For LED's to emit light in the visible region of electromagnetic light, it should have an energy band gap in the range of:

The wavelength of electromagnetic radiation for it to lie in the visible region should lie in the range of 4000Å to 7000Å. Using E=hc/λ and substituting the value of λ into the equation, we get the value of E to lie in between 1.7 to 3 eV