where is ambient light sensor

It is under the front glass just to the left side (volume and mute button side) of the Earpiece Speaker hole, when looking at iPhone. The sensor hole is clearly visible on a White iPhone 5 or picture of one.

what brad described is the proximity sensor.

the ambient light sensor is invisible even on a white iphone 5.

the ambient light sensor is above the earpiece speaker, to the left of the front camera

source:

https://developer.apple.com/resources/cases/dimensions/iPhone-5-dimensions.pdf