3.3 Microscopic measurements and calculations

Question 1

How can you measure the size of objects in a light microscope

Answer 1

. Use an eyepiece graticule

Question 2

What is a graticle and what is etched onto it

Answer 2

. A glass disc that is placed in the eyepiece of a microscope

. A scale is etched onto it which is typically 10mm long and is divided into 100 sub divisions

Question 3

Why can’t the scale on the eyepiece graticule be used directly to measure the size of objects under a microscopes objective lens

What must be done to fix this

Answer 3

Each objective lens magnifies to a different degree, so the graticule must be calibrated for a particular objective lens.

However once calibrated it can remain in place for future use as long as the same objective lens is used

Question 4

How do you calibrate the eyepiece graticle

Answer 4

. You use a special microscope slide called a stage micrometer
. This slide has a scale etched onto it and the scale is 2mm long and has sub divisions that are 0.01mm

. So as a result: for example
10 units on a qm scale are equivalent to 40 units on a graticule scale

So 1 unit on a qm scale is equivalent to 4 on a graticule scale

. So each unit on the qm scale equals 10qm, each unit on the graticule equals 10/4 which is 2.5

Question 5

How can you calculate the scale for different objective lenses

Answer 5

. Dividing the differences in magnification

Eg if an objective lens magnifying X40 gives a calibration of 25qm per graticule unit,
then an objective lens magnifying X400 which is 10 times more, gives a calibration of 25/10 which is 2.5qm.