 Light is our messenger from celestial objects-it carried the information we need to understand them What is light? Characteristics of a wave and a particle Wave Properties Wavelength – the crest to crest distance Frequency – the number of crests that pass by you each second Amplitude – the height of the wave tells you about the power Product of the wavelength and the frequency = velocity of the wave = vλν Constructive interference – the two waves are completely in phase Destructive interference – the two waves are completely out of phase Wave nature of light Diffraction: if you pass light through a slit, you get a diffraction pattern If light were made of particles, they would pass right through the slit, and youwould only see the bright spot in the middle not the side lobes James Clerk Maxwell Investigating the properties of electricity and magnetism in the late 19th century  He developed 4 mathematical equations describing electricity and magnetism He was able to solve these four equations and found the solution had the form of a wave with v=2.99x10^8m/s, which is the velocity of light He surmised light was a wave of alternating electric and magnetic fields Light Gathering Power and Magnification How much light can a lens gather?  Light gathering power ~diameter of the lens squared LGP aα2 Useful for comparing lenses (or mirrors) Magnification-ratio of the focal length of the primary lens and the eyepiece M=f/f Limitations of lens LGP scales with a2 , so to gather more light to see fainter objects, you need a bigger primary. Telescopes become too large Largest ever built was the 49.2 Great Paris Exhibition telescope Never used for astronomical observing, dismantled after the Paris Exhibition Chromatic aberration A bigger problem is chromatic aberration Refraction results from the slowing of the light as it passes form air to the material making up the lens. But remember v is a function of wavelength and frequency, so different wavelengths are slowed down to slightlydifferent velocities. Results in different wavelengths having a slightly different focal point. Images are slightly blurred and has collared fringes Reflection Angle of incidence=angle of reflection This does not bring the light to a focus Reflection from a curved surface Astronomers use a parabolic surface to bring the light to a focus Types of reflecting telescopes Resolution The ability of the telescope to distinguish between 2 closely spaced objects orfeatures on a single object. =(2.1x10α5)( /d) where d = diameter of the primary in meters and is λ λthe wavelength at which you are observing = 550nm for optical light.  Known as Dawes Criterion So far the 5m (200 in) Hale telescope at Palomar, the resolution is: =(2.1x10α5)(550nm)/5m) Units need to match 1 nm = 10-9 m, so  =(2.1x10α5)(550x10-9 m)/5m)=0.0231 arcseconds Where there are 60 arc minutes in a degree- 60 arcseconds in an arcminute- So there are 3600arcseconds in a degree- arcseconds usually abbreviated with “- arcminutes abbreviated with a So the size of the primary is critical-bigger primary mirrors allow you to see fainter objects and more details Magnification does not help, it simply takes what you see and makes it larger Electromagnetic Spectrum So we have established light is an electromagnetic wave- a wave of alternating magnetic and electric fields and that it travels at a speed=c Newton discovered that white light was actually composed of light of different colors Herschel in1800 found that the different colors of white light had different temperature, and just beyond the red end there was something unseen that was even hotter-infrared light. A year later, Ritter was experimenting with silver chloride, determining how it would react to the different colors of white light. When he placed it beyondthe violet end of the visible spectrum, he found an intense reaction. How is light generated? Lots of experiments were conducted on light and the spectrum by physicists and chemists in the 19th century  There were combined with the experiments that led to an understanding of the structure of the atom in the early 20th century and led us to an understanding of how light is generated So we need to look at the structure of atoms to discuss how we can generate light.  Atoms Protons – positively charged Neutrons – no charge Electron – negative