Astronomy 153 Lab 3: The Light Spectrum

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1 Astronomy 153 Lab 3: The Light Spectrum Solar Radiation Incident on the Earth Intensity v. Wavelength Image By: Astronomers use the light, or radiation, emitted by a celestial object to study it. The above image shows the radiation of the Sun as a graph of Intensity versus wavelength. Visible light, which is what is seen with the naked eye, makes up only a tiny percentage of the total light spectrum, but makes up most of the Sun's emission. Light is electromagnetic radiation and travels through space in the form of a wave. The particle nature of elctromagnetic radiation is the photon. Image illustrating the wavelngth, frequency, and amplitude of a wave. Image By: When an electron orbiting an atomic nucleus loses energy, it emits a photon. The energy of that photon is equal to the energy lost by the electron. The wavelength of light from that photon is propotional to 1/Energy of the photon. The frequency is the number of wave oscillations per second, and is also inversely proportional to wavelength, λ=c/f. Visible light is in the wavelength range of about 300nm to 700nm. From the first image on this page, you can see how most light from the Sun is

2 emitted in that range. This is why our eyes are tuned to those wavelengths, they make up the majority of the available light. But visible light is only a very small portion of light, as is illustrated by the graphic below. Every photon has a different energy, and so a different wavelength. The full spectrum goes from radio waves having long wavelengths, and low energies to the highest energy gamma rays with the shortest wavelengths, and everything in between. The graphic also has objects at representative scales for the light's wavelengths. Note the small space on the scale that visible light makes up. Graphic of the Electromagnetic Spectrum Image By: Every atom has electrons in different configurations at different energies. So they can preferentially absorb and emit light photons with energies corresponding to their electrons' possible configurations. The pattern of light emitted by atoms and molecules in this way is that atom's emission spectrum, and each atom has it's own. In fact the spectrum of a material is like a fingerprint. Each material has a different physical and chemical composition whose spectrum can be used to identify it. Image showing an unknown spectrum to be identified, with several possible spectra. Can you tell which atom the unknown material is made of? Image from:

3 NASA Graphic of Atmospheric Windows Image By: Of course, atoms can emit light in wavelengths away from the visible range, and these parts of the spectrum can be found and identified with instruments and computers. But there is another challenge to identifying these other wavelengths of light. The Earth's atmosphere absorbs and reflects most wavelngths! This helps us as a species because it keeps out most of the harmful, UV, X-Ray, and Gamma-Ray wavelengths that can cause serious cell damage. But it means to view these wavelengths and study the information they carry, we have to put our instruments above the atmosphere in space. The ranges of wvelengths that reach the Earth's surface through the atmospere are called atmospheric windows, and the graphic above shows where these windows are located on the spectrum. Notice the windows in the Visible and Radio wavelengths are the most pronounced. Lab Experiment. In today's lab exercise, you will draw the visible emission spectra lines for eight unknown elements, and identify all eight elements by that visible emission spectrum. Each element is in gas form contained inside a long thin bulb. These bulbs are in lamps, and the lamps are elevated by black wooden blocks. The large grey trianglular pieces on stands are the spectroscopes. Inside the specroscope is a prism that diffracts the light entering it so that each wavelengths is seperated and the spectrum can be distinguished. Without the spectroscope, you would only be able to see the combined colors. The lamps should be positioned so that the bulb touches the back of each spectroscope. When the students are ready to start you can flip on the lamps. The bulbs will start humming, this is normal. If you hear metallic vibrations though, the bulb is a little out of it's seat. Call over your TA to have it reseated before you continue. Now look through the eyepiece of the spectroscope, you should see colored individual lines above a solid rainbow. The rainbow block is just for comparison to the lines that make up the spectrum. There is a small red button attached to the spectrosope, when you press it, a scale should appear on the spectroscope's display to help you see where each line falls on the wavelength chart. Draw each line as accurately as you can, the worksheet boxes have lines every 25nm. Then use your spectra drawings to identify the elements. You can use the charts hanging up on the classroom walls, or use the online resources at: which has the National Institute for Standards official spectrum or every element.

4 Name: Date: Astronomy 153, Lab 3 Worksheet Element Identification As you are looking at each spectrum, record the positions of each emission line for numbers 1 through 8. It might help to write down the approximate wavelength in nm below your drawn lines. Then use the linked spectrum charts online ( and the charts hanging on the classroom walls to identify each atomic gas! 1. Element? 2. Element? 3. Element? 4. Element? 5. Element? 6. Element?

5 7. Element? 8. Element?