The well known Young’s experiment demonstrating
the wave nature of light was performed in ~1601. I
suppose that light from a large candle was focused
on the first single pinhole. The other two pinholes
were probably about 50 cm away from the first
pinhole. Thus the amount of light responsible for
fringes (perhaps on white paper situated about one
meter away from the pair of pinholes) was very low.
How was Young able to observe fringes without
using a photographic film (which would overcome
the difficulty by accumulating the effect)?
I suspect that textbook descriptions of his experiment
do not correspond to what he actually did. Perhaps
observations were made in two rooms separated by
a wall with two pinholes. This would allow Young to
stay it nearly total darkness for long time in order to
sensitize the eyes. Even with slits (instead of pinholes)
direct observation of fringes produced by light from a
candle in another room would be very difficult.
I hope that a person familiar with history of optics will
share with us what s/he knows about conditions under
which Young’s experiment was actually conducted.
Ludwik Kowalski