The floor and the four walls of Tom鈥檚 optical laboratory are each
rectangular, and each is covered in non-reflective material. The ceiling, too,
is rectangular鈥攂ut is a perfectly reflecting mirror. Within the room there
is a specific fixed point鈥攖he 鈥渙bject-point鈥濃�攁t which minuscule
objects may be placed for examination: the object-point is 8 foot 1 inch below
the nearest point of the ceiling. In the room there is also a fixed vertical
rod, from floor to ceiling: it meets the ceiling at a point distant 14 foot 1
inch from the object-point. On this rod there is mounted a point-source of light
(of equal illuminative intensity in all directions): this point-source can be
freely moved up and down the rod, but can not be moved in any other direction.
This point-source is the sole source of illumination in the room.
In order to have the greatest total illumination at the object-point, how far
from the top of the rod should Tom station the point-source?
A 拢10 book token will be awarded to the sender of the first correct
answer opened on Thursday 25 July. The Editor鈥檚 decision is final. Please send
entries to Enigma 880, New 杏吧原创, King鈥檚 Reach Tower, Stamford Street,
London SE99 0BB. The winner of Enigma 874 was Stephanie Clarke of Thane,
Oxfordshire.
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Answer to Enigma 874
Scamelot lottery
11, 19, 23, 31, 41 and 43.