Question

Can the scientists use magnified UV rays of sun which can produce lot heat energy that can used in nuclear fusion

Answer 1

Yes, you can focus the UV rays down, but you are limited by the sheer size of the optics needed. It simply isn't possible to build lenses and mirrors large enough. Plus I think you may be limited by some aspects of how optics works. For example, a magnifying glass doesn't focus ALL the light from the sun down to a single spot. It actually forms a very small image of the Sun on the ground. Larger lenses that have a longer focal length actually form a bigger image than a magnifying glass does, so you are limited to how much light you can actually focus onto an area, which is far far too little to do what you are suggesting.

Hope this helps:)