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Certain materials can refract nonvisible forms of EMR and, in effect, transform them into visible light. Certain fluorescent dyes, for instance, absorb ultraviolet or blue light and then use the energy to emit photons of a different color, giving off light rather than simply vibrating. This occurs because the energy absorption causes electrons to jump to higher energy states, after which they then almost immediately fall back down to their ground states, emitting specific amounts of energy as photons. Not all of the energy is emitted in a given photon, so the emitted photons will be of lower energy and, thus, of lower frequency than the absorbed ones. Thus, a dye such as Texas red may be excited by blue light, but emit red light; or a dye such as fluorescein isothiocyanate (FITC) may absorb (invisible) high-energy ultraviolet light and emit green light ( [link] ). In some materials, the photons may be emitted following a delay after absorption; in this case, the process is called phosphorescence . Glow-in-the-dark plastic works by using phosphorescent material.

An image shows a large cell in the foreground and other cells further in the background. Each cell has an irregular shape with a large blue circle in the center. Green lines surround the blue circle and span out towards the edges of the cell. The rest of the cell is red with a bright red edge. The background of the image is black.
The fluorescent dyes absorbed by these bovine pulmonary artery endothelial cells emit brilliant colors when excited by ultraviolet light under a fluorescence microscope. Various cell structures absorb different dyes. The nuclei are stained blue with 4’,6-diamidino-2-phenylindole (DAPI); microtubles are marked green by an antibody bound to FITC; and actin filaments are labeled red with phalloidin bound to tetramethylrhodamine (TRITC). (credit: National Institutes of Health)
  • Which has a higher frequency: red light or green light?
  • Explain why dispersion occurs when white light passes through a prism.
  • Why do fluorescent dyes emit a different color of light than they absorb?

Magnification, resolution, and contrast

Microscopes magnify images and use the properties of light to create useful images of small objects. Magnification is defined as the ability of a lens to enlarge the image of an object when compared to the real object. For example, a magnification of 10⨯ means that the image appears 10 times the size of the object as viewed with the naked eye.

Greater magnification typically improves our ability to see details of small objects, but magnification alone is not sufficient to make the most useful images. It is often useful to enhance the resolution of objects: the ability to tell that two separate points or objects are separate. A low-resolution image appears fuzzy, whereas a high-resolution image appears sharp. Two factors affect resolution. The first is wavelength. Shorter wavelengths are able to resolve smaller objects; thus, an electron microscope has a much higher resolution than a light microscope, since it uses an electron beam with a very short wavelength, as opposed to the long-wavelength visible light used by a light microscope. The second factor that affects resolution is numerical aperture , which is a measure of a lens’s ability to gather light. The higher the numerical aperture, the better the resolution.

Even when a microscope has high resolution, it can be difficult to distinguish small structures in many specimens because microorganisms are relatively transparent. It is often necessary to increase contrast to detect different structures in a specimen. Various types of microscopes use different features of light or electrons to increase contrast—visible differences between the parts of a specimen (see Instruments of Microscopy ). Additionally, dyes that bind to some structures but not others can be used to improve the contrast between images of relatively transparent objects (see Staining Microscopic Specimens ).

  • Explain the difference between magnification and resolution.
  • Explain the difference between resolution and contrast.
  • Name two factors that affect resolution.

Key concepts and summary

  • Light waves interacting with materials may be reflected , absorbed , or transmitted , depending on the properties of the material.
  • Light waves can interact with each other ( interference ) or be distorted by interactions with small objects or openings ( diffraction ).
  • Refraction occurs when light waves change speed and direction as they pass from one medium to another. Differences in the refraction indices of two materials determine the magnitude of directional changes when light passes from one to the other.
  • A lens is a medium with a curved surface that refracts and focuses light to produce an image.
  • Visible light is part of the electromagnetic spectrum ; light waves of different frequencies and wavelengths are distinguished as colors by the human eye.
  • A prism can separate the colors of white light ( dispersion ) because different frequencies of light have different refractive indices for a given material.
  • Fluorescent dyes and phosphorescent materials can effectively transform nonvisible electromagnetic radiation into visible light.
  • The power of a microscope can be described in terms of its magnification and resolution .
  • Resolution can be increased by shortening wavelength, increasing the numerical aperture of the lens, or using stains that enhance contrast.

Fill in the blank

When you see light bend as it moves from air into water, you are observing _________.

refraction

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Short answer

Explain how a prism separates white light into different colors.

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Questions & Answers

with the aid of a well labeled diagram describe the conducting system
Maridad Reply
what is cellular immunity
namugenyi Reply
Cellular Immunity. -Lymphocytes act against target cell. -Acts directly by killing infected cells.
abdinor
What are NK cells
Peter
Natural killer cells
Rahaba
what are Antigen determinant
mary
cellular immunity is the state where the lymphocytes destroy the infected or targeted cell
cynthia
any examples of oedema
cynthia
introduction of microbial diversity-1
Bhavanimangali Reply
List the type of micro organism arround us and how they can be seen and with what kind of instrument
clinton Reply
how is the arrangements of bacteria in bacilli
Vaidah Reply
Provide some examples of bacterial structures that might be used as antibiotic targets and explain why.
Vaidah
Coccobacilli, Club-Shaped bacilli, Bacilli with rounded ends, Fuilform bacilli, Bacilli with ends square.
Enoch
three main antibiotic targets in bacteria: The cell wall or membranes that surrounds the bacterial cell. The machineries that make the nucleic acids DNA and RNA. The machinery that produce proteins
Rushikesh
The bacterial cell wall. Protein production. and DNA synthesis. Why, this is because most drugs (antibiotics) affects the cell wall of the bacteria, which makes the bacteria weak or susceptible in human body.
Enoch
UV rays affecting the..
Mali Reply
what is microbiology
Baba Reply
Microbiology is the study of microorganisms, which are prokaryotic and eukaryotic cell which includes bacteria, fungi, viruses and pathogenic protozoa.
Enoch
Microbiology is the branch of Life science which deals with scientific study of many Microorganisms.
Rushikesh
what is types of microbiology
Alsheikh
Immunology, Bacteriology, Virology, Mycology, Algology etc
Enoch
Virology, Immunology, Bacteriology, Algology, Mycology, Protoozology etc
Enoch
and what is mycology
Alsheikh
Immunology, Serology, Virology, Microbial Genetics, Parasitology, Bacteriology, Mycology, Molecular, Cell Biology, Agricultural, Water,Soil, Food Industrial ,Pharmaceutical, Applied, Environmental, Clinical, Medical,Marine Microbiology, Microbial Systematics, Etc, are & many types of Microbiology.
Rushikesh
study of fungi is called mycology
Munna
Mycology is the branch of Microbiology which deals with scientific study of Fungi.
Rushikesh
Study of microorganisms,which we can't see with our naked eye is called microbiology
Munna
Mycology is the scientific study of Fungi.
Enoch
virology is the study of viruses
Oppah
what is microbiology? microbiology is the study of small microorganisms that we can not with our naked eyes.
Leticia
what is taxonomical classification of microbiology
Bami
The algae, protozoa, slime moulds, fungi, bacteria, archaea and viruses ,are taxonomic classification of Microorganisms
Rushikesh
We have Bacteria, Archaea, Protozoa, Algae, Fungi, Viruses.
Enoch
microbiology is the study of microbes too small to be seen by naked eyes
Maridad
microbiology is a branch of biology which deals with study of smallest living microrganisms such as bacteria protozoa fungi and viruses
Chaitra
microbiology is the study of microorganisms which can't be seen by our naked eyes
Nakaweesi
Micro - Minute Bio - Life Logus - Study
Roshan
what is the meaning of antimicrobial susceptibility testing
Devshree Reply
seven gram positive bacteria
Okocha Reply
seven examples of gram negative bacteria
Okocha
seven examples of gram negative bacteria
Okocha
Physical conditions that would enable selective Isolation of staphylococcus epidermis
shongile Reply
Nutritional requirements that would enable selective Isolation of staphylococcus epidermis
shongile
Nutritional requirements that would enable selective Isolation of staphylococcus epidermis
shongile
what is constant flux but
Jane Reply
Digestion of food is completed in __
Amina Reply
Small Intestine
Enoch
large inteatine
Abdiwali
small intestine
Betelhem
Small intestine
Abdirisaq
small intestine
Marah
small intestine specific in illum
Mariam
difference btwn hausteria and appears
Raviha Reply
numerical and molecular taxanomy
Dhanshri Reply
difference btwn hausteria and appesorium
Raviha
full life cycle of plasmodium parasite
Emmah Reply

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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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