Can we place giant reflectors in space to block enough of the Solar Energy from causing Global Warming?

Just like the Dyson Sphere collects energy from a star, can a simple program of reflectors hypothetically cool the Earth?

It seems possible that lightweight reflectors in orbit could be done.
Update: PS: I don't mean "reflectors" as in Newtonian/Gregorian etc, I mean something such as Mylar sheets with a reflective coating, a sort of giant Earth umbrella.
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