Transcript Lec5_2D
Fundamentals of Light Light as a Wave Light (or electromagnetic radiation) can be thought of as either a particle or a wave. As a wave, light has • a wavelength, (distance between waves) • a frequency, (number of waves passing you each second) • an energy, E = h (where h is just a constant) • a speed, c = (this is always the same: 300,000 km/s) Note that because the speed of light is a constant, , , and E are linked: if you know one, you know the other two. The Electromagnetic Spectrum Light waves can be any size, from the size of an atomic nucleus to larger than a football field. The eye only detects a small range of the electromagnetic spectrum. Atmospheric Windows Not all light from space makes it through the Earth’s atmosphere. In fact, only visible light, radio waves, and some infrared light makes it to the ground. The rest of the electromagnetic spectrum can only be observed from space. The Doppler Shift The wavelength emitted by an object is not always the wavelength you observe. If you are moving towards an object, you will see more waves per second (i.e., a higher frequency, like swimming upstream). Yellow light will appear bluer and be blueshifted. Conversely, if you are moving away from an object, the light will be redshifted. The Doppler Shift The wavelength emitted by an object is not always the wavelength you observe. If you are moving towards an object, you will see more waves per second (i.e., a higher frequency, like swimming upstream). Yellow light will appear bluer and be blueshifted. Conversely, if you are moving away from an object, the light will be redshifted. v c The faster the relative motion, the larger the red or blue shift. Light as a Particle Light can also behave as particle. Each packet of light is called a photon, and each photon carries a specific amount of energy (associated with the photon’s wavelength or frequency). Photons emitted from a source will spread out in all directions at the speed of light. Since the amount of area surrounding a source increases as the distance squared, the density of photons will decrease as 1 / r2. This is the inverse square law of light. Scattering of Light Dust (and molecules) in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength (blue) light gets scattered more than red light. That’s why the sky is blue. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. When you look away from the Sun, you see photons that have been scattered towards you. These are mostly blue. Scattering of Light Dust (and molecules) in the Earth’s atmosphere (or in space) can scatter light. In general, short wavelength (blue) light gets scattered more than red light. That’s why the Sun is red at sunset. The long path through the atmosphere means all the blue photons are scattered away. Ways of Creating Light There are 3 ways to produce light: • Through the blackbody process (a.k.a. thermal emission) • Through line emission • Through synchrotron emission (This last way is only for a few peculiar objects with strong magnetic fields. We will be ignoring this mechanism in this class.) The Blackbody Process Anything that is hot (i.e., above absolute zero) produces light at all wavelengths – a continuous spectrum. But the amount of light given off at each wavelength is very sensitive to an object’s temperature. Specifically, The hotter the object: • the more high-energy photons are created • the more light is created (MUCH more) L T4 The Blackbody Temperatures The color of an object reflects its temperature -- cool objects will produce more red light than blue light, while the opposite will be true for hot objects. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. The Blackbody Temperatures The color of an object reflects its temperature -- cool objects will produce more red light than blue light, while the opposite will be true for hot objects. QuickTime™ and a Graphics decompressor are needed to see this picture. The Blackbody Temperatures The color of an object reflects its temperature -- cool objects will produce more red light than blue light, while the opposite will be true for hot objects. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. The Blackbody Temperatures Temperatures Sun: 6000° (optical) People: 300° (IR) Rigel: 44,000° (UV) 1,000,000° gas: x-ray (all temperatures are in Kelvin)