Some important practical units

(i) To measure small distances,we use the following units:

(a) 1 millimetre i.e.,1 mm = 10-3 m = 10-1 cm.
(b) 1 micron = 10-6 m
(c) 1 angstrom = 10-10 m
(d) 1 fermi = 10-15 m
(e) 1nanometre = 10-9 m
(f) 1 barn = 10-28 m 2      [ used for measuring nuclear cross-section]

(ii) To measure very large distance we use the following units:

(a) 1 astronomical units (AU): It is defined as the mean distance of earth from sun.
                             1 AU = 1.496*1011 m ≅ 1.5*1011 m.
(b) Light year : It is the distance travelled by light in one year.

If velocity of light is 3*108 m/s in vacuum and one year is equal is 365 day *24 hr * 60 min * 60s.

                     distance = velocity * time

Therefore      1 light year = 3* 108*365 *24*60*60 = 9.46*105 m
                                         = 9.5*1015 m.

(c) Parsec (parallatic second ) : It is defined as the distance of the star whose parallex is one second i.e., the distance at which an arc of length one-astronomical unit subtends an angle of one second (1")

It is the unit of distance in astronomy.
                        1 Parsec =3.084*1016 m≌ 3.1*1016 m.
(iii) Relation between AU, light year and parsec
                          1LY = 6.3*104 AU.
                   1 parsec = 3.26 LY.
⟹ 1 parsec > LY > AU.