Surfaces, Illumination and The Dress

Surfaces multiply wavelength by wavelength

The eye sees only the product

Two readings of one photograph

What the ambiguity teaches

A beige wall photographed at sunset looks orange in the picture. Why can the photographed spectrum not tell you whether the wall is white or the light is orange?

Under the warm-illumination assumption, why does the dress read as blue and black rather than white and gold?

An incoming light has constant intensity $\ell_{\text{in}}(\lambda) = 2$ at every wavelength, and a surface reflects the fraction $s(\lambda) = 0.5$ at $500$ nanometers. With $k = 1$, write the reflected intensity $r$ at that wavelength.