Using a microscope to measure cell size
Cell size can be measured using an eyepiece A glass or plastic disc fitted into the eyepiece of a microscope. The graticule has a scale ruled on it and is used to estimate the size of a specimen when viewed with a microscope.. The graticule has a ruler on it.
You must find out the distance measured for each division of the graticule. You can then use the graticule to measure cells. The distance will be different for each objective lens of the microscope.
To do this, you will use a A glass slide with a scale etched on it. It is used to calibrate the eyepiece graticule of a microscope.. You will use this to To set an instrument or scale against a standard. the eyepiece graticule. Once it's calibrated the same measurements can be used each time you use the microscope.
- Place a stage micrometer on the stage of the microscope.
- Line up one of the divisions on the eyepiece graticule with a fixed point on the stage micrometer.
- Count the number of divisions on the eyepiece graticule that correspond with a set measurement on the stage micrometer.
- Calculate the distance in micrometres of one division on the eyepiece graticule.
The distance of 250 渭m on the stage micrometer lines up against divisions at 10 and 61 on the eyepiece graticule.
61 - 10 = 51 divisions on the eyepiece graticule are equivalent to 250 渭m on the stage micrometer.
One division on the eyepiece graticule is equivalent to 渭m on the stage micrometer:
= 4.9 渭m (to two Giving a number to a specified number of significant figures is a method of rounding. For example, in the number 7483, the most significant, or important, figure is 7, as its value is 7000. To give 7483 correct to one significant figure (1 sf), would be 7000. To 2 sf, it would be 7500.).
Therefore one graticule division is equal to 4.9 渭m.
Using the same calibrated eyepiece graticule to measure a cell:
The calibrated eyepiece graticule can be used to make measurement of any cells or other structures viewed with the microscope on that magnification.
The width of the cell highlighted = 52 - 40 = 12 eyepiece graticule divisions.
The real width of the cell is 12 脳 4.9 渭m = 59 渭m (to two significant figures).
Question
What would be the length of a plant cell, to two significant figures, that was 35 divisions on this graticule?
170 渭m.
Each eyepiece graticule division is 4.9 渭m.
The real width of the cell is 35 脳 4.9 渭m = 171.5 渭m,
To two significant figures, this is 170 渭m.
Question
How many graticule divisions would a single celled organism that was 240 渭m take up?
49
Each graticule division is 4.9 渭m.
An organism that measured 240 渭m would take up \( \frac{240}{4.9}\) divisions = 49 divisions.
| 0.1 | = \( \frac{1}{10}\) | = 10-1 |
| 0.01 | = \( \frac{1}{100}\) | = 10-2 |
| 0.001 | = \( \frac{1}{1000}\) | = 10-3 |
| 0.1 |
| = \( \frac{1}{10}\) |
| = 10-1 |
| 0.01 |
| = \( \frac{1}{100}\) |
| = 10-2 |
| 0.001 |
| = \( \frac{1}{1000}\) |
| = 10-3 |
If you increase a number by one order of magnitude, you are multiplying the number by 10.
If you decrease a number by one order of magnitude, you are dividing the number by 10, which is equivalent to multiplying by 0.1.
For instance, a 2 m tall person and a 20 m tall oak tree differ by one order of magnitude.
The person's height = 2 m = 2 脳 100 m
The oak tree's height = 20 m = 2 脳 101 m
Hence there is one order of magnitude between the height of a human being (2 m) and the height of an oak tree (20 m).
Notice that when dividing numbers in standard form, we subtract the powers.
When comparing orders of magnitude, actual distances can be approximated. It's the relative difference that is important.
Question
What is the difference in order of magnitude between a human hair and HIV?
The width of a human hair is 100 渭m, while the width of the HIV is 100 nm.
The difference in order of magnitude is 3.
Calculation:
The width of the human hair is 100 渭m = 10-4 m.
The width of the HIV is 100 nm = 10-7 m.
So, there are three orders of magnitude difference:
\( \frac{10 - 4}{10 - 7}\) = 3.
Or, more simply:
4 - (-7) = -4 + 7 = 3.