Why does it make a difference how far away the light sensor is from the object at which it is pointing?

I can think of two reasons:

1).  Air scatters and absorbs some light.  So the farther the sensor is
from the source, the more light is lost going through air, and the less
accurate the sensor will be.

2).  Light SPREADS out as it travels away from any source. 
Even from a LASER ... the spreading is very small, but it's there. 
This means that the farther the sensor is from the source, the
LESS light will enter the sensor, because the rest of the light will miss it.
More spreading out  ===>  more of the light misses the sensor.

(That's why, if the lamp is too dim to read what's on the paper, you
move the paper closer to the lamp, where the light is less spread out.
Then, there's more light hitting each square inch of the paper.)

Is this a question of an answer

Iassume the answer is 75 feet. inwards to outwards rather than outwards to inwards. probably be 75.hachures are stokes drawn in direction of the steepest slope. a series of short parallel lines drawn on a map to indicate topographic relief. it is actually called hachures on contour lines. the small marks in the center of the feature are called hachures.observe the picture attached