degree of freedom and spectrophotometer

Degree of Freedom:

I think the easiest way to see them is as follows:

The degree of freedom is the number of values in a calculation that we can vary.

(http://www.statsdirect.com/help/basics/degrees_of_freedom.htm)
Another way of thinking about the restriction principle behind degrees of freedom is to imagine contingencies. For example, imagine you have four numbers (a, b, c and d) that must add up to a total of m; you are free to choose the first three numbers at random, but the fourth must be chosen so that it makes the total equal to m - thus your degree of freedom is three.

(http://www.tufts.edu/~gdallal/dof.htm)
A single sample: There are n observations. There's one parameter (the mean) that needs to be estimated. That leaves n-1 degrees of freedom for estimating variability.
Two samples: There are n₁+n₂ observations. There are two means to be estimated. That leaves n₁+n₂-2 degrees of freedom for estimating variability.

Spectrophotometer:

Spectrophotometer can be used to measure absorbance or transmittance of light source with respect to the sample.

http://www.chm.davidson.edu/vce/spectrophotometry/Spectrophotometry.html

First, the intensity of light (I₀) passing through a blank is measured. The intensity is the number of photons per second. The blank is a solution that is identical to the sample solution except that the blank does not contain the solute that absorbs light. 

Second, the intensity of light (I) passing through the sample solution is measured. (In practice, instruments measure the power rather than the intensity of the light. The power is the energy per second, which is the product of the intensity (photons per second) and the energy per photon.)

Third, the experimental data is used to calculate two quantities: the transmittance (T) and the absorbance (A).

The transmittance is simply the fraction of light in the original beam that passes through the sample and reaches the detector. The remainder of the light, 1 - T, is the fraction of the light absorbed by the sample.

In most applications, one wishes to relate the amount of light absorbed to the concentration of the absorbing molecule. It turns out that the absorbance rather than the transmittance is most useful for this purpose. If no light is absorbed, the absorbance is zero (100% transmittance). Each unit in absorbance corresponds with an order of magnitude in the fraction of light transmitted. For A = 1, 10% of the light is transmitted (T = 0.10) and 90% is absorbed by the sample. For A = 2, 1% of the light is transmitted and 99% is absorbed. For A = 3, 0.1% of the light is transmitted and 99.9% is absorbed.

When we did our practical on abs, with the coloured solution, the darker the solution, more light will be absorbed and thus there will be greater absorbance/less transmittance.