Well, with the light spectrum there technically is no middle color. Both green and yellow meet up in the middle at 560 nm (wavelength interval) and 540 THz (frequency interval).
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Thus, the payback time for cavity wall insulation is 7.5 years
Free fall means your falling because of gravity and weight and will eventually hit the ground without any protection. weightlessness means a place or time where there is no gravity and you're floating and not falling.
