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Lecture 12

AS101 Lecture 12

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Wilfrid Laurier University
Patrick Mc Graw

Lecture 12 4/8/2013 2:44:00 PM Light and Telescope Part II Review: Properties of light:  Light is a wave and carries energy  Travels through space as a wave, but in some of its interactions with matter, acts more like particles  Visible light makes up a small part of the electromagnetic spectrum which also includes radio, infrared, ultraviolet, gamma rays, etc.  All of the above can be considered different types of light.  Like all waves, light waves have properties of wavelength, frequency and wave speed, which are related: o Wavelength: distance between two wave crests o Frequency: number of times per second that a wave vibrates up and down.  Wave speed: wavelength x frequency  Wave speed is fixed by the medium (for like in a vacuum, it is the speed of light), so that means  The higher the frequency, the shorter the wavelength, and the other way around.  For visible light, redder light = lower frequencies, longer wavelengths. Bluer light = higher frequencies, shorter wavelengths.  In the electromagnetic spectrum, radio waves have the longest waves/lowest frequencies, gamma rays have the shortest waves/highest frequencies.  Wave length comparisons: o Radio: can be as big as a football field, or bigger o Gamma: comparable to an atomic nucleus Photons: particles of light:  When it interacts with matter, light energy is emitted to absorbed in discrete chunks or packets: photons.  The size (energy) of a photon depends on its frequency: E=hf. o (h = Planck’s constant = 6.62x10^-34 J s)  in other words, photons of higher frequency light have more energy than for lower frequency light. Photons: higher frequency = higher energy:  Photons of higher frequency light have more energy than for lower frequency light.  Medical (and other) consequences of this: higher-energy photons can do more damage to living cells when absorbed. So, visible light is pretty harmless, but ultraviolet light (with its higher frequency/more energetic photons) can cause sunburn and skin cancer.  X-ray and gamma rays photons have even higher energies, so too much exposure to them can be more dangerous. Light: Our Messenger from the Universe:  Not all forms of light can get through Earth’s atmosphere: this is one of the reasons for building observatories in space.  On earth, we can only see astronomical objects through visible light and radio waves. How do telescopes help?  You probably know that they make small things look bigger (they magnify the apparent angular size of distant objects, allowing you to see more detail.) Limitations of the human eye: how telescopes help:  Remember that light intensity (brightness) describes the amount of energy falling on a surface per unit area.  Some objects (for example, far away stars) are hard to see because they are too dim. Not enough energy from them is reaching our eyes.  But you can increase the amount of energy collected by increasing the surface area.  Pupils of eyes have a small area: one thing telescopes do is concentrate light from a larger area. How do telescopes help: light-gathering power:  A telescope acts as a “light bucket”, or a “light funnel”.  Ability of a telescope to collect more light is called “light-gathering power”  Twice as much area  twice as much light.  10 times as much area  ten times as much light. Brightness and Magnitude of Stars:  Stars are classified by apparent visual magnitude, a number repressing their brightness.  Originally, there wee just six numbered classes: o First magnitude = brightest stars, sixth magnitude = faintest visible with the unaided eye.  Ptolemy used this magnitude system in his writing, others may have used it earlier.  More recently, it became possible to measure flux (energy per square meter per second) of light more precisely, allowing a more precise numerical measurement of brightness, mv. (apparent visual magnitude)  Smaller numbers mean brighter, larger numbers mean fainter.  With the new sale, some of the brightest objects have negative numbers.  Visual magnitude only counts visible light. Telescopes: why bigger is better:  A wider telescope means more light gathering power.  Another limitation of human eyes: diffraction.  Diffraction: o The bending of the edges of waves as they pass through an opening (or around an obstacle) Diffraction limit:  Diffraction blurs images, so if two stars are too close together in angular distance, their images blur into one. Diffraction limit (angular resolution):  This puts a limit on the smallest angular sizes we can observe with a telescope (or eye, camera, etc.) o Diffraction limit (degrees) = (25/3)^
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