Use an unambiguous diagram to illustrate how a beam of radiation is attenuated by an absorbing substance, and define the terms Pt and Po.

Imagine a light source emitting a beam of radiation. This beam, with an initial radiant power $P_o$, travels towards a sample cell (e.g., a cuvette containing a solution). As the beam passes through the absorbing substance within the sample cell, some of its energy is absorbed. The remaining radiation that exits the sample cell has a reduced radiant power, $P_t$. The diagram would show:

• An arrow representing the incident beam, labeled $P_o$, pointing towards a rectangular block (the absorbing substance/sample cell).
• The rectangular block representing the absorbing substance.
• An arrow representing the transmitted beam, labeled $P_t$, exiting the other side of the rectangular block.
• The length of the $P_t$ arrow would be shorter than the $P_o$ arrow to visually represent attenuation.

• $P_o$: The incident radiant power (or intensity) of the monochromatic beam of radiation before it enters and interacts with the absorbing substance. It represents the initial energy per unit time of the light beam.
• $P_t$: The transmitted radiant power (or intensity) of the monochromatic beam of radiation after it has passed through the absorbing substance. It represents the energy per unit time of the light beam that was not absorbed by the substance.