This spell is very
useful in crossing dangerous places; a
rainbow evoked by a druid can be used to
span fissures, rivers, and so on. The rainbow
springs into being, forming a multi-colored
arch of light 5’ wide, which can
support any amount of weight, and will
last as long as the druid maintains his
concentration upon it (or until the spell
duration elapses). A span up to 300’ may
be bridged by the rainbow, but the apex
of the arch will be one-fifth the length of
the span (60’ in the case of a 300’ span) in
height. Ceilings may not permit the
desired length, in which case the spell will
simply not function. Note also that the
rainbow makes upright travel across it
easy, regardless of the steepness of ascent
or descent; also, since there are no parapets,
it is possible to fall or be thrown off
of the rainbow.

Rainbow (illusionist cantrip)
Rainbow (5th level cleric spell)

The Natural Order,
By Arthur Collins