The sky appears blue due to Rayleigh scattering of sunlight in Earth's atmosphere. Shorter blue wavelengths of light scatter more than longer red wavelengths, so the sky looks blue. Clouds appear white because the water droplets that make up clouds are similar in size to light wavelengths, causing equal scattering of all visible light wavelengths. The skin may look blue in cyanosis due to increased deoxyhemoglobin, which absorbs more red light, making the reflected blue light appear more prominent.

1. DID
you ever wonder why the sky is
blue & cloud white ?

2. Basis for Blue color of the sky
• White light from the sunlight (which basically consists of 7
primary colors-VIBGYOR) before striking the earth’s surface
passes through earth’s atmosphere (which mostly consists of
atmospheric gases like N2, O2 that are smaller than the
wavelength of visible light ).
• When sunlight passes through particles smaller than the
wavelength of light, scattering of light occurs which is dependent
on the wavelength of light (Rayleigh scattering).
• And the intensity of scattered light is inversely proportional to the
4th power of wavelength of light. i.e
I α 1/λ4
• So, blue light with lesser wavelength scatters more, giving sky
blue color.
• But wait, isn’t violet the one with smallest wavelength &
shouldn’t the sky appear violet ? Well, the answer for that is :
Human eyes are more sensitive to blue color than violet, so we see
sky blue rather than violet.

3. Basis for white color of the clouds
• Besides atmospheric gases like N2,O2, our atmosphere also
contains particles of size similar or larger than the wavelength of
light (like dust particles, water droplets).
• But what happens when the sunlight passes through this particles?
• Well, when sunlight passes through particles of size similar or
larger than the wavelength of light, then scattering of light occurs
equally in all direction (Mie scattering)
• So, white light entering through clouds (water droplets) continues
to remain white, that’s why clouds are perceived as white to eyes.

4. Similar is the mechanism for bluish discoloration
of skin & mucous membrane in CYANOSIS.
1. Dermal collagen fibers play a major role in scattering of light
when light strikes human skin.
• Since the size of collagen fibrils are smaller than the
wavelength of visible light, Rayleigh scattering of light
occurs. i.e. blue light due to smaller wavelength scatters more,
giving blue skin color.
• Well you might think then, why is the skin not blue normally?
Its because other factors also play a role in skin color besides
scattering of light (pigments like melanin, oxyhemoglobin,
deoxyhemoglobin, carotene)
2. Another reason for blue skin color in cyanosis could be that :
• Deoxyhemoglobin is less red than oxyhemoglobin. So
deoxyhemoglobin absorbs more red spectrum of light, causing
blue spectrum to predominate in reflected light.
• Since cyanosis is most frequently due to increased
deoxyhemoglobin concentration in vasculature, this might be
one reason for blue skin color in cyanosis.

5. References
• https://www.ncbi.nlm.nih.gov/books/NBK367/
• https://content.sciendo.com/view/journals/sjdv/2/
4/article-p131.xml?lang=en