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Units specify how a particular quantity is measured. Tables 1.1 and 1.2 list the standard physical units for measurements.

Table 1.1 Units for fundamental quantities.

Quantity	Units	Abbreviation
mass	kilogram	kg
length	metre	m
time	second	s
electric current	Ampere	A
temperature	degree Celsius	°C

Table 1.2 Units for derived quantities.

Quantity	Units	Abbreviation
energy	Joule	J
power	Watt	W
intensity	Watt/cm2	W/cm2
pressure	Pascal	Pa
velocity	metre/sec	m/s
frequency	Hertz	Hz

It is common to use prefixes to modify these units to make them more convenient. For example, length may be measured in metres, centimetres or millimetres (abbreviated m, cm and mm) and time may be measured in seconds, milliseconds or microseconds (sec, msec, μsec). Table 1.3 defines the meaning of the commonly used prefixes.

Table 1.3 Common prefixes for units.

Quantity	Abbreviation	Meaning
giga	G	× 109
mega	M	× 106
kilo	k	× 103
centi	c	× 10-2
milli	m	× 10-3
micro	μ	× 10-6

Whenever you are making a calculation, make sure the units are consistent (e.g. if one length is in mm and one in cm, convert one of them so they are both in the same units).

You should also look at the units you are using to determine the units that your answer will be in. Write the units after each number or symbol, and multiply and divide units just as if they were numbers.

For example, you are asked to calculate

where a_p and λ are both measured in mm. Then

Dividing top and bottom by mm gives

and so the answer will be in mm.

If you want to change units, once again just write the units as if they were numbers. For example, if λ = 0.3 cm and we want to express it in mm instead, we simply replace [cm] with [10 mm] (since 1 cm = 10 mm) and then take the 10 out of the square brackets and multiply by it: