Goal: To understand how light is used to understand what humankind has discovered about astronomy. Objectives: 1) To learn the Properties of light 2) To understand how the 3 different types of spectra are created. 3) Play the GUESS THE GAS CONTEST!!! Properties of light • Light can act like either a particle or a wave. Most commonly acts like a wave. • Like waves you are familiar with, light waves have a wavelength and energy. • Energy = a constant * speed of light / wavelength Which of the following would NOT emit light waves? • • • • • • • Dentist X-ray machine An outgoing radio tower Radar machine The night side of the moon Neon sign All of the above emit light None of these emit light Spectrum of Electromagnetic Radiation Region Wavelength (Angstroms) Wavelength (centimeters) Frequency (Hz) Energy (eV) Radio > 109 > 10 < 3 x 109 < 10-5 Microwave 109 - 106 10 - 0.01 3 x 109 - 3 x 1012 10-5 - 0.01 Infrared 106 - 7000 0.01 - 7 x 10-5 3 x 1012 - 4.3 x 1014 0.01 - 2 Visible 7000 - 4000 Ultraviolet 4000 - 10 4 x 10-5 - 10-7 7.5 x 1014 - 3 x 1017 3 - 103 X-Rays 10 - 0.1 10-7 - 10-9 3 x 1017 - 3 x 1019 103 - 105 Gamma Rays < 0.1 < 10-9 > 3 x 1019 > 105 7 x 10-5 - 4 x 10-5 4.3 x 1014 - 7.5 x 1014 2-3 A radio wave is sent out from planet earth. How long will it take that radio wave to travel to Alpha Centauri B (4.3 light years away) • • • • • A) a little over 1 year B) a little over 2 years C) a little over 4 years D) a little over 8 years E) this radio wave can’t possibly get there. How is light created? • Light is created by electrons changing energy states – to conserve energy. • So, to understand light, you have to understand the atom. • Atoms are made of a nucleus (with + charged protons and uncharged neutrons) and shells of – charged electrons. • Much like different distances from an object creates a difference gravitational energy, so to does the distance from a nucleus. • However, there are only certain specific allowed energy states in an atom. Spectra, meet the spectra! • There are 3 types of spectra: • 1) continuous – a smooth light curve with no breaks and light at all wavelengths • 2) absorption – a continuum which has parts taken out (kind of like a hockey goalie). • 3) emission – either a continuum with emission added to it, or just a series of special lines. Condescend Light Bulb • Take the spectral glasses and take a look at the Condescend light bulb. What type of spectrum would you classify this as? • A) Continuous • B) Absorption • C) Emission Blackbody Spectrum – continuous • All objects which absorb most of the light which passes through them emit energy as a “blackbody”. The shape of the blackbody spectrum is always the same. • The strength of the spectrum (i.e. how much light it emits) and the peak wavelength of the spectrum depend on the temperature of the object. http://theory.uwinnipeg.ca/users/gabor/foundations/quantum/slide6.html Windex • • • • What type of spectrum does this produce? A) Continuous B) Absorption C) Emission Absorption Spectrum • As you can see, the Windex creates a spectrum where you have a continuous portion, but a huge chunk is taken out. • This chunk is absorbed by the Windex. • The Windex, not surprising, is blue because of the light it absorbs. Absorption spectra in Astronomy • Windex is a solution of very complex molecules though, and in Astronomy most Absorption spectra are created by gasses of a few atoms and few molecules. • Therefore, most Absorption spectra in Astronomy consist of thin dark lines, or thin areas where most of the energy is removed at just a few wavelengths. • Any “thin” substance which is in front of a continuous light source will produce an absorption spectrum IF it is cooler than the continuous source. Absorption – our use • When you find an emission or absorption spectrum, you know what the makeup of the gas or material that emitted it. • Each element and each molecule have their own specific energies they can emit. This is a fingerprint of the gas. • This is why we know the sun is made of mostly H and He. Fluorescent lights • • • • What type of spectrum does this produce? A) Continuous B) Absorption C) Emission Fluorescent lights • This is emission. • The electrons in the gas get “excited” by the energy put through the gas. • The electrons then “cascade” down to emit several photons of light. One is a optical photon. • There are different specific energy states, and the lines are produced by the different combinations of energy states (starting/ending). • Any thin gas will produce an emission spectrum if you JUST look at the gas. • Any thin gas which is HOTTER than a background continuous source will ALSO produce emission features! Guess the gas contest! • For each of the following tubes you may find it helpful to draw the pattern of light. You will guess which gas is in each tube. • Anyone who can get all of them gets a bonus 5 points! • Everyone who turns in with guesses and patterns on all tubes gets the normal points. • If you can get all but 1 I will give you a bonus 3 points. • If you get 4 of 6 you get 1 bonus point. Conclusion • We examined the properties of light. • We discovered the method of creating light. • We discovered the different spectrum and explored how to create each one of them (and therefore what we can use them for). • We looked at some gasses and found what most of them were from the light they emitted.