path length
/ PATH length /
Looking through a thin pane of tinted glass barely tints what you see, but looking down the length of the same glass — edge on — makes it look deeply coloured. The difference is how far the light had to travel through the colour. That distance is the path length.
Path length is the distance the light beam travels through the absorbing sample, the term l (or b) in Beer-Lambert law A = ε c l. In ordinary spectrophotometry it is set by the internal width of the cuvette, almost always 1 centimetre, which is why that value is so often assumed.
Because absorbance is directly proportional to path length, you can use it as a sensitivity dial. A longer cell amplifies weak signals from dilute samples, while a very short cell tames samples that would otherwise be off-scale — handy when dilution is impractical.
Swapping a 1 cm cell for a 5 cm one multiplies the absorbance of the same dilute solution fivefold, lifting a faint reading of 0.04 up to a comfortable 0.20.
A longer path multiplies absorbance, boosting sensitivity for dilute samples.
If you switch cuvette sizes, the path length in your calculation must change with them; assuming the usual 1 cm when the cell is not 1 cm is a common silent error.