From ERIC TURPIN
Andrew Haynes (Letters, 15 February) has brought up an interesting and
quite well-known phenomenon: the clue is that a normal fluorescent tube
is only light for short periods 100 times a second.
The initial light produced by the discharge in the mercury vapour is
blue, with a strong ultraviolet compound. This is the blue that Haynes observed.
The ‘gold’ is the orange afterglow of the fluorescent coating in the tube.
The blue is only visible for a short time every hundredth of a second. While
the orange is seen a short time after this, it continues for a while after
the discharge is extinguished. Here, if a rotating mirror (at 3000 rpm,
or sub-multiple) is used to reflect the opponent white light and the position
of the mirror is adjusted correctly, first blue then ‘gold’ is seen.
Eric Turpin Lewes, Sussex
