Microscopes Flashcards
(23 cards)
What is resolution?
Resolution: the ability to distinguish between 2 distinct points clearly. (The clarity of an image)
What is the result of high linear magnification?
Higher the linear magnification, the wider and longer the image is compared to the size of the actual object.
What is the result of high resolution?
Higher the resolution, the clearer the image.
Advantages of optical (light) microscopes.
- Relatively cheap.
- Easy to use.
- Portable (so can be used in filed and laboratories).
- Views living specimens.
Define photomicrograph.
Photomicrograph: a photograph of an image seen using an optical (light) microscope.
What is the magnification formula?
Magnification = image size / object size
How does a laser scanning (confocal) microscope work?
- Laser light used to scan an object tagged with fluorescent dye.
- Laser causes the dye to give off light which is focused onto a detector to a computer.
- Computer assembles pixels information into 1 image.
Advantages of laser scanning (confocal) microscopes.
- 3D image.
- Highest resolution of all microscopes.
- High contrast due to depth selectivity to focus on structures at different depths within the specimen.
Advantages of TEM.
- high resolution so used to look at a range of organelles.
- Highest magnification of all microscopes.
- High contrast in image.
What is magnification?
Magnification: the number of times larger an image appears compared to the size of the actual object.
How does a transmission electron microscope work?
- The specimen has to be dehydrated and stained with metal salts and placed into a vacuum.
- electromagnets focus a beam of electron which are then transmitted through the specimen. Some electrons pass through and are focused onto the screen/photographic plate.
- Denser parts of the specimen absorb more electrons so look more darker in the end image.
Define electron micrograph.
Electron micrograph: photograph of an image seen using an electron microscope.
Always in black and white but colour can be added afterwards.
How do scanning electron microscopes work?
- Specimen has to be placed into a vacuum and often coated with a fine film of metal salts.
- A beam of electrons scanned across the specimen. This knocks off electrons from the specimen which are collected in a cathode ray tube to form an image.
Disadvantages of optical (light) microscopes.
- Low resolution so usually used to look at tissues or whole cells due to short wavelength.
- Produces a 2D image.
- Specimens have to be thin and transparent.
Total magnification formula.
Total magnification = magnifying power of objective lens X magnifying power of eyepiece lens
Advantages of SEM.
- Image can be 3D
- High resolution.
- High magnification.
- Computer software programmes can add false colour to the image.
Disadvantages of TEM.
- 2D image.
- Black and white (gray scale) image.
- Only used on thin specimens.
- Large microscopes.
- Expensive.
- Need lots of training and skills to be used.
- Specimens have to be dead since viewed in a vacuum.
- Metallic salt stains are potentially hazardous to the user.
Disadvantages of SEM.
- Image shows surface of specimen.
- Large microscopes.
- Expensive.
- Need lots of training and skills to be used.
- Specimens have to be dead since viewed in a vacuum.
Magnification of
- Optical microscopes
- TEM
- SEM
Optical microscopes = x1500
TEM = more than x1000,000
SEM = less than x500,000
Resolution of
- Optical microscopes
- TEM
- SEM
Optical microscopes = 0.2 um
TEM = 0.0002um
SEM = 0.002um
Why staining is needed?
-If the specimen is transparent then the light just goes through rather than being absorbed so staining is used.
Staining in optical microscopes.
- Dye is used.
- Common ones are Methylene Blue and Eosin.
- Parts of specimen take up the stain more than others so the contrast makes the object show up.
Differential staining.
Some stains bind to specific organelles/cell structures, staining each cell structure differently so the structures can be easily identified in a single preparation.
E.g. Eosin stains cytoplasm.
