This study investigates how the acceleration and speed of a light-emitting object on an inclined plane may be determined using a smartphone's light sensor. A light-emitting object was released from the top of an inclined plane and its illuminance values were detected by a smartphone's light sensor during its subsequent motion down the plane. Using the illuminance and time values recorded by the smartphone, the distances between the sensor and the light-emitting object can be calculated. From these known distance and time values, the uniform acceleration and speed of the light-emitting object at any time could also be determined. A second method was also used to determine these values, i.e. the analysis of video-recordings of the light-emitting object during its motion. The values found by these two different methods were in good agreement. Thus, this experiment demonstrates that a smartphone's light sensor can be used as a reliable instrument to determine the constant acceleration and speed of light-emitting objects.