Final answer:
A feather appearing green without green pigment can be explained by diffraction, where microscopic structures in the feather cause constructive interference of light waves, making the feather appear green. This is due to the feather acting as a diffraction grating that causes green light to be diffracted and observed by the viewer.
Step-by-step explanation:
If a feather appears green but contains no green pigment, the explanation for this phenomenon involves a physical concept known as diffraction. Diffraction relates to how waves, including light waves, can interfere with each other when they encounter obstacles or slits that are comparable in size to their wavelength. In the case of the feather, microscopic structures within the feather can act as a diffraction grating, a surface with a series of closely spaced lines or grooves. These structures cause different wavelengths of light to interfere constructively or destructively.
Options a) and b) are incorrect because green light isn't absorbed nor refracted to cause the observed color. Option c) mentions scattering, which isn't responsible for the sharp, specific coloring that diffraction can create. The correct answer is d) Green light is diffracted by the feather. This happens when light encounters the microscopic structures, which then causes different wavelengths of light to constructively interfere at different angles. The angles at which this constructive interference occurs for green light is such that it is prominently reflected back to the observer, making the feather appear green despite the lack of green pigment.
Natural diffraction gratings within the feathers, similar to those in the scales of butterfly wings or the Australian opal, produce what's known as iridescence, a phenomenon where multiple colors may be seen from different viewing angles due to the interference effects of light waves being diffracted.