Or HaChaim on Exodus 10:23/אור החיים על שמות י׳:כ״ג

1 ולכל בני ישראל היה אור, all the Israelites had light, etc. The Torah makes a point of writing ולכל, "and for every Israelite," to tell us that whenever a Jew went to the house of an Egyptian he had light even within the dwellings of the Egyptians.

2Another possible meaning of that line could be in answer to the question "where did this light emanate from?" The answer is that it was light originating in the Egyptians' homes. The absence of the word אשר is not especially significant as we have many examples when the Torah omits the word אשר. The meaning of the verse is analogous to our sages telling us that the wicked envelop themselves in darkness. Accordingly, we may understand the darkness as being something subjective; the Egyptians who were evil experienced darkness whereas the Jews who were good experienced light in the very places the Egyptians experienced darkness. The Torah alludes to this idea by writing: במושבותם, within their dwellings.