Or HaChaim on Deuteronomy 9:10/אור החיים על דברים ט׳:י׳

1 כתבים באצבע אלוקים, "written with the finger of G'd." We need to understand what is meant by ועליהם, "and upon them." I have explained in Exodus 31,18 that the words: "written with the finger of G'd" mean that G'd's finger was pointed at the place on the Tablets where the particular letter was to be inscribed and that in this fashion the letter appeared engraved and this is the meaning of חרות על הלוחות, "engraved on the Tablets." Our verse tells us that whereas the area of the outline of the letter on the Tablets remained engraved, the hollow projected a light which emanated from the finger of G'd and appeared on the surface of the Tablets so that the letters appeared as if written on the Tablets though in fact they were engraved. This is what Moses meant when he spoke of ועליהם ככל הדברים אשר דבר השם, "and upon them like all the words G'd had said." The letter כ describes the result of G'd's engraving the Tablets, i.e. that the letters appeared visible as light on the surface of the Tablets.