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Microscopes

y The human eye, a telescope, a pair of binoculars a magnifying glass and a microscope arevarious types of optical instruments.

y A microscope is an optical instrument that is used to observe tiny objects, those that cannot beseen with the unaided human eye.

y Each optical instrument has a limit as to what can be seen..The limit is referred to as theresolving power or resolution of the instrument.

Simple Microscopes

y A simple microscope is one that contains only one magnifying lens.y A magnifying glass could be considered a simple microscope

With a magnifying glass, images appear 3-20 times larger than the objects actual size.

y Leeuwenhoeks simple microscopes had a maximum magnifying power of about 300X or 300times.

Compound Microscopes

y A compound microscope contains more than one magnifying lens.y Because visible light is the source of i llumination, the compound microscope , the compound

microscope is also referred to as a compound light microscope.

y Compound light microscopes usually magnify objects about 1000 times.Photographs taken through the lens system of a compound microscope are called photomicrographs.

y Its the wavelength of visible light(~0.45 micrometer ) that limits the size of object that can beseen.

y Objects cannot be seen if they are smaller than half of the wavelength of visible light.y Todays laboratory microscope contains two magnifying lens systems:

1. The eyepiece lens or ocular lens( usually X10)2. The objective (usually X4,X10,X40,and X100(oil immersion))

y For optimal observation of the specimen, the light must be properly adjusted. This isaccomplished using the condenser which is beneath the stage.

y Image clarity depends on the microscopes resolving power ( or the ability of the lens system todistinguish between two adjacent objects)

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The resolving power of the compound light microscope is ~1000 times better than that of the

unaided human eye.

y Increasing magnification without increasing the resolving power is called emptymagnification.

y Because objects are observed against a bright background or brightfield, the compound lightmicroscope is sometimes referred to as a brightfield microscope.

y If the condenser is replaced with what is known as a darkfield condenser ,illuminatedobjects are seen against a dark background or dark field, the microscope is now a

darkfield microscope.

y Other types of microscopes include:

y Phase contrast microscopes.y Fluorescence microscopes.

Phase contrast and Fluorescence light Microscopes

y Phase contrast microscopes are used to observe unstained living microorganisms.Organisms are more easily seen because the light refracted by living cells is different from the

light refracted by the surrounding medium.

y Fluorescent microscope contains a built-in ultraviolet (UV) l ight source.When UV light strikes certain dyes and pigments ,these substances emit a longer wavelength

light causing them to glow against a dark background.

Electron Microscopes

y Electron microscopes enable us to see extremely small infectious agents such as rabiesand smallpox viruses.

y Living organisms cannot be seen with an electron microscope- the processingprocedures kill the organisms.

y An electron beam is used as a source of illumination and magnets are used to focus thebeam .

y Electron microscopes have a much higher resolving power than the compound lightmicroscope.

y There are two types of electron microscopes-transmission and scanning.

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1. The Transmission Electron Microscope:y Using an electron gun to fire a beam of electrons through an extremely thin specimen(

less than 1 micrometer thick)

y The image of the specimen is produced on a phosphor-coated screen.y I

ts magnification is 1000 times greater than that of the compound light microscope.

2. The Scanning Electron Microscope:y Electrons are bounced off the surface of a specimen and the image appears on a

monitor.

y Used to observe the outer surfaces of specimens.y Resolving power is not as high as the transmission electron microscope.y Scanning and transmission electron micrographs are black and white images.