Malbim Ayelet HaShachar Klal 482
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וכשבא פעל 'הפך' על שינוי איכות בא אחריו שימוש הלמ"ד – "נהפך ללבן". וכשבא בלא שימוש הלמ"ד – "נהפך לבן" – מורה שלא נהפך כולו רק מקצתו. (שם סימן מח)
Ayelet Hashachar, translated by Betzalel Avraham Feinstein, 2021
And whenever the verb הפך (turn) is used with respect to a change in quality, the prefix letter ל will come afterwards – for example נהפך ללבן (turned to white). And when it comes without the prefix letter ל afterwards, for example נהפך לבן (turned white) – it teaches that it was not changed completely, only partially.