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Learning objectives

  • Identify and define the characteristics of electromagnetic radiation (EMR) used in microscopy
  • Explain how lenses are used in microscopy to manipulate visible and ultraviolet (UV) light

Part 1

Cindy, a 17-year-old counselor at a summer sports camp, scraped her knee playing basketball 2 weeks ago. At the time, she thought it was only a minor abrasion that would heal, like many others before it. Instead, the wound began to look like an insect bite and has continued to become increasingly painful and swollen.

The camp nurse examines the lesion and observes a large amount of pus oozing from the surface. Concerned that Cindy may have developed a potentially aggressive infection, she swabs the wound to collect a sample from the infection site. Then she cleans out the pus and dresses the wound, instructing Cindy to keep the area clean and to come back the next day. When Cindy leaves, the nurse sends the sample to the closest medical lab to be analyzed under a microscope.

  • What are some things we can learn about these bacteria by looking at them under a microscope?

Jump to the next Clinical Focus box.

Visible light consists of electromagnetic waves that behave like other waves. Hence, many of the properties of light that are relevant to microscopy can be understood in terms of light’s behavior as a wave. An important property of light waves is the wavelength , or the distance between one peak of a wave and the next peak. The height of each peak (or depth of each trough) is called the amplitude . In contrast, the frequency of the wave is the rate of vibration of the wave, or the number of wavelengths within a specified time period ( [link] ).

Figure a shows a wavy line with evenly repeating waves upwards and downwards. A straight line through the center of the wavy line indicates the base of the waves. The distance from the peak of one wave to another is the wavelength. The distance from the baseline to the peak of a wave or the distance from the baseline to the trough of a wave is called the amplitude. Figure b shows three waves with unit time labeled across the bottom. The top line has waves that are widely spread apart. Waves with a wide wavelength have a low frequency. The bottom line has waves that are close together. Waves with a narrow wavelength have a high frequency. The middle line has a medium wavelength and therefore a medium frequency.
(a) The amplitude is the height of a wave, whereas the wavelength is the distance between one peak and the next. (b) These waves have different frequencies, or rates of vibration. The wave at the top has the lowest frequency, since it has the fewest peaks per unit time. The wave at the bottom has the highest frequency.

Interactions of light

Light waves interact with materials by being reflected, absorbed, or transmitted. Reflection occurs when a wave bounces off of a material. For example, a red piece of cloth may reflect red light to our eyes while absorbing other colors of light. Absorbance occurs when a material captures the energy of a light wave. In the case of glow-in-the-dark plastics, the energy from light can be absorbed and then later re-emitted as another form of phosphorescence. Transmission occurs when a wave travels through a material, like light through glass (the process of transmission is called transmittance ). When a material allows a large proportion of light to be transmitted, it may do so because it is thinner, or more transparent (having more transparency and less opacity ). [link] illustrates the difference between transparency and opacity.

Figure a shows the tips of a person’s gloved hands holding a clear plate with a lid. The plate contains a reddish material in the bottom of the plate. Figure b shows a piece of metal in a person’s hands. The material is dark with some shiny regions.
(a) A Petri dish is made of transparent plastic or glass, which allows transmission of a high proportion of light. This transparency allows us to see through the sides of the dish to view the contents. (b) This slice of an iron meteorite is opaque (i.e., it has opacity). Light is not transmitted through the material, making it impossible to see the part of the hand covered by the object. (credit a: modification of work by Umberto Salvagnin; credit b: modification of work by “Waifer X”/Flickr)

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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