AP Chemistry Cell Phone Spectroscopy Lab Adopted from Alexander Scheeline Department of Chemistry University of Illinois at Urbana-Champaign

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AP Chemistry Cell Phone Spectroscopy Lab Adopted from Alexander Scheeline Department of Chemistry University of Illinois at Urbana-Champaign Back Ground Electromagnetic radiation Electromagnetic

1 AP Chemistry Cell Phone Spectroscopy Lab Adopted from Alexander Scheeline Department of Chemistry University of Illinois at Urbana-Champaign Back Ground Electromagnetic radiation Electromagnetic radiation (EMR) is the type of energy that encompasses light, heat, and x-rays. It can be conveniently described using a sinusoidal wave model, where the properties of the radiation depend on the wavelength, frequency, and other parameters of the wave. For some purposes, usually when discussing the absorption and transmittance of the energy of the radiation, it makes more sense to describe the energy as a stream of light particles called photons, where the energy of the photons is proportional to the frequency of the radiation. The wave/particle duality applies to all elementary particles, and should be used as a complementary, rather than contradictory, description of the movement of the radiation. Wave properties of electromagnetic radiation Some useful definitions and equations: Amplitude (A): The height of the wave (fig. 1) Wavelength (λ): The distance between two crests of the wave (fig. 1) Crest and trough: The highest and lowest points, respectively, of a wave (fig. 1) Speed of light (c ) - The velocity of radiation as it travels through a vacuum. This quantity is the same for all forms of electromagnetic radiation, from x-rays to light to radio waves, and is constant within a particular transportation medium. The speed of light in vacuum is x 10 8 m/s. The speed of light in air is only 0.03% slower, and c in either medium is usually just rounded off to 3.00 x 10 8 m/s. Frequency (ν): The number of waves that pass a fixed point per second Period (T): The number of seconds it takes for a wave to pass a fixed point Fig 1 ν = 1/T - The frequency of the wave is the reciprocal of the period. λ ν = c (or ν = c/λ) - the product of frequency and wavelength is the speed of light. Alternatively, the frequency of a wave is inversely proportional to the velocity. E = hν = hc/λ, where h is the Planck constant, x The energy of the radiation is equal to the Plank constant multiplied by the frequency of the radiation.

$The refractive index, n, is given by the equation n=c/v p where c is the speed of light in vacuum and v p is the speed of light in that particular substance.$

2 Movement of light between two substances When light moves between two substances, both the speed and the direction of the electromagnetic wave will change. The refractive index, n, is given by the equation n=c/v p where c is the speed of light in vacuum and v p is the speed of light in that particular substance. The refractive index is unitless and simply provides a means to compare the relative speeds of light in transparent substances; the larger the refractive index of a material, the slower light will pass through it. When the wave of light changes velocity, it also changes direction (figure 3). This refraction can be related to the speeds of light in the substances using Snell's Law, where θ 1 is the angle of incidence, θ 2 is the angle of reflection, and v 1 and v 2 are the speeds of light in the first and second media, respectively. Fig. 3 Dispersion of White Light in Gratings White light is a mixture of all of the wavelengths of visible light. When a beam of white light is sent from one medium to another, the different wavelengths of light that make up the beam are refracted at different angles because they are traveling at different speeds and have different refractive indexes. This causes the different wavelengths to be separated from each other into the visible spectrum as shown in Figure 4. Light can be separated using either prisms (Figure 4) or diffraction gratings (Figure 5). Fig. 4 Figure 4: A beam of white light dispersing into its component colors.

$A transmission diffraction grating is made up of a transparent material with regularly spaced grooves cut into it so that a beam of light passing through is separated out into the component$

3 A transmission diffraction grating is made up of a transparent material with regularly spaced grooves cut into it so that a beam of light passing through is separated out into the component wavelengths. Figure 5: Dispersion of light in a transmission grating The separation of the light is governed by the equation: Fig. 5 nλ = d(sin i + sin r) Where n is the diffraction order (a small whole number), d is the spacing between the grooves of the grating (usually calculated in nanometers/groove), and i and r are the angles of incidence and refraction, respectively. Because there are several values for 'n', there are a number of spectra, found at different angles of refraction, which can be formed from a diffraction grating. Ordinarily, however, the first-order line is the most intense, and only the +1, 0, and -1 spectra Spectrophotometers Spectrophotometers are instruments that measure the absorbance of wavelengths of light in solutions. The absorbance, A of a solution is a measure of how much light of a certain wavelength specific to the experiment passes through a solution versus how much is absorbed by the solution. Absorbance is defined using the Beer's Law; Where I 0 is the amount of the experimental wavelength of light present before the beam of light passes through the solution, and I is the amount of light present in the beam after it has passed through the solution. In general, the darker the solution, the less light that passes through the solution and the higher the absorbance.

4 Using our spectrophotometer, the students will be able to see which wavelengths of light in the visible spectrum are absorbed and which are transmitted. A deep red transparent solution, for example, allows red and orange light to pass through and absorbs the other colors in the spectrum. A pale red solution absorbs some of the blue and green light, but also lets some through, and a clear solution transmits the entire visible spectrum. This can be clearly seen in the diffracted spectrum through the grating. Materials LED (light emitting diode) light bulb/ source Battery Diffraction grating (500 line/mm linear) Base plate template A sample of colored liquid; primary colors give easily interpreted results. Suggested: Red Kool-aid, food coloring, or similar substance A cuvette, glass vial, small test tube Spectrometer frame (A way to keep the light, cuvette, and grating stationary) A camera; Cell Phone or Digital Pre-Lab Questions 1. What spectrum of light will we be examining? 2. What does a large refraction index mean in regards to the speed of light traveling through the medium? 3. What happens to the direction of light when its velocity is changed? 4. If light leaving a source (LED bulb was at 600nm before entering a solution, and the light leaving the solution was at 425nm what is the absorbance of the solution? Procedures 1. Prepare several samples of different concentrations. If using a powdered Kool-aid, measure the concentrations of the Kool-aid (in g/ml, scoops/l, or whatever other measurement system is convenient). If using food coloring, measure the number of drops that go into your sample. The important thing is to have a quantitative measurement of how much has gone into the sample. 2. Tape the light and power source to your sample holder, and position the grating about a foot away from the sample. In order to see the spectrum, the grating will need to be positioned at an angle to the light source. 3. Turn off as many lights in the room as you can while still being able to see what you are doing reasonably well. 4. Position your cuvette/ glass vial/ test tube in front of the light source so the light from the LED shines through when you look through the grating. Mark your position so that you can easily place all of your samples in the same spot. 5. Take a 'blank' reading: fill the cuvette with clear DI water, place it in your sample spot and take a picture of the resulting spectrum. 6. Take pictures of the spectra that come from each of your samples. You may need to adjust the flash mode (to off) and the shutter speed to a slower setting, ect.

5 Analysis - On the computer 1. Upload the pictures into the program. Load the picture of your water reading by going to 'file > load reference (right) > (your directory pathway)', Then do the same to your sample spectrum by going to 'file > load sample (left) > (your directory pathway)'. Depending on your camera, you may need to resize the pictures in a program such as Windows Photo Editor before you can see the entire spectrum; to do this in Photo Editor, simply import the files, select your picture, go to 'edit', choose 'resize picture' and shrink the picture to whatever percentage fits the screen. 2. Once the spectra are uploaded, simply click on one side of the sample spectrum on your screen. A box will appear, asking you to specify if it is the red or blue end ( visible light range) of the spectrum. Choose the correct answer and repeat the procedure at the other end of your sample. There should now be a green line connecting the red and blue ends of your spectrum. Run through the same procedure with your reference spectrum. 3. Spend a bit of time exploring the 'Plot trace selection' portion. Try overlaying the sample and reference spectrum and compare the two. 4. Click on the 'Make plot, compute T, compute A' button, then on the 'Generate CSV file for Excel' button. Save the Excel file anywhere that is convenient for you. 5. Open the Excel file and make plots (Graph) of the absorbance and transmittance versus the wavelength. How are the two plots related? 6. What are three (3) limitations of your spectrometer? 7. What are two (2) ways you could improve your spectrometer?

Spectrophotometer. An instrument used to make absorbance, transmittance or emission measurements is known as a spectrophotometer :

Spectrophotometer. An instrument used to make absorbance, transmittance or emission measurements is known as a spectrophotometer : Spectrophotometer An instrument used to make absorbance, transmittance or emission measurements is known as a spectrophotometer : Spectrophotometer components Excitation sources Deuterium Lamp Tungsten