Example: Light shining on a tomato Imaging light from a normal lightbulb hitting a tomato. The emission spectrum of the lightbulb looks approximately like this Incidentally, the amber/reddish glow of a lightbulb is fully explained by this emission spectrum. An incandescent lightbulb emits most of its visible light intensity in the range of 550nm to 750nm which we perceive as yellow and red. Of course most of the total emitted intensity is of course heat (i.e. radiation with a wavelength in the infrared regime of 750nm and higher). The tomato is an object and like all light interacting objects it has a reflectance spectrum. It is a complex task to describe the chemical and biological processes involved in causing such an effective reflectance spectrum but roughly speaking the surface layer of the tomato is composed of matter that preferencially absorbs blue light and thus reflects red and green to a lesser degree (depending on the ripeness of the tomato I suppose. We can now convolute the two graphs to get the effective spectrum of the reflected light. Convolution is the mathematical act of 'multiplying' two graphs such that each value for a particular wavelength is the result of the multiplication of the Intensity value at that wavelength from the emission spectrum of the lightbulb and the reflectance value of that wavelength from the reflectance spectrum of the tomato. We can see from the result that the light coming from the tomato after the reflection will be predominantly red and quite intense. If we had cast a light from a blue LED on the same tomato then we would have received almost no light back. The blue LED emits almost only blue light (400nm to 500nm) and the reflectance of the tomato in that wavelength range is very low.