Generally we have seen that UV-Vis Spectrophotometer scans from high to low wavelength. So it will scan first high wavelength and will come to low wavelength. For example, if we want to scan from 200 nm to 350 nm wavelength, then it will start scan from 350 nm and will come to 200 nm. Why it is designed like this? Let us understand it.
One sentence answer is: This mechanism minimize degradation of UV sensitive samples.
How?
When we set the wavelength, it will not immediately start scan without pressing start button to scan. First it will come to starting wavelength which you have set in method. Here this starting wavelength will be higher one. Higher wavelength has low energy and lower wavelength has more energy. Let's understand this by equation.
We all know that Energy, E = hv, where h = Planck constant and v = frequency.
Now frequency = c / λ ; where c = speed of light which is constant and λ = wavelength
So E = h c / λ; so if Wavelength is more, Energy is less and if Wavelength is less, Energy will be more.
Hope this will solve your query for the mechanism in which, wavelengths scanned from high range to low in UV-Vis Spectrophotometer.
