Biology - Unit 2 - Topic 1 - Magnification and Microscopes

what is the formula for magnification?

magnification = size of image/size of real object

define the term magnification

the number of times larger an image is compared with the real size of an object

define the term resolution

the ability to distinguish between two separate points

what is the limit of resolution of a light microscope?

• ½ the wavelength of the radiation used to view the specimen

• the shortest wavelength of light is 400nm so the max resolution is 200nmwha

what does a light microscope look like?

what is the maximum useful magnification of an optical microscope?

x1500

which organelles can you not see using a light microscope?

• ribosomes

• lysosomes

• mitochondria 

• endoplasmic reticulum

what is the other kind of microscope?

electron microscopes

what are the two types of electron microscope?

• transmission electron microscope

• scanning electron microscope

what is the maximum resolution of a transmission electron microscope?

0.1nm

what is the maximum resolution of a scanning electron microscope?

20nm

describe how a TEM works

• use electromagnets to focus a beam of electrons, which are transmitted through the specimen

• denser parts of the specimen absorb more electrons, making them look darker on the resulting image

describe how an SEM works

• scan a beam of electrons across a specimen

• this knocks off electrons from the specimen

• these are gathered in a cathode ray tube to form an image 

• the resultant images show the surface of the specimen and the 3-D

what are the advantages of TEMs?

gives the highest resolution images so shows small objects

what are the advantages of SEMs?

• can be used on thick specimens

• produce 3D images

• have a relatively high resolution

what are the disadvantages of TEMs?

• can only be used on very thin specimens

• can only be used on non-living specimens

• the specimen must be viewed in a vacuum

• images may contain artefacts which can make it difficult to identify organelles

what are the disadvantages of SEMS?

• give lower resolution images than TEMs

• can only be used on non-living specimens

• need to be used in a vacuum

• images may contain organelles

describe how to prepare a temporary microscope slide

• place a small drop of water on the centre of the slide

• use tweezers to place a thin section of your specimen on top of the water drop

• add a drop of iodine on top

• place a coverslip on top

• blot off any excess iodine using filter paper