Lab 5 Spectroscopy Key - Answers To Lab

Lab Activity 5 – Atomic Spectra Objective: The purpose of this activity is for the student to use a diffraction grating to observe spectra from various sources and to apply what they have learned to describe the composition of stars. Introduction: Light is described as being both a wave and a particle. In the particle model, light travels via massless individual particles that travel at the speed of light called photons. Photons can be emitted or absorbed by electrons within an atom. Each photon carries with it an amount of energy equal to hf, where h is Planck’s constant = 6.63x10-34 J·s and f is the frequency of the light wave (units??). Therefore, when an electron absorbs a photon, it gains an amount of energy equal to: E = hf When an electron emits a photon, it loses hf of energy. Astronomers can observe these processes via the emission and absorption spectra discussed in class. Recall that each element has its own unique “spectral fingerprint” which can be used identify it. For example, the spectral lines that arise as the result of emission and absorption of hydrogen are different from the spectral lines we observe from helium. In this activity, we will use a “spectrometer”, or “diffraction gratings mounted in a particular way in a snazzy box”, to observe light from various gases. Prior to observing these gases, it is important to realize how different types of spectra are created. Light from hot sources or gases will pass through a diffraction grating you will use. The diffraction grating is just like a prism, breaking light up into different wavelengths. Procedure: 1)#Consider#a#hot#source,#such#as#a#star#or#light#bulb.##Kirchoff’s#1st#Law#states#that#hot#sources#such#as#these# produce#what%type%of%spectra#(think#of#the#kinds#of#spectra#that#we#spoke#about#in#class)? 1pt Continuous, or Black-body # 2)#Now#turn#your#attention#to#the#situation#where#a#hot#cloud#of#gas#stands#alone#without#any#hot#star# nearby.#The#gas#emits#radiation…and#this#creates#what%type%of%spectra?#1pt Emission If#you#were#looking#at#this#type#of#spectrum,#how#would#you#describe#it? 1pt Discrete lines at various colors, against a black/less luminous background. 3)#A#cool#gas#clouds#sits#in#front#of#a#hot#star.#Electrons#within#the#gas’#atoms#gain#energy#and#jump#to#higher# energy#levels.#What#type#of#spectrum#is#produced? 1pt Absorption If#you#were#looking#at#this#particular#spectrum,#how#would#you#describe#it? 1pt Continuous spectrum with black absorption lines 4) Hold the spectrometer close to your eye and look towards the incandescent light bulb in the front of the class. The grating should have diffracted light (broken up light) into its component wavelengths – a spectrum. The spectrum you will see is the entire visible spectrum – you can ONLY see the ‘visible’ spectrum. a)#What#do#you#see?#Are#there#any#spectral#lines?#1pt No b)#This#is#an#example#of#what#type#of#spectrum?#1pt Continuous c)#You#can#only#see#the#visible#light#portion#of#the#electromagnetic#spectrum#here.#List#from#longest# wavelength#to#shortest#wavelength#the#6#regions#of#the#entire#electromagnetic#spectrum#[microwaves#are# included#within#the#radio#region].#6pts Radio – Infrared – Visible – Ultraviolet – X-rays – Gamma Rays 5) Now, turn your spectrometer towards the fluorescent light in the back of the class. a)#What#do#you#see?##Are#there#any#spectral#lines,#or#do#you#see#a#continuum? 1pt Yes, spectral lines. b)#This#is#an#example#of#what#type#of#spectrum? 1pt Emission 6) Now, let’s look at the two tubes in Station A, with one containing krypton and the other containing mercury. They are very, VERY similar, but there are some subtle differences. Draw their spectra below (indicate which side is red and which side is blue). Using your drawings, try to identify what gases you observed. 4 pts Gas A: Krypton (note the brighter purple line on the LEFT of purple band). Also, more reds in spectrum. Blue/Red Blue/Red Gas B: Mercury (note the brighter purple line on the RIGHT of purple band). Also, less reds in spectrum. Blue/Red Blue/Red 7) One of them is similar to the fluorescent bulb. Which one do you think it is (feel free to look back at the bulb)? 1pts The Hg lamp should look similar. The spectrum after fluorescing should look like this: You can see three sets of lines that are similar, but the other fluorescent bands of light blue and red are different. Red in particular is not prominent in the Hg lamb, nor is that turquoise band. 8) What’s the deal, Neal? Why does the tube in #7 look somewhat different from the fluorescent bulb? Let’s explore that. Go to station B and look at my demo with the regular water and the tonic water. After seeing that demo, explain in your own words what is going on with the fluorescent bulb.5pts # Fluorescent bulbs are essentially like the glass tubes in the front – a voltage energizes the gas, and the tube emits certain spectral lines (the coating on the tubes, incidentally, plays a role in the exact lines. The mercury vapor emits UV, which excites the fluorescent material). The difference in appearance is the fluorescing material, just like the UV laser going through the tonic water. UV is “waveshifted” down to a set of red emission frequencies. 9) Let’s look at the overhead projector demo. First, look at the light from the primary bulb through the spectrometer. What type of spectrum is this? 1pt Continuous 10) Now, look at the spectrum through each of the four vials of liquid, using the spectrometer. Please describe in general what is happening to the light before it gets to your spectrometer. 1 pt Light is getting absorbs pretty severely, at particular frequencies. 11) One of the vials transmits light composed of a very small band of frequencies (or colors). Which one is it? 1 pt Green 12) Another one of the vials has a composite color resulting from a large combination of different colors. Which one is it?1 pt “Yellow” for sure. “Red” has some orange in it. Blue also has a wide range of colors. 13) In the front of the classroom, there are different tubes of gas. The tubes can be placed in the electric device that delivers electricity through the gas, exciting the atoms (this is how a “neon sign” works, by the way). The electricity makes electrons jump to higher energy levels, but what happens next is what we are concerned about. Electrons want to be at the lowest energy levels possible, so after the electricity excited the electrons up a level, the electrons fall back down a level resulting in a photon of light being emitted. The photon has the same energy as the difference between the energy levels. A photon of light has an energy given by the following: E = hf = hc/λ where h=6.63 x 10-34 Joules x seconds. Consider a beam of red light with a wavelength of 700 nanometers (700 x 10m). 9 What#is#the#energy#that#a#photon#of#this#wavelength#has?##MAKE#SURE#THAT#YOUR#UNITS#FOR#EACH# COMPONENT#ARE#APPROPRIATE.#2pt E=hc/lambda=(6.63x10-34J*sec)(3x108 m/sec)/700x10-9 m =2.84x10-19 J 14)#We#will#now#observe#five#other#different#gases#through#the#diffraction#gratings#(within#our# spectrometer).#Remember,#look#just#to#the#side#of#the#gas#tube#to#get#the#best#effect.#For#each#gas,#you#should# see#the#full#visible#spectrum#range#(blue#to#red).#However,#you#will#not#see#all#the#colors.#In#the#boxes#below# draw#what#you#see#for#each#gas#(there#will#be#4#pure#elemental#gases#and#ONE#SURPRISE!).##Hint:#Try%to%draw% the%major%lines%that%you%see.%%Indicate#the#blue#and#red#side#of#your#drawings#by#circling#the#proper#color. All spectra drawings are 1pt…must be sufficiently close to the pictures (i.e., includes prominent lines). Gas C: Neon !!Blue/Red!!!!! ! ! ! ! ! ! ! ! !! ! Blue/Red! Gas D: He !!Blue/Red!!!!! ! ! ! ! ! ! ! ! !! ! Blue/Red! ! ! ! ! ! ! ! !! ! Blue/Red! Gas E: Argon !!Blue/Red!!!!! ! Gas F: CO2 !!Blue/Red!!!!! ! ! ! ! ! ! ! ! !! ! Blue/Red! Gas G: Hydrogen !!Blue/Red!!!!! ! ! ! ! ! ! ! ! !! ! Blue/Red! # 15)#Now#that#you#have#seen#all#five#gases#and#have#sketched#them#in#the#boxes#above,#you#have#to#figure#out# what#gases#are#which.#Spectra#of#different#gases#will#be#shown#in#the#front#of#the#class.##Using&your& drawings,&try&to&identify&what&gases&you&observed.&Label&these&above&each&spectrum&you&created.&& What#is#the#non-element#gas#mixture’s#composition#(hint:#plants#require#what#we#respire!#–#it’s#spectrum#is# NOT#on#the#board)?##Is#it#simple#or#complex#compared#to#the#others,#considering#the#number#of#lines? Each id is 1pt. Complex (1pt) !