As an object approaches the speed of light, the relativistic Doppler effect becomes significant, causing a shift in the observed frequency of light. However, the Doppler effect alone does not directly result in perceiving incoming light as ionizing radiation.
The Doppler effect describes the change in frequency and wavelength of light as observed by an observer in relative motion to the source. When an object is moving towards an observer, the observed frequency is higher, resulting in a "blue shift." Conversely, when an object is moving away from an observer, the observed frequency is lower, resulting in a "red shift."
In the case of light, as an object approaches the speed of light, the observed frequency can indeed be significantly shifted towards higher frequencies due to the relativistic Doppler effect. However, this does not cause non-ionizing light (such as visible light) to become ionizing radiation (such as ultraviolet, X-rays, or gamma rays).
Ionizing radiation possesses higher frequencies and energies than non-ionizing radiation, and it has the ability to ionize atoms or molecules by removing electrons from them. Visible light does not have sufficient energy to cause ionization, even with significant blue shifting due to relativistic effects.
If an object were moving at extremely high speeds, such as a substantial fraction of the speed of light, the Doppler effect would shift the observed frequency towards higher values. However, the incoming light would still fall within the same non-ionizing range of the electromagnetic spectrum, such as visible, infrared, or radio waves.
To perceive incoming light as ionizing radiation, you would need to observe light with frequencies higher than ultraviolet, such as X-rays or gamma rays. These frequencies are well beyond the range of visible light, and their perception as ionizing radiation is not a result of the relativistic Doppler effect alone but rather the inherent properties of the light itself.