To resolve details of a cell using an ordinary microscope, you must use a wavelength that is about the same size, or smaller, than the details of the cell you want to observe. Suppose you want to be able to see the ribosomes, which are about 20 nm in diameter. 

(a) To use an ultraviolet microscope, what minimum photon energy is required? (As a practical matter, lenses that are effective at such short wavelengths are not available.) 

(b) If you use an electron microscope, what is the minimum kinetic energy of the electrons? 

(c) Through what potential difference should the electrons be accelerated to reach this energy?