The nursery rhyme “London Bridge is Falling Down” includes a line
Iron and steel will bend and bow
To me as a non-native speaker, both “bend” and “bow” evoke essentially the same kind of mental image in this context. Is there a difference which can be put into words? Or are they fully synonymous here, and just combined for the sake of the rhyme?
Writing this I realized that while apparently both words work for iron and steel (perhaps with some artistic liberty?), I’d only bow to a king and bend rays of light using a lens, not the other way round. But I don’t know why, except that I’m copying phrases I have read in these contexts. So perhaps an explanation of situations where just one word is appropriate can help point out distinguishing nuances for situations where both are possible.
Bend is a much more common word than Bow.
There are several related words written “bow”
There is is /bau/ rhyming with cow (noun, verb) which is a lowering your body to show humility, for example before a King.
There is /bau/ (noun) The front part of a boat.
There is /bou/ rhyming with know (noun) a knot made with loops of string, for example to tie up a shoes or a gift.
There is /bou/ (noun) Used to shoot an arrow, or play a violin
There is /bou/ (verb) meaning “bend”. This is quite rare, as it can nearly always be replaced by the word “bend”.
The verb “bow”, meaning “bend”, is normally used without an object:
The beams holding up the roof are bowing
Bend can be used both with or without an object:
The robot bent the steel bar with ease.
The beams holding up the roof are bending.
(Transitive use of “bow” is possible, but even rarer)
Some slight difference is that “bowing” suggests gentle curving, whereas “bend” might suggest an angular bend, or a fold.
In the rhyme, they are nearly fully synonymous. They are used for the alliteration. Using redundant words like this is technically called “pleonasm”. For example if I say “You will get a free gift.” The word “gift” implies “free”, but I might choose to use the redundant word for style or emphasis.