Sunlight and its Properties

[Pages:48]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.

Solar radiation intensity of other planets

Planet

Distance D (x 109 m)

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto

57 108 150 227 778 1426 2868 4497 5806

Solar Radiation wo(W/m2)

9228 2586 1367 586

50 15 4 2 1

Effect of Atmosphere on Sunlight

? As sunlight enters the earth's atmosphere, some of it gets absorbed, some is scattered and some passes through and reaches the earth's surface.

? Different molecules in the atmosphere absorb significant amounts of sunlight at certain wavelengths ? Water vapor and carbon dioxide absorb sunlight in the visible and infrared region ? Ozone absorbs sunlight in the UV region

Scattering of incident light

? Blue light has a wavelength similar to the size of particles in the atmosphere ? thus scattered.

? Red light has a wavelength that is larger than most particles ? thus unaffected

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