Why does shaving produce a greenish tinge on face/head of people with fair complexion?
S.M. Chidambaram, Tirunelveli, Tamil Nadu
Answer: The appearance of the freshly shaven skin has a few more features than the green tinge. A person with yellowish fair complexion and balk beard only exhibit the greenish tinge on the freshly shaven cheeks. On the other hand, the persons with dark complexion or with white beard do not show this tinge.
The particular combination of yellowish fair skin and black beard on it makes the appearance of the tinge.
The skin of such a person would appear yellowish because it reflects yellow light predominantly. However, at location of the shaven beard stub is highly polished like a tiny mirror, making it reflect all light to some extent. But, being black, the stub absorbs most of the light, preferentially the long wavelength light in the red-orange-yellow region. The green - blue - violet light is reflected by these spots of the freshly cut beard stubs. Thus the total reflected light received by the distant observer is a combination of the yellowish light from the skin and the bluish light from the stub spots. This combination produces the feeling of green light leading to the appearance of the greenish tinge. Such feeling is experienced when we look at a thin mixture of yellow and blue pigments like the "Holy colours".
The white hair stubs do not absorb any colour preferentially and so do not offer any specific tinge. Similarly those people who have a dark complexion, do not reflect yellow light from their skin, so do not offer any greenish tinge, normally offered by yellow skinned people with black beard.
Chennai Mathematical Institute, Chennai,
Homi Bhabha National Institute