Exodus

The painting depicting the Israelites crossing on dry land through the Red Sea bed is by Hiraku Khan. Crossing the Red Sea

There were at least 10 plagues not 7.

See:
 Plagues of Egypt
 https://en.wikipedia.org/wiki/Pl...

Water into blood ( ): Ex. 7:14-24
Frogs: Ex. 7:25-8:15
Lice: Ex. 8:16-19
 Mixture of Wild Animals or Flies-: Ex. 8:20-32
Diseased livestock -: Ex. 9:1-7
Boils: Ex. 9:8-12
Thunderstorm of hail: Ex. 9:13-35
Locusts: Ex. 10:1-20
Darkness for three days : Ex. 10:21-29
Death of firstborn: Ex. 11:1-12:36

Towards the end of these plagues the Egyptians described Egypt as in ruins (Exodus 10:7). The Israelites left and a large number of other peoples described as a Mixed Multitude left with them (Exodus 12:38). Pharaoh got his men together and pursued the Israelites. The Hebrews went through the Red sea which had split in two for them. The Egyptians followed after them. The Israelites had safely passed through the Sea on dry land. The Sea then returned to its former state, catching the Egyptians who were still travelling over the dry bed. Consequently the pursuing Egyptians were drowned (Exodus ch. 15).
One opinion says that all, or nearly all, the original Egyptians had been wiped out. At all events Egypt was in shambles and took a while to recover from the disasters that have overtaken them. We hear that God had hardened the heart of Pharoah and his servants so that they would be able to receive the plagues (Exodus 9:14, 16).
cf.

Exodus (NASB) 10:
 1 Then the LORD said to Moses, 'Go to Pharaoh, for I have hardened his heart and the heart of his servants, that I may perform these signs of Mine among them....'
7 Pharaoh's servants said to him, "How long will this man be a snare to us? Let the men go, that they may serve the LORD their God. Do you not realize that Egypt is destroyed?"

Exodus 12:
 37 Now the sons of Israel journeyed from Rameses to Succoth, about six hundred thousand men on foot, aside from children. 38 A mixed multitude also went up with them, along with flocks and herds, a very large number of livestock.

The Israelites experienced what had happened and they did believe because of that.

Exodus 14:
 31 When Israel saw the great power which the LORD had used against the Egyptians, the people feared the LORD, and they believed in the LORD and in His servant Moses.

Jethro did hear of the miracles and did come and convert (Exodus 18:1).

The question was, why did they [i.e. the Egyptians] not convert to Judaism after such a clear display of 'the reality of the Jewish deity'? If jethro could do it why could not they? If the Israelites beleived because of what had happeend why did the Egyptians not also do so?
Well, how do we know they did not? How many of them were left after all what happened? What condition were they in? What would they convert to? The Israelites had left and were wandering about the Wilderness. 
 Also people do not necessarily change, at least not all at once.
 In our own time the German people followed Hitler into World War 2. They murdered millions of Jews and millions of others and caused much suffering. Even after the war was already lost and there was no hope of anything else they kept on killing Jews.
 Why did they do that?
 I do not know, but they did.
So too with the Egyptians. Why the Egyptians reacted in one way and not another is a matter for speculation and philosophical consideration.
 The bottom line is that they were there. So were the Hebrews. Yehudah Ha-Levi, Maimonides, and others made the point that since millions of people were involved thus involving the whole nation of Israel then this in itself is testimony. 
This is a proof in its own right since it was not an experience of a single individual but of the whole people. It could not have been invented since those concerned would not have accepted it unless they knew about it.
 If there are those who do not wish to accept this then the onus of proof is on them. They have to explain why they deny it. They who believe do not have to explain anything since the testimony is on their side.