The phenomenon of redshift and blueshift is a result of the Doppler effect, which occurs when there is relative motion between a source of waves (in this case, light) and an observer. Redshift refers to the lengthening of wavelengths, resulting in a shift towards longer wavelengths (lower frequency), while blueshift refers to the shortening of wavelengths, resulting in a shift towards shorter wavelengths (higher frequency).
In the context of cosmology, the observed redshift of light from distant celestial objects is attributed to the expansion of the universe. As the universe expands, the space between galaxies or other light-emitting objects stretches, causing the wavelengths of light to stretch as well. This leads to a redshift, where light is shifted towards longer wavelengths.
The reason why we observe redshift more commonly in the universe, rather than violet shift, is due to the nature of the visible spectrum and the relative motion of objects.
The visible spectrum ranges from violet to red, with violet having the shortest wavelength and red having the longest. When objects move away from us in space, the expansion of the universe causes the space between us and the object to stretch, resulting in a redshift. The effect is more pronounced for objects that are farther away, as the stretching of space increases with distance.
Since the visible spectrum already spans from violet to red, when an object's light is redshifted, it gets shifted towards longer wavelengths and moves further into the infrared spectrum. However, there is no room for further shifting beyond red, as the visible spectrum ends there. Thus, we do not observe a violet shift beyond the visible spectrum because there are no wavelengths shorter than violet that we can detect within the visible range.
It's important to note that outside the visible spectrum, there are phenomena where ultraviolet (UV) light can be redshifted into the visible range, but this occurs at shorter wavelengths than violet. However, such observations typically require specialized instruments and are not directly visible to the human eye.
In summary, the absence of a violet shift beyond the visible spectrum is primarily due to the limited range of the visible spectrum itself and the fact that the universe's expansion predominantly causes redshift rather than a shift towards shorter wavelengths.