Please explain the theory also. I need to understand the concept. Thank you in advance. A light of wavelength 620 nm is emitted from 5 Schwarzschild radii from a 20 solar mass black hole. What wavelength is observed for this light by an observer a long distance away? (The objects are not moving with respect to the observer)

Please explain the theory also. I need to understand the concept. Thank you in advance. A light of wavelength 620 nm is emitted from 5 Schwarzschild radii from a 20 solar mass black hole. What wavelength is observed for this light by an observer a long distance away? (The objects are not moving with respect to the observer)