Chapter 4: Microscopy Flashcards
What is a simple magnification system? How is it different from a compound magnification system?
A simple magnification system is a single lens used to form an enlarged image of an object. Examples include the projection of a slide in a lecture hall, pocket magnifier and hand lens.
A compound magnification system is where magnification occurs in two stages. The total magnification is the product of the magnification of the first lens and the second lens. The observer looks at the first image with a lens that produces a virtual image.
What is a virtual image?
A virtual image is the enlarged image produced by a lens in a compound magnification system. The real image does not exist where the virtual image appears to be and is only visible due to the compound magnification system.
What is focal length?
Focal length is the distance between two points of focus on either side of the lens. It determines much of the image quality.
In humans, our eyeballs are curved so we cannot maintain a clear point of focus for all distances. 10 in or 25cm is the distance that a human eye can easily distinguish between two objects next to each other.
What is resolution?
Resolution is the minimum distance between two objects that can be separated and still be seen as two objects.
The human eye resolution is between 0.15 and 0.3mm.
Why is resolution more important than magnification?
An object will not be visible if the resolution is not high enough for the magnification. A large image with no resolution would be very difficult to see and interpret.
What are the main parts of a microscope?
The eyepiece can be monocular or binocular.
The objective lens is the closest to the specimen.
The stage is the platform the specimen is placed on, and can move up or down.
The condenser is a lens below the stage used to obtain a bright field of view. They have their own condenser diaphragm that eliminates excess light and adjusts for contrast in the image.
The field diaphragm is a control that allows more or less light into the lens system of the microscope.
If you saw “10x/0.54/170/0.17” on an objective, what would it mean?
10x is the magnification.
0.54 is the numerical aperture: the angular measure of the lens’ light-gathering ability and its resolving quality.
170 is the tube length: the distance from the lowest part of the objective to the upper edge of the eyepiece.
0.17 is the recommended thickness of the coverslip: the thin glass plate that goes on top of the mounted specimen.
What is astigmatism?
Astigmatism (spherical aberration) results from a lens not being properly spherical. It makes specimen images seem to be pulled in one direction when focusing through it.
What does a condensing lens do?
A condensing lens is used to obtain a bright field of view. It has its own condenser diaphragm that eliminates excess light and adjusts for contrast in the image.
What’s the difference between a real image and a virtual image?
A real image is an image that can be seen on the screen or is projected onto the screen.
A virtual image is an enlarged image produced by a compound magnification system in which the image appears to be larger and in space where it actually is not.
What is RI ? What is the RI of air, water, and diamonds?
Refractive index (RI) is the relative speed at which light moves through a material with respect to its speed in a vacuum.
The RI of air is 1.0008, water is 1.330, and diamonds are 2.417.
Why is microscopy important to forensic science?
Microscopy allows us to visualize details that are invisible to the naked eye. It allows us to compare and contrast evidence and can be extremely helpful in solving a case.
What is a mounting medium?
A mounting medium (mountants) are materials commercially available to be used as a medium in which a sample is viewed in transmitted light. Samples need to be viewed in a material with an RI similar to their own. It is the medium your sample is in while you view it under a microscope.
Name three materials besides air that are isotropic.
Isotropic materials demonstrate the same optical properties in all directions. Gasses, liquids, certain glasses and crystals all exhibit isotropic qualities.
What does a polarizing filter do?
A polarizer is a filter that only lets light pass through if it vibrates in the “preferred” direction. Light that only vibrates in one direction is called polarized light.
