Having just informed us that total darkness descended over Egypt, is it not obvious that they could not see each other?
Ramban and Seforno (to 10:21): It means that even by means of a lamp or a torch they could not see each other. 1
Refer to 10:21:1:3*.
Why does the Torah see fit to mention here that Yisrael had light... ?
Targum Yonasan: Hashem gave the opportunity of the darkness sent upon the Egyptians, to enable Yisrael to bury the Resha'im who died then, 1 without the Egyptians being aware of it; and to enable Yisrael to perform Mitzvos. 2
Rashi (to 10:22): Because Yisrael searched the Egyptians' property, and discovered where they kept their precious vessels and clothes. Consequently, when, before leaving Egypt, they asked to 'borrow' them, they were able to counter the Egyptians' claims that they had nothing, by telling them where everything was.
Penei David (to 10:3): The Makos came also on Yisrael for a moment. They had darkness, and then immediately light. The Resha'im, whose deeds are called darkness, were afflicted (died) in the moment of darkness.
What is the significance of the double expression, "Lo Ra'u Ish ... v'Lo Kamu ...?
Rashi: This refers to two sets of three days - one during which they were only unable to see; the second, during which they could not even move. 1 They remained rooted to the spot where they found themselves; someone who was sitting could not stand up, and someone who was standing could not sit down. 2
Oznayim la'Torah (also Gur Aryeh to 10:22): This two-fold plague was unique to Makas Choshech and was not to be found by any of the other plagues. The first three days were necessary to enable Yisrael to bury the Resha'im who died. The second three made it possible for Yisrael to search the Egyptians' houses to see where they kept their valuables (had the Egyptians still been mobile, they would have been able to protect their valuables even without being able to see). See Oznayim la'Torah.
Why did the darkness need to be so thick that they could not rise?
Moshav Zekenim: Because had they been able to rise, they would have lit lamps. 1
He assumes that had they lit lamps, they would have been able to see. Why would lamps help them to see more than sunlight? Refer also to 10:22:1.2:1 (PF).
Rashi (to 10:22) writes that the light enabled Yisrael to see the Egyptians' property, and to later ask to borrow it. Why was this necessary? Hashem gave Yisrael grace in the Egyptians' eyes (11:3 and 12:36)!
Moshav Zekenim: Granted, they had grace, but had they not seen the property, they would not have known what to ask for [and the Egyptians would not have offered it on their own volition]. 1
I heard that Hashem gave Yisrael grace in the Egyptians' eyes through the light. An Egyptian heard Yisraelim in his house talking about all the valuables there. When the plague ended, he saw that they did not take anything, and admired their integrity (PF).
Rashi (to 12:36): The Egyptians gave more than Yisrael requested. 'You say one - take two and leave!' (However, if they did not know that they had a certain kind of property, the Egyptian would not have offered it on his own (PF).
