What is the minimum potential difference applied to an x-ray tube if x-rays of wavelength 0.250 nm are produced? E=hf f=c/wavelength f=3e8/.250e-9 =1.2e18 then i applied it to the first equation using plancks constant as h and my final answer came out to be 7.95e-16 KV. the answer is wrong. can you please help me out? thanks.