frequency's unit is Hz which mean cycles per second
given frequency * wavelength = speed of wave
speed of light = 3x10^8 m/s
wavelength = 680nm = 680x10^-9 m
frequency * 680x10^-9 m = 3x10^8 m/s
frequency = 3x10^8 / 680x10^-9
= 4.41x10^14 Hz
= 441x10^12 Hz
= 441 THz
which is the frequency for red light
Definitely cell phone reception! However books could be argued for since they also are a good more than a necessity. However the last 2 i believe would be classified more as necessities than goods.
~Hope this helps!~
