Light and Spectroscopy

Chemistry 128

Light and color are familiar concepts. Light is a form of energy that travels freely though space. Light generated in the sun reaches the earth where it provides us with heat, the energy used in photosynthesis and the daylight we use to see our surroundings. Clearly the light interacts with chemical substances-- it can cause dyes to fade and lose their color, it causes tanning and sunburn, it cause cameras and eyes to record images and it is responsible for almost all life on earth through photosynthesis. The study of chemistry logically includes a study of how light affects materials.

Light is a form of electromagnetic radiation. This is actually a form of electrical and magnetic energy that travels through space and through some materials. The term "light" usually refers that portion of the spectrum that is detectable by the human eye. This is often called visible light. Chemists prefer to use a broader definition, including light beyond the range of the human eye; we include the Infrared and Ultraviolet regions of the spectrum. Beyond this we have high energy radiation (X-rays and gamma rays) and lower energy radiation (microwaves and radio waves.)

The term "spectrum" often means that there is a wide and continuous variation of a property. The light from most light sources includes a wide range of colors. . Sometimes we can separate the light by colors and we see a clear display of colors that we also call a spectrum. The best known spectral display is a rainbow, where you can clearly see colors ranging from red to yellow to green to blue. A similar effect is seen when you look at a CD, at a film of oil on a pool of water or a soap bubble. Early scientific studies (Newton, for example) used a glass prism; modern laboratory work is more likely to use a diffraction grating.

Associated with each color is a wavelength, generally reported in nanometers (10^-9 m.) When light is absorbed, it occurs in small packets of energy called quanta or photons. Shorter wavelength light has more energy per photon. Ultraviolet and Blue light is more energetic than Green or Red light.) The amount of energy in a photon of visible light is enough to cause an electron in a molecule to be excited and jump into a higher energy orbital. This is enough energy to start some chemical reactions.

Colors and Wavelength (each color extends over a range of wavelengths)

• Ultraviolet Light (shorter than 400 nm)
• Violet 400-420 nm
• Blue 420-490 nm
• Green 490-560 nm
• Yellow 560-585 nm
• Orange 585-650 nm
• Red 650-750 nm
• Infrared (longer than 800 nm)

Color is actually a complex relationship between the light, the retina of the eye, the optic nerve and the vision centers in the brain. We can see and recognize a wide range of colors and we can make subtle distinctions of color and intensity. This is quite amazing considering that the retina only has three types of color sensors, each detecting light over a moderate range of color. Color information is derived from the relative signal from each of the three sensors. This is basically the same for color film and video cameras. They have three light sensitive materials, each responding to a different region of the spectrum. Likewise the color displays on television, computer terminals and ink jet printers build a full range of colors from three colors.

When a relatively balanced mixture of light is present, the eye/brain recognizes this as "white" and treats it as a neutral or colorless form of light. If the intensity is low we might label the image as "gray." At the other extreme, the absence of all light is recognized as "black."

If light of one color is removed from white light we see a new color, called the complementary color. Dyes, colored glass and paints selectively remove one region of the spectrum and transmit or reflect the remaining light. (The first laboratory exercise will let you examine complementary colors.)

Light Sources

1.Incandescent sources -- objects that emit light because they are hot

• examples: the sun, an ordinary light bulb, a candle flame, iron in a blacksmith's forge.
  • such sources emit a continuous range of wavelengths
  • the hotter the source, the more intense the light and the richer in the blue region of the spectrum. Only very hot objects appear to glow white.
    • at 650-900 ^oC objects glow red (faint dark red at 550^oC)
    • at 1000-1150 ^oC objects glow yellow
    • candle flame (heated carbon particles) operates at 1700^oC
    • ordinary light bulbs operate at 2500^oC (orange cast due to missing blue)
    • tungsten halide bulbs operate at 2900^oC (whiter light; richer in blue)
    • sun's surface is 5800 ^oC (our usual concept of balanced white light)
• to a chemist, the light from an incandescent lamp provides no information about the heated object other than its temperature..
  • however, an incandescent lamp is a convenient source when we want light of all wavelengths.

2. Atomic Emission Sources --

• Atoms or ions with electrons in high energy orbitals. When the electron returns to the lower level it releases the extra energy as light. One electron -- one photon
  • examples: Neon lights, mercury and sodium vapor lamps, many lasers; fireworks are colored by adding specific elements
  • the light is characterized by having only one or a few wavelengths of light present; all other regions of the spectrum are missing.
    • Since a spectral display shows up as very thin lines this is called a line spectrum.
  • We need atoms in excited states, so these sources generally involve either a flame or an electrical discharge in a gas.
• To a chemist it is important to note that the color or wavelength is specific to the chemical element. It is possible to identify a specific element by the presence of light of that wavelength. Astronomers routinely identify elements present in different stars by detecting light of specific wavelengths.
  • In the laboratory we will identify elements by light emitted from a flame. By measuring the intensity of the light we can also determine the concentration of the elements in the sample. This is a technique known as Flame Atomic Emission Spectrometry.

3. Other Light Sources, based on emission by molecules

• Examples:
  • Fluorescence lamp (actually a mercury discharge lamp with an inner coating of material that absorbs UV light and fluoresces in the visible.)
    • Chemiluminescence: fireflies, chemical light sticks.

Spectrometer

.

• First-- the light enters from an optical fiber. This allows us to aim the fiber at a light source and to keep the spectrometer several feet from the actual light source.
• Inside, the light is directed on a small grating and the light disperses to form a spectrum.
• The light sensor is called a CCD Array (CCD = charge Coupled Detector)
  • basically this is a small electronic chip with 2000 individual light sensors
  • each sensor covers a different wavelength in the spectrum
  • together, they will report the intensity of light at 2000 different wavelengths from 300 to 1100 nm. (UV through Infrared)
• The entire device is small-- ours is on a small board that fits into a computer. Computer software sends the spectral information to the screen.
• The simplest display is a plot of Intensity vs. wavelength (a spectrum of the light source.)

EXPERIMENTAL WORK

You should work in groups of four persons.

The experiment is divided into seven parts; you may work on these parts parts in any order.

Each experiment is at a separate work station, distributed throughout the lab.

• Everything is done at these work stations, not at your work bench.
• Each work station has a computer with a short tutorial providing additional description.

• You are expected to have a dedicated notebook for lab (bound, blank pages)
  • You are expected to briefly describe your experimental work so someone who has not read the experiment can follow your work. You need to start with a brief description of what you did.
    • What equipment did you use?
    • What did you observe?
    • What numerical results were recorded?
      • What calculations did you do?
      • What conclusions did you draw?
  • Your notebook will be collected and reviewed.They will serve as your report and will be graded.

• You are expected to use your notebook to answer questions about your work
• In part, this is intended as a self test of the accuracy and completeness of your notes.

• Note-- there is one setup and there may be waiting lines-- (this need not be done first.)
• We have a spectral display set up in the lab.
  • A slide projector provides a beam of white light
  • A diffraction grating disperses the light into a spectrum
  • A screen is used to display the spectrum
• You are to place colored Solutions and Filters in front of the slide projector lens
  • You can observe the changes in the spectrum (absorption of light)
  • You are to determine a set of complementary colors

• In your lab notebook, briefly describe or sketch the experimental setup
• record your observations for five different samples
  • there may be a slight distortion or overall loss in intensity due to the bottle holding the solution
  • strong solutions may completely absorb one color region; weaker solutions will only decrease the intensity. Since the room is not completely dark the effect may not be complete.
• Record your observations and conclusions; use a table such as the example below:
  • sample sample color spectrum conclusion
  • KMnO₄ _________purple___________green is missing____ green is complement of purple
  • phenolphthalein___red/pink ... ....
  • cyan filter________cyan ____________xxx is missing____xxx is the compl. color to cyan .... ....
  • yellow filter_____yellow ... ...

• One of the computer based spectrometers is set up with a series of light sources.
• Spectra can be recorded and printed.
  • In your notebook briefly describe the setup and then write your observations
    • Hint: it may be useful to treat the light of an incandescent lamp as the reference.
    • Hint: a sketch of the spectrum may be more appropriate than a verbal description
  • Light Sources we are likely to provide
    • incandescent lamp
    • mercury vapor light
    • fluorescent lamp --these produce a complex pattern--
      • compare to the mercury lamp spectrum
    • light emitting diodes; (red, green, yellow, orange, blue)
    • red laser (laser pointer)
  • neon or argon bulb
    • We have an incandescent lamp with an adjustable power supply.
      • You can vary the filament temperature (by adjusting the voltage)
      • At lower voltages the light is less intense, but the sprectrum also shifts towards more red and less blue.
        • from the electrical resistance we could determine the temperature of the filament and we could compare the color (spectrum) at several different temperatures

A infrared thermometer looks at the light emitted by a sample and determines the intensity and the spectral distribution. From this, the temperature of the emitting object can be determined. When the temperature is between 0 and 500^o C, there will be no visible light and the measurements are made in the infrared. (The instrument ignores visible light.) A similar device is now commonly used medically to measure temperature, probing inside the ear.

The instrument we have is a handheld unit that is aimed at the sample and it reports the temperature on a digital display. This instrument uses a small laser to project a circle of red dots to help you determine where it is aimed. Temperature can be measured from objects 10-20 feet away.

• Use the instrument and measure the temperature of several objects we have provided
  • Beaker of ice water
  • Beaker of water on a hot plate
  • Someone's hand (Do not aim the laser at someone's face or eyes)
  • Room's ceiling
  • In your notebook, record the temperature of these samples
  • If a thermometer is provided, compare the accuracy of the infrared measurements
  • Measure one object again, but through a transparent window (glass or plastic)
  • Measure one object again, but expose it to bright light. Any change?

EXPERIMENT-- Part 4. Atomic Emission Spectrometry

• First, perform simple flame tests
  • clean a wire by dipping it in HCl and placing it in flame of a bunsen burner
  • then dip the wire loop into solution and place it into a Bunsen burner flame
  • observe and record the color
    • try solutions of Na⁺, K⁺, Ca⁺, Li⁺, Sr²⁺, Ba²⁺, Cu²⁺
      • solution: tap water
    • sample: Sensodine tooth paste (can you identify a major ingredient)

Atomic Emission Spectrometry --

the spectrometer is set up as an assembly of a torch, an aspirator and a fiber optic spectrometer

• a propane torch serves as the light source
• an aspirator is used to make a mist of solutions and deliver that mist into the flame
• the spectrometer display will show a single strong peak
  • each solution will take about 15 seconds for a reading; record the height of the peak

We will focus our work on samples containing potassium and on the spectral line at 770 nm.

• for each of the prepared solutions (known concentration)
  • measure the signal (peak height, a number between 0 and 4095)
    • record the reading and solution concentration in your notebook
  • plot signal (Y axis) vs. solution concentration (X axis)
    • when the set of data is recorded, draw the best curve
      • use the graph paper provided or a computer spreadsheet; tape the graph into your notebook
  • measure a suitable sample (tap water, fruit juice)
    • record the intensity (amplitude )
    • if too concentrated, dilute (see your instructor)
  • fit this signal to the graph and determine the corresponding concentration.

EXPERIMENT-- Part 5. Fluorescence and "Black Light"

• a. place several solid samples below the violet/UV light source
  • observe and record emission (color)
  • hold sample up to white light and see if it is colored(transmission of light)
• b. select one of the solutions; place it in the light patha beam of blue light comes from a light emitting diode
  • it passes through your test solution
  • observe the beam
    • the sample will absorb light and emit light of another color
    • you see a colored beam in the water where light is emitted
  • in your notebook, briefly describe your observations

• c. Use the photographic flash gun and expose one of the plastic insects to a burst of light.
  • Observe the duration of light emission. This is an example of phosphorescence.

• This is set up in a slightly darkened region of the room
  • mix together 2 ml of solution A and two ml of solution B
  • observe the light emitted
    • note color, duration and relative intensity in your notebook
  • we will also have an example of a commercial light stick-- this involves similar chemistry but it adds a coloring agent and it emits light for a longer period of time.

This is a preview of a future experiment. We can use the absorption of light to determine the concentration of a solution. This is a very fast method of chemical analysis and it can be applied to a wide range of chemical species. Absorption spectrometry is perhaps the most common method of quantitative chemical analysis.

• One of our miniature spectrometers is set up to display absorption spectra
  • In this case, white light passes through the sample
  • The spectrum recorded is the RATIO before and after
  • We often report the %transmission at each wavelength
    • It is actually more useful to display the ABSORBANCE
    • Absorbance = - log₁₀ (100 x %T)

• a. Compare the absorption spectrum of a sample from Part I with reading on this instrument
  • the regions of high absorbance should match the regions where you saw light vanish
• b. Record the absorbance of two solutions and add your data to the posted calibration curve
  • objective: we should see a straight line plot of Absorbance vs. concentration.

some Web sites related to the experiment: (a few no longer work)

Incandescent Lamps

Light Bulbs (many of the links here no longer work)

color and the eye

(shows response of eye's cones and rods)

(manufacturer of our spectrometers)

• Na 589.6, 589.0 nm
• K 769.9, 766.5 nm
• Li 670.8 nm
• Ca 618.2, 620.3 nm (bands)
• Sr 606.0 (bands)
• Mg 518.0, 517.3 (bands)
• Hg 404.7, 435.8, 491.6, 496.0, 577.0, 579.1, 623.4, 690.7 nm