Electrical/optical engineers (should) know logs, or more precisely dBs (10log10), i.e. a logarithmic measure of ratio. That is because many powers, e.g. electri

Electrical/optical engineers (should) know logs, or more precisely dBs (10log10), i.e. a logarithmic measure of ratio. That is because many powers, e.g. electrical and optical power as well as loss is often expressed in dB(m) (power in relation to 1 mW) or dB/m (as loss/gain). However it is not necessary to memorize so many. It's easier to explain in dB. 0dB = 1, (that's really definition), 3 dB =2 and 5 dB =3 (that's a bit less precise), maybe we can add 7 dB = 5. From those ones we can deduce everything else: 4=22=3 dB+3dB, 10=3 dB + 7 dB= 10 dB, similar negative dBs i.e. 1/3 = 0 - 3 dB...