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

been developed. The prefix for 10 -9 is nano abbreviated as n. Hence the wavelength of yellow light can be written as 600 nanometers or 600 nm. Another term also used with electromagnetic wavelengths is the “Angstrom” abbreviated as Å which is 10 -10 m. Hence the wavelength of yellow light can also be written as 6000 Å. The list below summarizes commonly used metric prefixes, their names, and abbreviations. Metric Prefix Name Abbreviation 10 -9 nano n 10 -6 micro m 10 -3 milli m 10 -2 centi c 10 3 kilo k 10 6 Mega (million) M 10 9 Giga (billion) G 10 12 Tera (Trillion) T 10 100 googol (Yes! That is the source of the name “Google” that you are familiar with. Strictly speaking, this is not a metric unit, but a whimsical word given by mathematicians.) This lab will show another important property of electromagnetic waves, which is that the range of each of the regions (radio, IR, visible, UV, X-ray, and gamma ray) is not equal. Also, some regions include many familiar terms that you may not connect to an astronomy course, so here is an opportunity to learn how this course affects your daily life! Spectroscopy is a very important tool for astronomers. Each chemical element has its own distinctive fingerprint or bar code revealed by its spectral lines. Chemical compounds made up of two or more elements will show lines from each element, and the width and brightness of spectral lines gives additional information about the chemical constituents of objects. Since light is the only information we get from the stars, it is through spectral analysis that astronomers have figured out everything we know about stars, like their temperature, mass, size etc. When sunlight passes through a prism or a diffraction grating, it breaks up into its component colors, which is the familiar band called a “spectrum.” In the 19th century, scientists learned to make many different types of spectra which were examined in detail with spectroscopes, which are instruments consisting of a prism or grating to 3

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