Hi guys. I have a question. See figure 2.1 in the PDF file, page 13. Basically we have a radiation sensor (sensitive only to infrared light) and we have a light bulb in front of it. We must make some measurement to see that the inverse square law holds and so we must measure the "intensity" (in reality it's a difference of potential that the sensor can measure but anyway that's the idea) of light at different distances from the light bulb. My question is: Why do they consider the distance between the sensor and the light bulb to be the distance between the sensor and the filament instead of the distance between the sensor and the glass bulb? I understand that the filament is probably over 1500K while the glass bulb might be over 400K which is much less... but still. Why not an intermediate distance between the glass bulb and the filament? Doesn't the glass bulb stops the infrared light from the filament and since the glass bulb is heated by the filament, it emits the infrared light? I know it's a totally different story with visible light (the glass bulb doesn't really stop the visible light of the filament.) but infrared light is still stopped by the glass bulb I think... What do you think?