שמות פרק י , כא וִיהִי חשֶׂךְ עַל-אֶׂרֶׂץ מִצְרָּיִם וְיָּמֵש חשֶׂך
מכילתא פרשת בשלח פרשה ד
מגיד הכתוב שהיה המצרי עומד ולא היה יכול לישב יושב ולא היה יכול לעמוד. פורק ואינו יכול לטעון. טוען אינו יכול לפרוק . מפני שהוא מש באפלה שנאמר וימש חשך
The Midrashim explain that the darkness of this plague was as thick as the size of a dinar coin. And this is difficult to understand, since when does darkness have a measurable size. Further, Rashi says that during the days of this plague it was all night and no day. How is this possible to change the nature of creation?
HaShem had already promised to Noach and his children that day and night would not cease. Plus, how was it dark to the Egyptians and not to the Jews?
Therefore, Rabbi Epstein ventures a unique explanation. This concept of darkness was not in the outside environment, but was limited to the eyes of the Egyptians. This means that G-d caused a stretching of the cataract over the pupil of the eye, and the thickness of the cataract became the size of a dinar coin, which resulted in total darkness only to the Egyptians.
