Chapter 3 Section 1: Microscopes

Cells Under the Microscope

Objectives:• Describe how scientists measure the length of objects.

• Relate magnification and resolution in the use of microscopes.

• Analyze how light microscopes function

• Compare light microscopes with electron microscopes.

• Describe the scanning tunneling microscope.

Cells Under the MicroscopeMeasuring Cell Structures:

Measurements taken by

scientists are expressed in

metric. The official name of

metric system is the

International system of

measurements, abbreviated

SI.

Cells Under the Microscope• Magnification is the

quality of making an image appear larger than its actual size.

• Resolution is a measure of the clarity of an image.

Cells Under the Microscope

• Both high magnification and good resolution are needed to view the details of extremely small objects clearly.

Types of Microscopes

Light Microscopes• Light Microscopes form an image when light

passes through one or more lenses to produce an enlarged image of a specimen.

Electron Microscopes• Electron Microscopes form an

image of a specimen using a beam of electrons rather than light.

• The electron beam and specimen must be in a vaccum so that the electron beam will not bounce off of gas molecules.

• Live organisms cannot be viewed with an electron microscope.

Transmission Electron Microscopes

• An electron beam is directed at a very thin slice of a specimen stained with metal ions. Some structures become more heavily stained than others.

Transmission Electron Microscope

• The heavily stained parts absorb electrons, those that are lightly stained allow electrons to pass through.

• The electrons that pass through strike a fluorescent screen, forming an image.

Scanning Electron Microscopes• An electron beam is focused on a specimen

coated with a very thin layer of metal.• The electrons that bounce off the specimen

form an image on a fluorescent screen.• The image shows a three-dimensional details

of the surface of specimen.

Scanning Tunneling Microscope• A needle-like probe measures

difference in voltage caused by electrons that leak, or tunnel, from the surface of the object being viewed.

• A computer tracks the movement of the probe across the surface of the object.

Scanning Tunneling Microscopes• The image shows a

three-dimensional details of the surface of a specimen.

• Live specimens and objects as small as atoms can be viewed.