My guess would be.. it was notated that way to avoid confusion since we are in G major which does not have F natural in its scale... But then again: why not just use the natural sign then?
Difficult to go into all the theory behind it here however
The short of it is essentially the same reason there is a d# in the previous measure going to e
It is a leading tone and acts as a sort of raised 7th to pull the ear
To the goal note.
If you play a major score built on an f# tonic you would notate it 7th as e#
Hi Dean and Visitor,
I think that's good points.
The question of the OP I myself asked me a long long time ago, and it's a very interesting one, and many resources may exist which describe this in different ways ( and NONE of these ways can, as should be clear, justify one or the other ( e sharp or f natural ) completely for OUR NORMAL WELL-TEMPERATED PIANOS. Since it's the same key.
But: There ARE reasons, and they lead to the facts you already mentioned.
Prof. Erich Wolf, who taught at Detmold Conservatory, Germany, writes in his "Harmonielehre", (vol. II of his "Die Musikausbildung"), which I have, the following: I try to translate:
The Material:
[...] The gradual relation to next tones, especially "leading tone"-effects, are of importance as criteria. An "ear-pulling", leading effect do tones have then, if they stand in the offset of "minor seconds" to the neighboring grades. (Requirement for this is, that the harmonic relations bring it forward. )
The "major-scale" contains the "leading-tones-by-nature" of the IIIrd and VIIth level ( leading upwards ), or VIIIth and IVth level ( leading downwards ).
ARTIFICIAL leading tones can be created by augmenting ( = "setting a semitone higher" / "sharpening" ) a natural tone level ( eine Stammtonstufe ) (upwards ) or by flattening ( downwards ).
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E sharp to F sharp, here, should be such a diatonic semi-tone-step. The 6th level of G major, E, is sharpened, leading to the 7th level of G-major, wich is F sharp. F sharp, itself, is, according to Wolf and perhaps our knowledge, itself a "leading-tone-by-nature", of G-major, leading and "pulling" the ear to the "G" ( which isn't directly existing in the pic given. )
Please note, that the temperament of the instrument given plays a major role: Only if instruments are well-temperated, the aforementioned conditions may be valid, because, then, a "g sharp", for example, sounds exactly the same as an a-flat, and it's the same key on the piano, which is, at first, confusing, and can lead to the questions of the OP, and the comments by Wolf as explanation, or, at least, one explanation.
Careful should we be if it comes to the VIOLIN, for example. If it's not well-temperated, the "g sharp" would sound slightly DIFFERENT compared to an a-flat (the "sharp" sounding a little brighter, the flat a little...flatter.)
Cordially, 8_oct!