Spectra Student Guide

Introduction:

In this lab you’ll use a high quality spectrometer, made by Project STAR, to examine the spectra of a variety of light sources. The main goals are to practice accurately observing and recording the appearance of a spectrum, and to think about the different types of spectra and the characteristics of the objects that create them.

Background material:

Astronomers are very interested in spectra – graphs of intensity versus wavelength for an object. They basically tell you how much light is produced at each color. Spectra are described by Kirchoff's Laws:

1.A hot opaque body, such as a dense gas or a solid, produces a continuous spectrum – a complete rainbow of colors.

2.A hot, low-density gas produces an emission line spectrum – a series of bright spectral lines against a dark background.

3.A cool, low-density gas in front of a source of a continuous spectrum produces an absorption line spectrum - a series of dark spectral lines among the colors of the continuous spectrum.

Emission and absorption lines have a characteristic pattern that is determined by the composition of the gas. For a given gas, the bright lines in the emission spectrum occur at exactly the same wavelengths as the dark lines in the absorption spectrum. One can think of the absorption spectrum as a continuous spectrum minus an emission spectrum.

Review the background material on light and spectra: http://astro.unl.edu/naap/hydrogen/light.html http://astro.unl.edu/naap/blackbody/spectra.html

1 of 9 AST101: Our Corner of the Universe Lab 5: Spectra

1 Introduction

Objectives

Materials

A Project STAR spectrometer and various spectral tubes.

Using the Spectrometer Using the Spectrometer:

Eye Hole

Calibrated Scale

Source

Figure 1: The STAR Spectrometer. Note the locations of the eye hole, the calibrated scale that you look at through the eye hole and the position of the source with respect Figure 1: Theto STARthe spectrometer. Spectrometer. This is Note explained the locationsin detail below. of the eye hole, the calibrated scale that you look at through the eye hole and the position of the source with respect to the spectrometer. This is explainedHold in detail the spectrometer below. so that you can look through the grating in the narrow end. You should be able to see two rows of calibration marks and numbers. Pay attention to the lower row, which gives the wavelength (in nanometers, or nm) of the light in the Hold the spectrometerspectra above so it. that you can look through the grating in the narrow end. You should be able to see two rows of calibration marks and numbers. Pay attention to the lower row, which To observe a spectrum, keep holding the spectrometer up to your eye, and turn your gives the wavelengthwhole body (in until nanometers, the slit at the or right-hand nm) of theside lightof the in front the is spectra pointed aboveat the source it. of light you want to examine. (This is the most counter-intuitive part of the whole To observe aprocedure. spectrum, Most keep people holding are tempted the spectrometer to just aim the up middle to your of the eye, spectrometer and turn at your the whole body until the slitlight at the source. right-hand Aim the side right of side the instead front is.) When pointed you athave the the source spectrometer of light aimed you want to examine. (This is the mostproperly, counter-intuitive a spectrum of the part light ofsource the wholeshould appear procedure. above the Most wavelength people scale. are tempted to just aim the middleThis procedure of the spectrometer takes a little practice. at the lightIf you source.need help, Aim ask your the rightTA. side instead.) When you have the spectrometer aimed properly, a spectrum of the light source should appear above the wavelength scale.Observing Spectra This procedureYour takes TA will a littleset up practice.a variety of If light you sources need help, for you ask to yourstudy TA.in the lab, similar to the ones we saw in class. We will use:

• Two light bulbs with different brightnesses.1

• An emission tube of hydrogen gas.

• Two emission tubes labeled “Source A” and “Source B.”

• A white fluorescent light (i.e. a regular white strip light).

2 of 9 In this lab, you will be asked to make observations of the spectra of these different sources and answer questions about them. You can make the observations of the spectra in any order you like (it will help to prevent crowding if people do these in different orders). You can answer the questions at any time during the lab, but you should make sure you have observed all the spectra before you leave the lab. Observe each source with your eyes and through the STAR spectrometer and answer the questions below.

White Light Bulbs.

In part one of this lab, we will study a common blackbody in everyday use: a light bulb. You will observe the light bulb at two different brightnesses (which correspond to two different temperatures).

Start by observing either light bulb:

1. What type of spectrum do you see when you look at a white light bulb through the spectrometer?

2. Where in the light bulb does the light come from? Describe the nature of this source of light.

Observe the light bulb on the bright setting:

3. What is the smallest wavelength of light you can see when you view this source through a spectrometer (in nanometers) and what is it color?

4. What is the longest wavelength of light you can see when you view this source through a spectrometer (in nanometers) and what is it color?

5. Which color appears the brightest? What is its approximate wavelength?

3 of 9 Now observe the light bulb on the dim setting:

6. What is the smallest wavelength of light you can see when you view this source through a spectrometer (in nanometers) and what is it color?

7. What is the longest wavelength of light you can see when you view this source through a spectrometer (in nanometers) and what is it color?

8. Which color appears the brightest? What is its approximate wavelength?

Compare your two observations:

9. Describe the changes between the two light bulb observations. What happened to the spectrum as the brightness and temperature of the light bulb increased? Specifically, what happened to the relative amount of light at different wavelengths?

10. Betelguese is a Red Giant Star found in the constellation Orion. Sirius, the brightest star in the sky, is much hotter than Betelguese. Describe how you might expect the colors of these two stars to differ.

4 of 9 11. Wein’s law relates light source temperature to the location of the peak of a blackbody light intensity curve – the most intense wavelength of light.

λmax = (2.89 × 10-3 K m)/T

Estimate the temperature of the light-emitting source on the bright setting and on the dim setting. Note that 1 nm = 10−9 m. Show your work below:

Hydrogen Gas Lamp. A gas lamp is filled with a diffuse gas. Electricity is used to excite the gas, adding energy to the electrons. As the electrons return to their original energies, they emit light.

1. What color does the hydrogen lamp appear to be when you view it with your eyes?

2. What type of spectrum do you see when you look through the spectrometer at the hydrogen gas lamp?

5 of 9 3. Carefully make a sketch of the spectrum that you see through the spectrometer on the scale below. Draw a vertical pencil line at each wavelength where you observe a line in the hydrogen spectrum. Label each line with its color. 3. Carefully make a sketch of the spectrum that you see through the spectrometer on the scale below. Draw a vertical pencil line at each wavelength where you observe a line in the hydro- Type: gen spectrum. Label each line with its color. Source: Composition:

700 600 500 400 nm

Type: 4. 4.IdentifyIdentify the line the corresponding line corresponding to the 1.9 to eV the transition 1.9 eV thattransition we discussed that we in discussed class. Remember in that class. Calculate the wavelength (in nm) that corresponds to 1.9 eV using the 19 Energy in eV 1.6 10 = h frequency, Source: following relationships: ⇥ ⇥ ⇥ and Composition: −19 Energy inc =eVfrequency × 1.6 × 10 wavelength, = h × frequency, ⇥ 34 2 8 where h = 6.626 10 m kg/sc = isfrequency Planck’s constant× wavelength. and c = 2.998 10 m/s is the speed ⇥ 9 ⇥ of light. Recall that 1 nm = 10 m. Circle the line in your drawing above and show your workingThe700 below. constants and unit 600 conversions you will 500 need are: h = 6.626 400 × 10 −34 m n2 mkg/ (Planck’s constant), c = 2.998 × 108 m/s (speed of light), and 1 nm = 10−9 m. Type: Show your work below, then circle the line in the drawing above. Source: Composition:

700 600 500 400 nm

Type: Source: Composition:

700 600 500 400 nm6 of 9 Type: Source: 3 Composition:

700 600 500 400 nm

Type: Source: Composition:

700 600 500 400 nm

5 Sources A and B. SourcesSources A andA and B. B. Sources A and B are emission tubes set up near the front of class. One of these tubes contains SourcesSources A A and and B B are are emission emission tubes tubes set set up up near near the the front front of class. of class. One One of these of these tubes tubes contains mercury vapor and the other contains helium gas. They are both hot gas sources, so they both mercurycontains vapor mercury and thevapor other and contains the other helium contains gas. Theyhelium are gas. both They hot gasare sources,both hot so gas they both produce emission spectra. Make a careful sketch of the emission spectra viewed through the producesources, emission so they spectra.both produce Make emission a careful sketchspectra. of Make the emission a careful spectra sketch viewed of the throughemission the spectrographspectra viewed on the through scales below.the spectrograph Make sure you on dothe not scales mix upbelow. sources Make A andsure B, you or youdo not will not spectrograph on the scales below. Make sure you do not mix up sources A and B, or you will not bemix able up to sources answer theA and questions B, or you below. will not be able to answer the questions below. be able to answer the questions below. Type: Source A Source: Source A Composition:

700700 600600 500500 400400 nnmm Type: Type: Source: Source B Source: Source B Composition: Composition:

700700 600600 500500 400400 nnmm 700700 600600 500500 400400 nnmm Type: Type: TheThe tables tables below below shows show a listsome of someof the of visible the visible spectral spectral lines lines emitted emitted by by the the elements elements helium Source: The tables below shows a list of some of the visible spectral lines emitted by the elements helium Source: andhelium mercury: and mercury: and mercury: Composition: Composition: Wavelength (nm) Element Wavelength (nm) Element Wavelength (nm) Element Wavelength (nm) Element 447 Helium 436 Mercury 447 Helium 436 Mercury 700700502 600600Helium 546 500500 Mercury 400400 nnmm 700588502 600HeliumHelium 579 546 500 MercuryMercury 400 nm 700588 600Helium 579 500 Mercury 400 nm Type: 668 Helium Type: 706668 HeliumHelium Source:Type: 706 Helium Source: Composition:Source: 1. Using your drawings of the spectra and the tables above, determine which element is in each Composition: 1.1. UsingemissionUsing your your tube drawings drawings A and B.of of Circlethe spectraspectra the correct and and the answersthe tables tables above,below. above, determine determine which which element element is in each is Composition: in eachemission emission tube tube A and AB. and Circle B. Circle the correct the correct answers answers below. below. Source A: Mercury Helium 700Source A: 600 Mercury 500 400 Helium nm 700Source A: 600 Mercury 500 400 Helium nm 700 Source 600 B: Mercury 500 Helium 400 nm Type: Source B: Mercury Helium Type: Source B: Mercury Helium Source: Source: Composition: Composition: 4 7 of 9 700 600 4 500 400 nm 700700 600600 500500 400400 nnmm 700 600 500 400 nm Type: Type: Source: Source: Composition: Composition: 700 600 500 400 nm 700700 600600 500500 400400 nnmm 700 600 500 400 nm Type: Type: Source: Source: Composition: Composition: 700 600 500 400 nm 700 600 500 400 nm

5 5

5 5 Fluorescent Lamp. Observe the white fluorescent lamp through your spectrometer. Notice that although the spectrum appears somewhat continuous, like the white light bulb, there are bright lines in the spectrum. A fluorescent tube is a gas-discharge lamp that uses electricity to excite a gas inside the lamp. The gas emits visible light as well as high-energy ultraviolet light, and the ultraviolet light causes a phosphor coating on the inside of the lamp to fluoresce, producing more visible light. In this lab, we want to determine which gas is inside the lamp making the phosphor coating fluoresce.

1. Carefully make a sketch of the spectrum that you see through the spectrometer on 3. Carefullythe scale makebelow. a sketchDraw ofa vertical the spectrum pencil that line you at seeeach through wavelength the spectrometer where you on observe the scale below.the brighter Draw alines vertical in the pencil spectrum line at each of the wavelength fluorescent where lamp you observe a line in the hydro- Type: gen spectrum. Label each line with its color. Source: Composition:

700 600 500 400 nm

Type: 4.2. IdentifyOnce you the have line correspondingobserved all the to the spectra 1.9 eV in transition the lab that(the we white discussed light, insource class. A Remember and B thatand the hydrogen lamp), use your drawing and the spectra from the previous 19 exercises to determineEnergy what ingas eV is inside1.6 10 the fluorescent= h frequency, lamp. Circle your answer Source: ⇥ ⇥ ⇥ below. Composition: and c = frequency wavelength, Hydrogen Mercury⇥ Helium 34 2 8 where h = 6.626 10 m kg/s is Planck’s constant and c = 2.998 10 m/s is the speed ⇥ 9 ⇥ of light. Recall that 1 nm = 10 m. Circle the line in your drawing above and show your working below. Explain 700your reasoning: 600 500 400 nm Type: Source: Composition:

700 600 500 400 nm

Type: Source: Composition: 8 of 9

700 600 500 400 nm Type: Source: 3 Composition:

700 600 500 400 nm

Type: Source: Composition:

700 600 500 400 nm

5 Star spectrum: Below is a spectrum that was measured from a particular star. The intensity of light at different wavelengths is plotted for a range of wavelengths including visible light. Several dips in the intensity are labelled. Note that the wavelength increases from left to right in this plot, while the spectrometer showed you wavelength decreasing from left to right.

[nm][

5. What type of spectrum is plotted in the figure?

6. At approximately what wavelength is the star emitting the most light?

7. How do you think the temperature of this star compares to the light bulbs you observed earlier? Why?

8. One dip in intensity is labelled “Na”, for sodium: this star has sodium gas in its atmosphere. If you made a sodium vapor lamp, it would emit light at the same wavelength as this dip. What wavelength is this, and what color would a sodium vapor lamp appear?

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