7.1.2 Absorption and Emission

Question 1

absorption and emission

Answer 1

• The color of light depends on the wavelengths of which it is composed and the observed color of an object is due to the wavelengths that were not absorbed by the object.
• An absorption spectrum is a plot of the intensity of light absorbed by a sample versus wavelength.
• An emission spectrum is a plot of the intensity of light emitted by a source versus wavelength.
• Absorption and emission spectra are important tools for probing at the atomic level of a sample.

Question 2

note

Answer 2

• The color of light depends on the wavelengths of which it is composed and the observed color of an object is due to the wavelengths that were not absorbed by the object.
• White light is composed of light with all wavelengths present.
• An absorption spectrum is a plot of the intensity of light
  absorbed by a sample versus wavelength.
• The absorption spectrum of a substance is obtained by passing white light in a narrow beam through the substance. The light emerging from the substance is then passed through a prism. The prism separates the wavelengths (colors) of light present in the beam and allows a detector to measure the intensity of the emerging light by wavelength.
• In the example, the red sample absorbs the green portion of the spectrum, which is absent at the point of the detector.
• An emission spectrum is a plot of the intensity of light emitted by a source versus wavelength.
• In an emission spectrum the source and the sample are the same. The emission spectrum is obtained by passing the light emitted by a substance in a narrow beam through a prism. The prism separates the wavelengths of the light present in the beam and allows a detector to measure the intensity of the light by wavelength.
• Absorption and emission spectra are important tools for
  probing at the atomic level of a sample.
• Different elements have different emission spectra. This can be used to identify the components of a sample.
• In the example, the emission spectra produced by exciting the elements hydrogen and neon show characteristic bands. The absorption spectra of these two colorless gases are similar.
• The discrete wavelengths emitted by the hydrogen and neon gas give information about energy levels in the atoms.

Question 3

Why does a sheet of paper appear white?

Answer 3

because it reflects all wavelengths of visible light

Question 4

Suppose you obtain a visible emission spectrum of a sample. What change would you have to make to this experiment to obtain the sample’s ultraviolet emission spectrum?

Answer 4

Use a different type of detector.

Question 5

A solution that appears violet is used in an absorption experiment. The spectrum that results includes all wavelengths except those corresponding to yellow light. What can you conclude from this experiment?

Answer 5

The absence of yellow is perceived as violet.

Question 6

A bulb with a tungsten filament is used as the light source in an absorption experiment. The spectrum that results includes all wavelengths except those corresponding to blue. What is most likely to occur if the experiment is repeated with the same sample, but with a tube of hydrogen gas as the light source rather than the tungsten-filament bulb?

Answer 6

The new spectrum will consist of a few separate lines, none of which are blue.

Question 7

Photosynthesis is the process in which plants use energy from visible light along with carbon dioxide and water to produce glucose. The main chemical that absorbs visible light in plants is called chlorophyll. Chlorophyll gives plants their green color. However, plants also have light-absorbing chemicals called carotenoids. These chemicals are usually yellow or orange. Why is it advantageous to plants to have carotenoids in addition to chlorophyll?

Answer 7

so they can absorb violet and blue light

Question 8

Where should the sample be placed in the following diagram in order to obtain the best emission spectrum?

Answer 8

A

Question 9

In which of the following situations would it make the most sense to perform an emission experiment?

Answer 9

You wish to determine which wavelengths of light are produced by the light-generating molecules in fireflies.

Question 10

In order for a cat to become invisible, its body would have to __________ all wavelengths of visible light.

Answer 10

transmit

Question 11

Suppose you have two samples, each consisting of a reddish-brown gas but having different chemical compositions. Which of the following would be the best way to tell the samples apart?

Answer 11

Use the gases in an emission experiment by applying a high voltage to a glass tube containing each gas.