Sunlight and its Properties

Sunlight and its Properties

EE 495/695 Y. Baghzouz

The Sun

? The sun is a hot sphere of gas whose internal temperatures reach over 20 million deg. K.

? Nuclear fusion reaction at the sun's core converts hydrogen to helium.

? A nuclear fusion releases a tremendous amount of thermal energy according to Einstein's formula:

E = mC 2

? The radiation from the inner core is not visible since it is absorbed by a layer of hydrogen atoms closer to the sun's surface.

Plank's Black Body Formula

? The energy at the surface of the sun is estimated according to Plank's formula:

w

=

2h-5

hc

e kT -1

? w: Energy density (W/m2/unit wavelength in m) ? h: Plank's constant = 6.63 x 10-34 Ws2 ? c: speed of light = 3 x 108 m/s ? T: temperature of black body (K) ? K: Boltzmann's constant = 1.38 x 10-23 (J/K) ? At the surface of the sun, T 5,800 K, resulting in w = 5.961 x

107 W/m2.

Solar radiation in space

? The energy radiated on an object in space decreases as the object moves further away from the sun.

? The energy density w on an object some distance D from the sun is found by

wo

=

R D

2 w

where R = 6.955 x 108 m is the radius of the sun.

How much solar radiation doe the earth receive?

? By the time this energy travels 150,000,000 km to the earth's outer atmosphere, it is reduced to nearly 1,367 W/m2.

? By the time this energy travels through the earth's atmosphere to the earth's surface, it is reduced to nearly 1,000 W/m2

? In one hour, the earth receives enough energy from then sun to meet its energy needs for nearly one year.

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