How can we produce a wavelenght of 400-700 nanometres?

Answer 1

400 - 700 nm is roughly the range of visible light. So any source that creates white light will do this (ie a filament bulb, a fluorescent tube).

To create a specific wavelength, you can pass white light through a prism to seperate the components, or you can use specific lamps, ie sodium for yellow, thallium for green, etc.

You could also use lasers to create light of very specific wavelengths. For instance, HeNe lasers have output wavelengths of 633 nm.

Answer 2

this is visble lighth.

Any lamp could serve this purpose