A point source of light moves in a strai...

A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as

`Iprop1/r`

`Iprop1/r^2`

`Iprop1/r^3`

`Iprop1/r^4`

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