Explain :
(i) Three photo diodes `D_1, D_2, and D_3,` are made of semiconductor having band gaps of 2.5 eV, 2 eV and 3 eV respectively. Which one will be able to detect light of wavelength 6000 Å ?
(ii) Why an elemental semiconductor cannot be used to make visible LEDs?

Explanation

Use the relation `E=(hc)/lambda` and find wavelength `lambda`.