Does a Telescope Do?
Magnifies –Important when viewing planets, nebulae, and other objects in space. Magnification equals the focal length of the telescope divided by the focal length of the eyepiece. Or, in equation form… Mag = FLT ÷ FLE The longer the eyepiece focal length, the less the magnification. What a Telescope Does:
What a telescope does: Resolves –Separates two objects at a distance so they appear separate.
Resolution In the figure right, two lights are imaged by telescopes of different apertures. Apertures are decreased by a factor of two each step to the right. Notice the decrease in the ability to resolve the two lights as separate objects. Bigger aperture = Better resolution Note also the wavelength dependence. Bluer light = Better resolution. This is due to the Dawes Limit r (arcsec) = 2 x 10 -3 λ/d λ - wavelength in Angstroms d - diameter of the aperture in cm.
Important: Because of air turbulence, the best resolution from ground- based telescopes is limited to ½ arcsec. 1degree = 60 arcmin 1 arcmin = 60 arcsec ½ arcsec = the width of a human hair across two football fields (~ 200 yards). The limiting factor of air turbulence in Earth’s atmosphere is why the Hubble Space Telescope (and other orbiting observatories) are placed into space.
Refractors vs Reflectors Primary/Objective lens Secondary lens or eyepiece Primary/Objective mirror Secondary mirror Eyepiece
RefractingTelescope Refraction – the bending of light, as when it travels from one medium to another. Focal Length – Distance at which the light rays converge.
Chromatic Aberration – Different colors are focused at different points. This causes blurry images. Chromatic Aberration can be solved using compound lenses, but this costs light and makes images dimmer. It also makes telescopes heavier and more expensive.
Reflectors Rule But Why?????? Large diameter refractors are expensive to make. Mirrors are easier to mount. Large mirrors can be supported from behind. Refractors suffer from chromatic aberration. Also, large lenses sag under gravity.
However, reflectors can suffer from Spherical Aberration Parallel light rays do not image at the same point.