Light and Spectra Laboratory

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Astronomy

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Dec 6, 2023

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Name- Gurleen Kaur. Date- 11-05-2023 Section- 101 O ACTIVITY 7 Light and Spectra Learning Goals In this activity, you will explore the properties of continuous and line emission spectra. By working through this activity, you will be able to I. explain how the temperature of an incandescent lightbulb affects the intensity and colors of the observed spectrum. 2. compare the observed continuous spectrum to a series of Planck (blackbody) curves. 3. examine emission spectra of five elements, noting the patterns and intensities of the lines. q. identify three "unknown" elements. 5. state the significance of each element having its own unique spectral signature of emission lines. Key terms: incandescent, luminous, blackbody, spectrum, continuous spectrum, wavelength, visible wavelength, infrared light, ultraviolet light, line emission, intensity Step I—Background Incandescent bulbs pass an electric current through a resistive coil ofwire, usually made of tungsten because of its high melting temperature. As the wire heats up, it starts to glow. The hotter the bulb is, the more luminous (brighter) it becomes. Incandescent radiation is thermal radiation (also called blackbody radiation). When an incandescent bulb is hot, it shines with the whole rainbow of colors. Splitting light into all of its separate colors creates a spectrum. These colors become brighter as the bulb gets hotter. Energy-inefficient incandescent bulbs are being phased out and replaced by the much more energy-efficient fluorescent and LED bulbs that do not depend on thermal radiation. The way incandescent bulbs shine mimics stars; fluorescent and LED bulbs do not shine in the same way. When we see a rainbow in the sky, with all of the colors of the visible part of the spectrum— violet, blue, green, yellow, orange, red—we are looking at a continuous spectrum, one without a break in the colors. Light is traveling from the rainbow to us as waves of energy, waves characterized by wavelength, the distance from the peak of one wave to the peak of the next wave. The wavelengths we can see are known as visible wavelengths, and they range from about 350 nanometers nm, 350 billionths of a meter) to about 750 nm. Light with wavelengths a bit longer than we can see is called infrared light. Light with wavelengths a bit shorter than we can see is called ultraviolet light. If you were to look at the spectrum of a sodium-vapor street lamp (by shining its light through a prism or by reflecting its light carefully off a CD), you will see that only specific colors or wavelengths of light are present. The streetlight's spectrum has just a few curved, bright lines. We call those bright lines line emission. We could describe this by saying "the colors are more intense" at those wavelengths; the intensity of the light is greater. Astronomers study the continuous and line emission spectra of stars to determine the temperatures and compositions of astronomical objects. In this activity, you will explore a few earthbound examples.

34 ACTIVITY • Light and Spectra 1. Rainbows show Emission spectra, while street lamps will have continuous spectra. 7 Step 2--lncandescent Bulbs and the Continuous Spectrum Turn to Figure 7.1 in the appendix. The image shows spectra (Figure 7. la) from an incandescent bulb (Figure 7.1b), at its brightest at the top of the image and at its dimmest at the bottom of the image. These are continuous spectra because the colors run smoothly across the wavelengths. Use colored pencils—or work with shading with regular pencils—to reproduce the brightest spectrum in Figure 7.2a and the dimmest in Figure 7.2b. If you use a regular pencil, label each wavelength with the color that appears in the spectrum at that wavelength. (a) 350 400 450 500 550 600 650 700 750 Wavelength Observed (b) Wavelength Observed FIGURE 7.2 2. Is a dim incandescent bulb hotter or cooler than a bright incandescent bulb? 3. Write a sentence to describe how the temperature, as shown in Figure 7.1c, affects the bulb's intensity. – Intensity of light is directly proportional to the fourth power of temperature. 4. There are obviously colors missing in the spectrum when the bulb is at its dimmest compared with the spectrum of the bulb when it is at its brightest. Which colors are missing? – When the bulb is at its brightest, there are some colors missing which are present when the bulb is at its dimmest. 5. Write a sentence to describe how the temperature of the bulb affects the colors. – Wavelength emitted by blackbody is proportioned inversely to the temperature, meaning depending on the wavelength the color being emitted changes. 0 2019 by W. W. Norton & Company, Inc. Learning Astronomy by Doing Astronomy Second Edition 350 750

6. Combine your answers to questions 3 and 5 to write a sentence describing how the temperature affects BUN-I the intensity and the colors in the spectrum of an incandescent bulb. – The intensity of light from a dim light will be smaller compared to a bright light, but wavelengths from a dim bulb will be larger than a bright light. This is due to wavelengths decreasing when temperature increses. ACTIVITY 7 • Light and Spectra 35 Step 3 —Relating the Results to Blackbody Curves The spectrum of light from a hot object can be graphed, with intensity (brightness) on the y-axis, and wavelength (related to color) on the x-axis. For objects' that shine because they are hot, the resulting curve is called a "blackbody curve." A series of blackb6dy curves are shown in Figure 7.3, for an object at four different temperatures. These curves are "theoretical"; they were generated mathematically. Blackbody curves for temperatures 3500—2000 K FIGURE 7.3 7. Do these theoretical curves support the summary you provided in question 6? Explain your answer, bringing in the intensity of the light at wavelengths in the visible part of the spectrum (300—700 nm). – Yes, it does support the summary since it shows the hottest curve has the highest intensity and emits light over all wavelengths, while the coolest curve has the lowest temperature and hardly emits any light. 8. The peaks in intensity for these curves occur in which region of the spectrum? a. ultraviolet b. visible c. infrared Wavelength (nm)

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36 ACTIVITY • Light and Spectra The melting point for tungsten, the element for the filament of an incandescent bulb, is about 3700 K, which means that the filament probably does not get much hotter than about 3500 K. Examine Figure 7.4, which gives an expanded view of the blackbody curve for an object with a temperature of 2000 K. 7 Blackbody curve for temperature = 2000 K Wavelength X (nm) FIGURE 7.4 9. Assume that the temperature of the filament in the bulb at its dimmest was around 2000 K, so that its spectrum would look like the curve in Figure 7.4. Explain how we were still able to see some red in the spectrum of the incandescent bulb, even though the peak of the spectrum is well outside of the visible wavelength region. - There is still a little bit of light being emitted at red wavelengths, shown on the graph it’s about 0.1. The bulb would be much brighter if we could see infrared wavelengths. 10. If we could see at infrared wavelengths, would the bulb appear _ brighter or _ dimmer? 11. There are many stars that have surface temperatures in the 3500—2000 K range. Would they appear bright or dim to us? Explain your answer by contrasting a 3500 K star with a 2000 K star. - It would appear dimmer because astronomers currently use infrared telescopes to view the stars. 0 2019 by W. W. Norton & Company, Inc. Learning Astronomy by Doing Astronomy Second Edition

ACTIVITY 7 • Light and Spectra 37 Step 4—Emission Spectra and Identifying "Unknown" Elements Turn to Figure 7.5 in the appendix. This series of images contains the true-color emission spectra of five elements. In Figure 7.6, use colored or regular pencils to reproduce these spectra. Be careful to note the wavelength at which each color appears the intensity of each line. Indicate the intensity of a line in Figure 7.6 by making wider lines on your paper for brighter lines in the spectrum. Element names: (a) 350 400 450 500 550 600 650 700 750 Wavelength Observed (b) Wavelength Observed (d) Wavelength Observed (e) Wavelength Observed 2019 by W. W. Norton & Company, Inc. Learning Astronomy by Doing Astronomy Second Helium Mercury Hydrogen Krypton Neon 750 WavelengthObserved 750 400 450 500 550 350 750 350 750 350

38 ACTIVITY • Light and Spectra FIGURE 7.6 @ Edition 7 12. Study the spectra that you sketched. Did any of the elements have the same emission spectrum? If so, which ones? If not, why not? – The spectra of helium, hydrogen, neon, and Krypton are matching. 13. Comment on both the similarities and differences you observed among these spectra. – In the case of helium and krypton, the violet and blue lines are present while the neon spectra is dominated by orange and red spectrums. Turn to Figure 7.7 in the appendix. This figure shows the lines of "unknown" elements. Consider the emission spectra you sketched in Figure 7.6 to be the reference spectra obtained in the laboratory. Identify each unknown element in the final series of images. The pattern of the lines, colors, and spacing are all important. 14. Based on your comparisons between the known and unknown elements, what are the mystery elements? – a= helium, b= hydrogen, c= neon Step 5 —Putting It Together 15. Describe how we can estimate the temperature of stars based on their blackbody (thermal) curve. – To estimate the temperature of stars, we know as the temperature increases the peak of the black body radiation curve shows lower intensity but longer wavelengths. 16. From your results in Step 4, summarize how spectra can be used to find the composition of a gas. Include in your summary the significance of each element having a unique spectrum. Be sure to include at least three key terms in your explanation. This method forms the basis for how astronomers determine the stars—in other words, 0 2019 by W. W. Norton & Company, Inc. Learning Astronomy by Doing Astronomy Second Edition

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the stuff of which stars are made. – Spectra can be used to find the composition of a gas because each element emission spectrum is specifically unique. Meaning we can use the spectra to identify all the elements of the gas and deduce from there. APPENDIX

40 ACTIVITY • Light and Spectra FIGURE 7.1 (a) 0 2019 by W. W. Norton & Company, Inc. Learning Astronomy by Doing Astronomy Second Edition

APPENDIX FIGURE 7.5 (a) (b) (c) (d) APPENDIX

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FIGURE 7.7 (a) (b)

Light and Spectra Laboratory

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