The optimum wavelength is 450 nm because that is the wavelength of maximum absorbance by FeSCN2+(aq) 
you should choose a wavelength with maximum absorbance. In this case, you are using the scattered light, not the absorbed light as your signal. So you should avoid wavelengths where there are absorption peaks.
<h3>What is wavelength ?</h3>
A waveform signal that is carried in space or down a wire has a wavelength, which is the separation between two identical places (adjacent crests) in the consecutive cycles. This length is typically defined in wireless systems in metres (m), centimetres (cm), or millimetres (mm) (mm).
- The distance between two waves' crests serves as an illustration of wavelength. When you and another person have the same overall mindset and can easily communicate, that is an example of being on the same wavelength.
Learn more about Wavelength here:
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Empirical evidence is information acquired by observation or experimentation. This data is recorded and analyzed by scientists and is a central process as part of the scientific method.
As example of not empirical evidence:
Language, letters, words, truths, numbers, logic, mathematics, on and on.
All of these things do not have empirical existence. They exist only in the mind.
        
             
        
        
        
Answer:
Do = density of obect      Dw = density of water
Weight of object = V g Do
Weight of water = V g Dw
F = m a = V Do a = V g Do - V g Dw
Acceleration of object = mass * force on object
Do (g - a) = Dw g
Do = Dw ( 1 / (1 - a / g) = 1000 * 1 / (1 - 3.75 / 9.80)  kg/m^3
Do = 1441 kg / m^3
If no acceleration then density of object = density of water