Lab-02_Electromagnetic-Spectrum2

Lab 02: The Electromagnetic Spectrum Objectives This exercise will allow you to  visualize the range of the electromagnetic spectrum so that you can appreciate the width of all its different parts.  see the differences between continuous, emission and absorption spectra.  observe spectral lines and identify the wavelengths of emission lines formed by different elements. Marketable Skills: This course assesses the following Core Objectives. In this assignment, you will develop the following marketable skills: Critical Thinking  Analyze Issues  Anticipate problems, solutions, and consequences.  Apply knowledge to make decisions  Detect patterns/themes/underlying principles  Interpret data and synthesize information Communication  Summarize information  Use proper technical writing skills Personal Responsibility  Accept responsibility  Exhibit Time Management  Show attention to detail  Learn and grow from mistakes Empirical Quantitative  Communicate results using tables, charts, graphs  Contextualize numeric information/data  Demonstrate logical thinking  Draw inferences from data, use data to formulate conclusions Equipment  Ruler with centimeter markings  colored pencils. 1

Please print page #14 only . DO NOT print this entire lab as it will waste your printer’s ink! The last two pages of this lab shows spectra from various elements. Please DO NOT print page 15 and 16 (last 2 pages) as it will waste your printer’s ink! Simply view page 15 and 16 online when asked to make measurements. Introduction The electromagnetic spectrum is the entire range of electromagnetic waves which are divided into different regions named as radio, infrared, visible, ultraviolet, x-rays and gamma rays. While all these waves travel at the speed of light (3 x 10 8 m/s) they do not have the same wavelength or frequency. Recall that the equation relating speed (c), frequency ( f ), and wavelength (λ) is c = f λ. Hence if any two variables are known, the third can be calculated. Any one of the following equations can be used to find the unknown quantity: c = fλ f = c λ λ = c f c = 3.0 × 10 8 m / s When these equations are used, it is important to keep track of units. If speed c is measured in meters per second (m/s), frequency will be in Hertz (Hz) and wavelength will be in meters (m). For example, let’s calculate the wavelength of yellow light if its frequency is given as 5 x 10 14 Hz. λ = c f = 3.0 × 10 8 m / s 5.0 × 10 14 Hz = 6 × 10 − 7 m The answer above can also be written as 60 x 10 -8 m or 600 x 10 -9 m or 6000 x 10 -10 m There is a reason why we are introducing all these different exponents. It is inconvenient to keep saying “10 -7 ” so prefixes (shortcut words for the exponents) have 2

produce the spectrum and a small telescope to enlarge the colored spectrum and see its details. Three different types of spectra are summarized below. 1. A continuous spectrum shows a continuous band of colors, red merging into yellow, green and blue. It is produced by a dense gas or a luminous solid. You can easily see a continuous spectrum if sunlight passing through a hanging crystal makes a “rainbow” on a wall, or if you hold up the shiny surface of a CD to a light source. 2. An emission spectrum consists of a series of brightly colored lines, and each element shows specific colors in specific positions. It is produced by a low- density gas if it is heated sufficiently. These types of spectra are easy to produce in the lab by passing electricity at a high voltage through a discharge tube containing the gas. Scientists have made accurate photographs of these types of spectra, and the position of the colored lines has been measured very accurately and converted to give their wavelengths. Examples: a) Emission spectrum of Iron b) Emission spectrum of Hydrogen 3. An absorption spectrum looks like a continuous spectrum, but it has dark lines on it. It is produced when a cool, low-density gas is placed between the light source and the spectroscope. It is noticed that the location of the dark lines depends on the nature of the intervening gas. For example, if the intervening gas is hydrogen, the absorption spectrum will show dark lines in the same position showed by an emission spectrum of hydrogen. Absorption spectrum of Hydrogen 4

To understand how spectra are produced, recall that each chemical element has its own number of protons, neutrons, and electrons. The protons and neutrons are tightly bound in the tiny nucleus and do not contribute to forming spectra. It is the movement of electrons which produces spectra. The electrons in an atom are organized in different orbitals or shells and each orbital has its own energy level. The energy levels are also like rungs on a ladder, so an electron can be in level 1 or level 2, but not on level 1.5. The energy levels of electrons in atoms are said to be quantized, i.e. each level has a discrete value associated with it. Also, the energy increases the further away the electron lies from the nucleus, meaning that energy levels further away from the nucleus have a higher value. Just as it takes energy to climb a ladder from rung 2 to rung 4, an electron has to absorb energy to go from energy level 2 to energy level 4. Similarly, just as you decrease your potential energy if you climb down from rung 5 to rung 2, the electron decreases its energy if it moves from energy level 5 to energy level 2. The excess energy between level 5 and 2 will be emitted as a photon. A photon is a tiny packet of electromagnetic energy, or simply a particle of light. The photon’s energy is related to its frequency by the equation Energy E = h×frequency f ∨ E = hf frequency f is related to wavelength by c = fλ wherec = 3.0 × 10 8 m / s Let’s first explain how an emission spectrum is produced. If oxygen gas is placed in a discharge tube and a high voltage is applied to the tube, the electrons in the gas will be energized and move to higher energy levels. But not wanting to stay there, they will move back to their original energy levels and emit the photons they had previously absorbed. This produces an emission spectrum with many colored lines at specific positions. Each line’s position indicates its wavelength, which can be related to the frequency and the difference in the energy level of the electron. 5

c) 6 Megahertz d) Both b and c are correct 3. Use the equation λ = c/ f to calculate the wavelength of ultraviolet light whose frequency is 2 x 10 15 Hz. a) 1.5 x 10 7 m b) 1.5 x 10 -7 Hz c) 1.5 nm d) 1.5 x 10 -7 m 4. Which of the following units is the smallest? a) Millimeters b) Centimeters c) Nanometers d) Kilometers 5. The wavelength of blue light is 450 nm. This can be written as a) 450 x 10-9 m b) 4.50 x 10-7 m c) 4500 Å d) All answers are correct 6. An emission spectrum shows a) Brightly colored lines b) A rainbow of colors merging into each other c) Only red and orange bands d) Only green and blue bands 7. The spectrum of a star has dark absorption lines of helium superimposed on a continuous spectrum. What can you conclude from this? a) The star is made up of helium b) There is a great deal of helium in the Earth’s atmosphere c) The dark lines have absorbed helium d) The light from the star has passed through a cloud of helium 8. If an electron moves from a lower energy level to a higher energy level, what type of spectrum will be produced? a) Continuous b) Emission 7

c) Absorption d) All are possible Lab Exercise Note: Steps to be performed are in alphabetic order (A, B, C, …). Questions to be answered are in numerical order (1, 2, 3, …). A. We will begin the lab by first viewing some spectra as shown on page 16. This will allow you to visualize the concepts more easily. B. Fig 1 is a continuous spectrum. C. Fig 2 is the emission spectrum for Hydrogen. 1. How many emission lines do you see for hydrogen? a. 2 b. 3 c. 4 d. 5 D. Fig 3 is the emission spectrum for Helium. 2. How many emission lines do you see for helium? a. 2 b. 3 c. 4 d. 5 3. Is the red line in hydrogen at the same location as the red line in helium? a. Yes b. No 4. Which red line has a higher wavelength? a. Hydrogen b. Helium E. Fig 4 is the hydrogen absorption spectrum. 5. How many hydrogen absorption lines do you see? 8

I. The table on page 9 lists the range of all electromagnetic waves. Since each range merges smoothly into the adjacent one, the values given are approximate. There may be some overlap in numbers used depending on the source. Colors seen with the human eye are also subjective, as some people may not be able to distinguish between shades of yellow and orange or blue and green. The quality of printers, inks, and screen resolutions also determines how accurately colors are evident. Wave Type Frequency (Hz) Frequency X 10 14 Hz Wavelength nm Radio 1 to 10 12 1 to 0.01 x 10 14 3000 nm to 3 x 10 5 nm Infrared 10 12 to 10 14 0.01 x 10 14 to 1 x 10 14 3 x 10 5 nm to 3 x 10 3 nm Visible Red 4.3 x 10 14 to 4.8 x 10 14 4.3 x 10 14 to 4.8 x 10 14 698 nm to 625 nm Visible Orange 4.8 x 10 14 to 5.1 x 10 14 4.8 x 10 14 to 5.1 x 10 14 625nm to 588 nm Visible Yellow 5.1 x 10 14 to 5.4 x 10 14 5.1 x 10 14 to 5.4 x 10 14 588 nm to 555 nm Visible Green 5.4 x 10 14 to 6.2 x 10 14 5.4 x 10 14 to 6.2 x 10 14 10

Wave Type Frequency (Hz) Frequency X 10 14 Hz Wavelength nm Visible Blue 6.2 x 10 14 to 7 x 10 14 6.2 x 10 14 to 7 x 10 14 Visible Violet 7 x 10 14 to 7.4 x 10 14 7 x 10 14 to 7.4 x 10 14 428.6 nm to 405.4 nm Ultraviolet 8 x 10 14 to 10 16 8 x 10 14 to 100 x 10 14 X-rays 10 16 to 10 20 100 x 10 14 to 1,000,000 x 10 14 Gamma rays More than 10 20 More than 1,000,000 x 10 14 < 0.003 nm J. Look at the numbers in the data table shown in the previous step. The second column lists all the frequencies, while the third column has the same frequencies, but they are all recorded in powers of 10 14 . The fourth column has some wavelength values already worked out, while some are missing. You will fill in the missing values later in the lab. K. Next you need a printed copy of page 14 of this lab. This is called Chart # 1 and you will use it to mark various parts of the electromagnetic spectrum and attach it when you submit the lab. L. The top line of the Chart shows 25 intervals with 1 cm per interval. Notice how each tick mark rises by a factor of 10. 11

for which the cream offers protection. Mark the following on the middle line of the chart if you can: UVA = 7.5 x 10 14 – 9.4 x 10 14 Hz UVB = 9.4 x 10 14 – 10 x 10 14 Hz UVC = 10 x 10 14 – 30 x 10 14 Hz 24.Can you mark the location of UVC, X-rays and gamma rays on the middle line of your chart? Give a reason for your answer. How long should the paper be to plot the position of gamma rays? Keep in mind that each notch on the middle line was 1 cm on the chart. 25.The scale used on the second line was a “linear scale” where each centimeter was equal to the previous and following segment. How is this scale different from the first line where you used a logarithmic scale? What advantages do you see in each scale? U. Using spectroscopes, it is possible to measure the wavelength of spectral lines. E.g., if the red frequency ( f ) is 4.3 x 10 14 Hz, the corresponding wavelength (λ) is: λ = c/f = (3 x 10 8 m/s) / (4.3 x 10 14 Hz) = 6.98 x 10 -7 m = 698 x 10 -9 m = 698 nm. It is preferable to express the wavelength in nm as it is the standard unit used for wavelengths. V. The data table in step H has many wavelength values already worked out. You can practice using λ = c/f since you have the answers. Also notice how the pattern of wavelength numbers repeats in each succeeding row. Calculate the values for the missing wavelengths and enter the values in the data table. 26.What is the wavelength range for green light? a. 484 nm to 429 nm b. 555 nm to 484 nm c. 429 nm to 405 nm d. 700 nm to 610 nm 27.What is the wavelength range for ultraviolet rays? a. 30 nm – 375 nm 13

b. 4000 nm – 40,000 nm c. Less than 10 nm d. More than 500 nm W. Use the bottom line on the chart to mark the wavelengths of the visible colors. Since the wavelengths range from 400 to 700 nm choose any scale you wish. Remember to label your chart so it is clear to the person reading it. 28. Scan your chart and save it as a pdf file on your computer. You will upload it using the Browse button when you are ready to enter your answers on eCampus. 29.Summarize what you learned from this experiment. GRADING RUBRIC Questions 1-14= 0.5 points each = 7 points Question 15 = 1 point Questions 16-21 = 0.5 points each = 3 points Questions 22-27 = 1 point each = 6 points Question 28 (chart) = 6 points Question 29 = 2 points 14

Fig 3 Emission Spectrum of Helium Fig 4 Absorption Spectrum of Hydrogen Fig 5 Absorption Spectrum of Helium Spectra image credits: Spectrum Explorer UNL Animations Continuous Spectrum Hydrogen Helium 16

Oxygen Nitrogen Sodium Spectra image credits: AST115H: Basic Astronomy 17