a). frequency = (speed) / (wavelength)
The speed of light is around 3 x 10⁸ m/s.
For 380 nm (violet light), frequency = (3 x 10⁸ m/s) / (380 x 10⁻⁹ m)
Frequency = 7.89 x 10¹⁴ Hz
For 760 nm (red light), frequency = (3 x 10⁸ m/s) / (760 x 10⁻⁹/s)
Frequency = 3.94 x 10¹⁴ Hz
The ratio is 2 .
That's 1 octave, or 0.3 of a decade.
b). The ratio of highest/lowest sounds is (20,000 Hz/20 Hz) = 1,000
That's 3 decades, or about 10 octaves.
===> Speaking logarithmically ( ! ), ears are sensitive to a range of sound frequencies that's 10 times as wide as the range of light frequencies that eyes can detect.