Sun/Sunlight 101 and Human Eyes 101

by Brian Mullin on January 30, 2010

I have been doing some testing and reviewing of sunglasses lately, and I thought I would pass along some research I did while investigating sunlight and the human eye.

Sun/Sunlight 101
Our Sun, is a yellow dwarf, and specifically a G2V star, which means it’s a spectral type G2 (based on color and surface temp) and luminosity class V (based on surface area).

Surface temp: 5800 ºK or 9980 ºF
Core temp: 15.6 million ºK or 28.1 million ºF
Mass: 4.38 x 10^30 lbs.
Diameter: 864,000 miles
Age: 4.57 billion years
Composition: 74% Hydrogen/24% Helium/2% other
Color: White (appears yellow due to refraction)

The Sun generates its energy by nuclear fusion of hydrogen nuclei into helium, with a net output of 383 yottawatts (10^24). The Sun lays approximately 93 million miles from us, and its light travels that distance in 8 minutes and 19 seconds, which is about my average speed while mountain biking. Sunlight is the total spectrum of the electromagnetic radiation given off by the Sun, and its wavelengths vary from 100 nanometers to 1 millimeter, and are divided into five regions:

Ultraviolet C or UVC: spans 100 to 280 nm
Greatly absorbed by the atmosphere, it has germicidal properties, and is more powerful than UVA and UVB, but they can be fatal to an organism.

Ultraviolet B or UVB: spans 280 to 315 nm
Mostly absorbed by the atmosphere, overexposure can cause sunburn and some forms of skin cancer, and they cause irritation to the cornea.

Ultraviolet A or UVA: spans 315 to 400 nm
Less damaging to the DNA, but are responsible for premature aging of the eye.

Visible light: spans 400 to 700 nm.
It is this range that is visible to the naked eye, and it represents 45% of Sunlight.
Bright and intense light, can lead to fatigue, temporary reduction of sight, and eye pain over prolonged exposure, and even with brief exposure the retina may be damaged and suffer vision impairment.

Infrared light: spans 700 nm to 1 mm
It is largely responsible for the warmth or heat that the sunlight carries, and it represents 48% of Sunlight. They heat the tissues, dry out tears reducing their protective qualities and can cause eye disorders.

Human Eyes 101
The human eye is a significant human sense organ. It allows humans conscious light perception, vision, which includes color differentiation and the perception of depth. The human eye has a 200° viewing angle and can see 10 million colors.

Eye Parts
Cornea: transparent protective envelope, 300 times more sensitive to sunlight than the skin.
Iris: colored membrane surrounding the pupil, determines eye color.
Pupil: central part of the eye varying according to brightness.
Crystalline lens: a transparent positive lens which adapts itself to object distance, reflects light.
Vitreous body: gelatinous liquid located between the crystalline lens and the retina.
Retina: a plate which is hypersensitive to light, transmits information to the optic nerve and to the brain.
Optic nerve: it transforms the images perceived by the retina into nerve impulses.

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