What is the best interpretation of $f(11)=300$?

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The function

f (x) = 600 (0.5)^{\frac{x}{11}}

gives the predicted intensity of a beam

f (x)

, in number of photons in the beam, x millimeters below the surface of a certain material.

What is the best interpretation of

f (11) = 300

A beam at the surface of the material has a predicted intensity of 11 photons in the beam.

A beam at the surface of the material has a predicted intensity of 300 photons in the beam.

A beam 11 millimeters below the surface of the material has a predicted intensity of 300 photons in the beam.

A beam 300 millimeters below the surface of the material has a predicted intensity of 600 photons in the beam.