Wavelength and frequency are related by speed of light (in vacuum) as:
`lambdanu = c`
i.e., product of wavelength and frequency is equal to speed of light in vacuum.
Using the given value of wavelength (4.257 x10^-7 m) and known value of speed of light (3 x 10^8 m/s), we can calculate the frequency as:
`nu = c/lambda = (3xx10^8)/(4.257xx10^-7)`
`= 7.047 xx 10^14 hz`
hope this helps.