THE SUN LIGHT
The sun light
Nature of light and quantum theory
The early theories describing the atomic structure are based
on classical physics. However these theories could not explain the behavior of
atom completely. The modern view of atomic structure is based on quantum theory
introduced by Max Planck.
Before learning the quantum theory, it is necessary to
understand the nature of light.
Light
Light is considered as an electromagnetic radiation. It
consists of two components i.e., the electric component and the magnetic
component which oscillate perpendicular to each other as well as to the
direction of path of radiation.
Picture of electromagnetic radiation
The electromagnetic radiations are produced by the
vibrations of a charged particle. The properties of light can be explained by
considering it as either wave or particle as follows.
Wave nature of light
According to the wave theory proposed by Christiaan Huygens,
light is considered to be emitted as a series of waves in all directions. The
following properties can be defined for light by considering the wave
nature.
Wavelength (λ): The distance
between two successive similar points on a wave is called as wavelength. It is
denoted by λ.
Units: cm, Angstroms (Ao), nano meters (nm),
milli microns (mµ) etc.,
Note:
1 Ao =
10-8 cm.
1 nm= 10-9m = 10-7cm
Frequency (ν): The number of
vibrations done by a particle in unit time is called frequency. It is denoted
by 'ν'.
Units: cycles per second = Hertz = sec-1.
Velocity (c): Velocity is
defined as the distance covered by the wave in unit time. It is denoted by
'c'.
Velocity of light = c = 3.0 x 108 m.sec-1 = 3.0 x 1010 cm.sec-1
Note: For all types of electromagnetic radiations, the
velocity is a constant value. The relation between velocity (c), wavelength (λ)
and frequency (ν) can be given by following equation.
velocity = frequency x wavelength
c = νλ
Wave number(ϋ): The number of waves spread in a length
of one centimeter is called wave number. It is denoted by ϋ. It is the
reciprocal of wavelength, λ.
ϋ = 1/λ
units: cm-1, m-1
Amplitude: The distance
from the midline to the peak or the trough is called amplitude of the wave. It
is usually denoted by 'A' (a variable). Amplitude is a measure of the intensity
or brightness of light radiation.
Particle nature of light
Though most of the properties of light can be understood by
considering it as a wave, some of the properties of light can only be explained
by using particle (corpuscular) nature of it. Newton
Planck’s quantum theory
Black body: The object
which absorbs and emits the radiation of energy completely is called a black
body. Practically it is not possible to construct a perfect black body. But a
hollow metallic sphere coated inside with platinum black with a small aperture
in its wall can act as a near black body. When the black body is heated to high
temperatures, it emits radiations of different wavelengths.
The following curves are obtained when the intensity of
radiations are plotted against the wavelengths, at different
temperatures.
Black body diagram
Following are the conclusions that can be drawn from above
graphs.
1) At a given temperature, the intensity of radiation
increases with wavelength and reaches a maximum value and then starts
decreasing.
2) With increase in temperature, the wavelength of maximum
intensity (λmax) shifts towards lower wavelengths. According to
classical physics, energy should be emitted continuously and the intensity
should increase with increase in temperature. The curves should be as shown by
dotted line.
In order to explain above experimental observations Max
Planck proposed the following theory.
Quantum theory:
1) Energy is emitted due to vibrations of charged particles
in the black body.
2) The radiation of energy is emitted or absorbed
discontinuously in the form of small discrete energy packets called quanta.
3) Each quantum is associated with definite amount of
energy which is given by the equation E=hν.
Where
h = planck's constant = 6.625 x 10-34 J. sec = 6.625 x10-27 erg. sec
ν= frequency of radiation
4) The total energy of radiation is quantized i.e., the
total energy is an integral multiple of hν. It can only have the values of 1 hν
or 2 hν or 3 hν. It cannot be the fractional multiple of hν.
5) Energy is emitted and absorbed in the form of quanta but
propagated in the form of waves.
Einstein’s generalization of quantum theory
Einstein generalized the quantum theory by applying it to
all types of electromagnetic radiations. He explained photoelectric effect
using this theory.
Photoelectric Effect: The ejection of electrons from the surface of a metal, when
the metal is exposed to light of certain minimum frequency, is called photoelectric
effect.
The frequency of light should be equal or greater than a
certain minimum value characteristic of the metal. This is called threshold
frequency, νo.
The photoelectric effect cannot be explained by considering
the light as wave. Einstein explained photoelectric effect by applying quantum
theory as follows:
1. All electromagnetic radiations consists of small discrete
energy packets called photons.
These photons are associated with definite amount of energy given by the
equation E=hν.
2. Energy is emitted, absorbed as well as propagated in the
form of photons only.
3. The electron is ejected from the metal, only when a
photon of sufficient energy strikes the electron. When a photon strikes the
electron, some part of the energy of photon is used to free the electron from
the attractive forces in the metal atom and the remaining part is converted
into kinetic energy.
hν = W + K.E
Where
W = energy required to overcome the attractions
K.E = kinetic energy of the electron
Since the frequency corresponding to the minimum energy
required to overcome the attraction is called threshold frequency, νo,
the above equation can be written as:
hν = hνo +
K.E or K.E = hνo- hν = h (νo- ν)
The final step to our current understanding of the nature of
light came in the early 1900's when Max Plank developed quantum theory, which
proposed extremely small units of matter called photons which contain small units of energy measured in quanta. Albert Einstein
expanded on this theory, as did R.A. Millikin.
The color ranges of visible light
The result of centuries of theories and proofs and arguments
is that light is a portion of the electro-magnetic spectrum. Sometimes its
nature can best be described using small packets of energy/matter called
photons, and at other times its nature can best be described using wave models.
We say that visible light has a DUALISTIC nature.
The kinds of light
Light is part of the electromagnetic
spectrum, the spectrum is the
collection of all waves, which include visible light, Microwaves, radio
waves (AM, FM, SW), X-Rays, and Gamma Rays.
Cosmic rays
Cosmic rays come from stars and interstellar space, and the
ones that bombard earth are trapped in the magnetic field around earth. Their
interaction with the atmosphere produces the aora borealis or northern lights.
They may also be referred to as ultragamma rays. Ultra-
means 'beyond', so these are 'beyond gamma rays'. One more item about cosmic
rays for now, and we will come back to them later. The little packets of energy
called photons, if they are in the cosmic ray range of the electromagnetic
spectrum, have more energy than any of the other elements of the spectrum.
Cosmic rays
contain:
X-RAYS: They travel through things that are not transparent to
visible light. Based on the comment under cosmic rays, they act more like
particles, less like waves. They can do damage to biological tissues.They come
in three types: Alpha rays, beta rays, and gamma rays. In a diagram of the electromagnetic spectrum, the
gamma rays are to the left, the alpha rays are to the right. That is why cosmic
rays are 'ultragamma' instead of 'ultraalpha'.
The gamma rays with three times faster than the alpha rays
and they are the most damaging and the highest energy of the X-rays; the alpha
rays are the least damaging and the least energetic of the X-rays.
ULTRAVIOLET or
UV: Ultra- means 'beyond', so these rays
are beyond violet.
UV is still more into particles than waves. But waves are
gaining. They have less energy than X-rays. And in small doses they don't do
much. But if you have been reading much, or have been in the optical business
long, you
know that the build-up of UV exposure over time leads to damage (or at least
accelerated ageing) of biological tissues, i.e., your skin and eyes.
VISIBLE LIGHT: The visible
portion of the electromagnetic spectrum is generally considered to be from
380/400nm to about 760nm, give or take a bit. How does this segment of the
spectrum differ from UV or IR? It differs only in that there are some
specialized cells in the retinas of our eyes that contain a pigment that gets
excited when it encounters waves of these wavelengths. Cats have a pigment that
gets excited at shorter wavelengths, so they see a little bit into the UV-A
range, and therefore the 'visible range' for a cat is different from the
'visible range' for a human. In fact, very young children can see 380nm, where
elderly people cannot see 400nm; so the 'visible range' changes even for
humans.
Within the visible range we have different colors, because
we have three different types of cells in the retina that get excited by three
different RANGES of wavelength.
The following picture is a listing of major color names and
the APPROXIMATE wavelength that these colors represent. The colors are ranges,
and not just one wavelength. Do not memorize color names and wavelengths.
Rather, memorize the order of the colors and the relative position in the
spectrum.
We use the acronym ROY G. BIV to help remember the order of
the colors from long wavelength (around 700nm) to short wavelength (around
400nm).
At this area of the electromagnetic spectrum, sometimes the
waves act like waves, and sometimes like particles. In the way they travel they
are more wavelike, in the way they interact chemically and electrically they
have more of a photon nature.
INFRA RED or IR: So the left end of above bar is the 'high energy' end.
INFRA RED or IR: So the left end of above bar is the 'high energy' end.
IR-A is the short-wave IR; the -A is the end close to
visible for both IR and UV.
IR-A ranges from about 800nm to about 1500nm, and it is
absorbed by the aqueous, lens, and vitreous of the eye. About 4% of the IR
exposure reaches the retina.
IR is converted to heat when it is absorbed. Our bodies are
great dissipaters of heat. The IR that reaches the retina and is converted to
heat is dissipated by the blood vessels at and near the retina. But if there is
an exposure that is so high in IR that the tissues cannot dissipate it fast
enough, we get a retinal burn. What happens if you have a quick intense
exposure to heat? You kill cells. In the skin, the result is pain, dead cells,
regrowth of cells, and eventually new skin cells. In the retina there are no
pain nerves, so when a person stares at the sun there is no pain. But there is
to much heat, and retinal cells die. Problem is, the retinal cells do not
regrow. Once the cell is dead, no other cell replaces it, and the person has a
permanent blind spot. This is what happens when people look directly at a solar
eclipse: because the moon is blocking the brightest of the sun's rays, the
sensation of discomfort that comes from staring at the sun is absent, and those
little IR rays can kill cells with no interference, and the person has a
permanent blind spot that is a ring where the intensity of the IR waves were
absorbed by the retina.
Longer wave IR, IR-B and IR-C, are partially absorbed by the
cornea and may cause corneal and conjunctival irritation.
One type of cataract, called the glassblowers cataract, is
caused by prolonged exposure to IR.
RADAR, MICROWAVE, RADIO, TV: Notice that we refer to all of them as waves. So most of the attributes of TV and radio transmission can be explained using wave theories. This is the low energy end of the spectrum.
RADAR, MICROWAVE, RADIO, TV: Notice that we refer to all of them as waves. So most of the attributes of TV and radio transmission can be explained using wave theories. This is the low energy end of the spectrum.
There are just two points we
must to remember:
First is that the shorter the wavelength, the higher the energy.
The fact is, the only
difference between a cosmic ray and a radio wave is the amount of energy in the
photon/wave!
Second is that the higher the energy and therefore the shorter the
wavelength, the more the particle/wave acts like a photon. The lower the energy
and therefore the longer the wavelength, the more the particle/wave acts like a
wave. And visible light is right in the middle of the transition between
particle and wave. No wonder the argument over whether light is a wave or a
particle continued for centuries and occupied the time of the greatest
philosophers and scientists in history!
The Sunlight
Introduction
The sunlight shining through the clouds
Sunlight, in the broad sense, is the
total frequency spectrum of electromagnetic radiation given
off by the Sun, particularly infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through the Earth's atmosphere, and solar
radiation is obvious as daylight when the Sun is above the horizon.When the
direct solar radiation is not blocked by clouds, it is experienced as sunshine,
a combination of bright light and radiant heat. When
it is blocked by the clouds or reflects off of other objects, it is experienced
as diffused light.The World Meteorological Organization uses the term "sunshine
duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts per
square meter. Sunlight may be recorded using a sunshine recorder, pyranometer or pyrheliometer.
Sunlight takes about 8.3 minutes to reach the Earth.
On average, it takes energy between
10,000 and 170,000 years to leave the sun's interior and then be emitted from
the surface as light.
Direct sunlight has a luminous efficacy
of about 93 lumens per watt of radiant flux.
Bright sunlight provides illuminance of approximately 100,000 lux or lumens per square meter at the Earth's
surface.
Sunlight is a key factor in photosynthesis, a
process vital for many living beings on Earth.
Composition
Solar irradiance
spectrum above atmosphere and at surface
The spectrum of the Sun's solar radiation is close to that of
a black body with a temperature of about
5,800 K. The Sun emits EM
radiation across most of theelectromagnetic spectrum. Although the Sun produces Gamma rays as a result of thenuclear fusion process, these super high energy photons are
converted to lower energy photons before they reach the Sun's surface and are
emitted out into space. As a result, the Sun doesn't give off any gamma rays.
The Sun does, however, emit X-rays, ultraviolet,visible light, infrared, and even radio waves. When
ultraviolet radiation is not absorbed by the atmosphere or other protective
coating, it can cause damage to the skin known assunburn or trigger an adaptive change in human skin pigmentation.
The spectrum of electromagnetic radiation striking
the Earth's atmosphere spans
a range of 100 nm to about 1 mm. This can be
divided into five regions in increasing order ofwavelengths:
Ultraviolet C or (UVC) range, which spans a range of
100 to 280 nm. The term ultraviolet refers to the fact that the radiation
is at higher frequency than violet light (and, hence also invisible to the human eye). Owing
to absorption by the atmosphere very little reaches the Earth's surface (Lithosphere). This
spectrum of radiation has germicidal properties, and is used in germicidal lamps.
Ultraviolet B or (UVB) range spans 280 to
315 nm. It is also greatly absorbed by the atmosphere, and along with UVC
is responsible for the photochemical reaction leading
to the production of the ozone layer.
Ultraviolet A or (UVA) spans 315 to 400 nm. It
has been traditionally held as less damaging to the DNA, and hence used intanning and PUVA therapy
for psoriasis.
Visible range or light spans 380 to 780 nm. As the name
suggests, it is this range that is visible to the naked eye.
Infrared range that
spans 700 nm to 106 nm
(1 mm). It is
responsible for an important part of the electromagnetic radiation that reaches
the Earth. It is also divided into three types on the basis of wavelength:
-Infrared-A: 700 nm to
1,400 nm
-Infrared-B: 1,400 nm to 3,000 nm
-Infrared-C: 3,000 nm to 1 mm
-Infrared-B: 1,400 nm to 3,000 nm
-Infrared-C: 3,000 nm to 1 mm
-Infrared-B: 1,400 nm to 3,000 nm
-Infrared-C: 3,000 nm to 1 mm
-Infrared-B: 1,400 nm to 3,000 nm
-Infrared-C: 3,000 nm to 1 mm
Calculation
To calculate the amount of sunlight
reaching the ground, both the elliptical orbit of the Earth and
the attenuation by
the Earth's atmosphere have
to be taken into account. The extraterrestrial solar illuminance (Eext),
corrected for the elliptical orbit by using the day number of the year (dn), is
given by:
where dn=1 on January 1; dn=2 on
January 2; dn=32 on February 1, etc. In this formula dn-3 is used, because in
modern times Earth's
perihelion, the closest approach to the Sun and
therefore the maximum Eext occurs around January 3 each year.
The value of 0.033412 is determined
knowing that the ratio between the perihelion (0.98328989 AU) squared and the
aphelion (1.01671033 AU) squared should be approximately 0.935338.
The solar illuminance constant (Esc),
is equal to 128×103 lux. The direct normal illuminance (Edn),
corrected for the attenuating effects of the atmosphere is given by:
Solar constant
The solar
constant, a measure of flux density, is
the amount of incoming solar electromagnetic radiation per
unit area that would be incident on a plane perpendicular to the rays, at a
distance of one astronomical unit (AU) (roughly the mean distance from the
Sun to the Earth). The "solar constant" includes all types of solar
radiation, not just the visible light. Its
average value was thought to be approximately 1.366 kW/m²,varying slightly
with solar activity, but
recent recalibrations of the relevant satellite observations indicate a value
closer to 1.361 kW/m² is more realistic.
Total (TSI) and spectral solar irradiance
(SSI) upon Earth
Total Solar Irradiance upon Earth (TSI)
was earlier measured by satellite to be roughly 1.366 kilowatts per square meter (kW/m²),
but most recently NASA cites TSI as "1361 W/m² as compared to ~1366 W/m²
from earlier observations [Kopp et al., 2005]", based on regular readings
from NASA's Solar Radiation and Climate Experiment(SORCE)
satellite, active since 2003,[11]noting
that this "discovery is critical in examining the energy budget of the
planet Earth and isolating the climate change due to human activities."
Furthermore the Spectral Irradiance Monitor (SIM) has found in the same period
that spectral solar irradiance (SSI) at UV (ultraviolet) wavelength
corresponds in a less clear, and probably more complicated fashion, with
earth's climate responses than earlier assumed, fueling broad avenues of new
research in "the connection of the Sun and stratosphere, troposphere,
biosphere, ocean, and Earth’s climate".
Intensity in the Solar System
Different bodies of the Solar System receive light of an intensity
inversely proportional to the square of their distance from Sun. A rough table
comparing the amount of solar radiation received by each planet in the Solar
System follows :
Perihelion - Aphelion
distance (AU) |
Solar
radiation
maximum and minimum (W/m²) |
|
0.3075 – 0.4667
|
14,446 – 6,272
|
|
0.7184 – 0.7282
|
2,647 – 2,576
|
|
0.9833 – 1.017
|
1,413 – 1,321
|
|
1.382 – 1.666
|
715 – 492
|
|
4.950 – 5.458
|
55.8 – 45.9
|
|
9.048 – 10.12
|
16.7 – 13.4
|
|
18.38 – 20.08
|
4.04 – 3.39
|
|
29.77 – 30.44
|
1.54 – 1.47
|
The actual brightness of sunlight that
would be observed at the surface depends also on the presence and composition
of anatmosphere. For
example Venus' thick atmosphere reflects
more than 60% of the solar light it receives. The actual illumination of the
surface is about 14,000 lux, comparable to that on Earth "in the daytime
with overcast clouds".
Sunlight on Mars would be more or less
like daylight on Earth wearing sunglasses, and as can be seen in the pictures
taken by the rovers, there is enough diffuse sky radiation that shadows would
not seem particularly dark. Thus it would give perceptions and "feel"
very much like Earth daylight.
For comparison purposes, sunlight on
Saturn is slightly brighter than Earth sunlight at the average sunset or
sunrise (see daylight for comparison table). Even on Pluto
the sunlight would still be bright enough to almost match the average living
room. To see sunlight as dim as full moonlight on the Earth, a distance of
about 500 AU (~69 light-hours) is
needed; there are only a handful of objects in the solar system known to orbit
farther than such a distance, among them 90377 Sedna and (87269) 2000 OO67.
Surface illumination
The spectrum of surface illumination
depends upon solar elevation due to atmospheric effects, with the blue spectral
component from atmospheric scatter dominating during twilight before and after
sunrise and sunset, respectively, and red dominating during sunrise and sunset.
These effects are apparent in natural light photography where the principal source of
illumination is sunlight as mediated by the atmosphere.
According to Craig Bohren,
"preferential absorption of sunlight by ozone over long horizon paths
gives the zenith sky its blueness when the sun is near the horizon".
See diffuse sky radiation for
more details.
Climate effects
On Earth, solar radiation is obvious as
daylight when the sun is above the horizon. This is
during daytime, and also in summer near the poles at night, but not at all in
winter near the poles. When the direct radiation is not blocked by clouds, it
is experienced assunshine, combining the perception of bright white light
(sunlight in the strict sense) and warming. The warming on the body, the ground
and other objects depends on the absorption (electromagnetic radiation) of the electromagnetic radiation in
the form of heat.
The amount of radiation intercepted by
a planetary body varies inversely with the square of the distance between the
star and the planet. The Earth's orbit and obliquity change with time (over thousands of
years), sometimes forming a nearly perfect circle, and at other times
stretching out to an orbital eccentricity of
5% (currently 1.67%). The total insolation remains almost constant due to Kepler's second law,
(2A/r2) dt=dθ
where A is the "areal velocity"
invariant. That is, the integration over the orbital period (also invariant) is
a constant.
If we assume the solar radiation power P as
a constant over time and the solar irradiation given by the inverse-square law, we obtain also the average insolation as a
constant.
But the seasonal and latitudinal distribution
and intensity of solar radiation received at the Earth's surface also varies. For
example, at latitudes of 65 degrees the change in solar energy in summer &
winter can vary by more than 25% as a result of the Earth's orbital variation.
Because changes in winter and summer tend to offset, the change in the annual
average insolation at any given location is near zero, but the redistribution
of energy between summer and winter does strongly affect the intensity of
seasonal cycles. Such changes associated with the redistribution of solar
energy are considered a likely cause for the coming and going of recent ice ages.
Effects on human health
The body produces vitamin D from sunlight (specifically from the
UVB band of ultraviolet light), and excessive seclusion from
the sun can lead to deficiency unless adequate amounts are obtained through
diet.
Sunburn can have mild to severe inflammation effects on skin; this can be avoided
by using a proper sunscreen cream or lotion or by gradually
building up melanocytes with increasing exposure. Another detrimental effect of
UV exposure is accelerated skin aging (also called skin photodamage),
which produces a difficult to treat cosmetic effect. Some people are concerned that ozone depletionis
increasing the incidence of such health hazards. A 10% decrease in ozone could
cause a 25% increase in skin cancer.
A lack of sunlight, on the other hand,
is considered one of the primary causes of seasonal affective disorder (SAD), a serious form of the
"winter blues". SAD occurrence is more prevalent in locations further
from the tropics, and most of the treatments (other than prescription drugs)
involve light therapy,
replicating sunlight via lamps tuned to specific (visible, not ultra-violet)
wavelengths of light or full-spectrum bulbs.
A recent study indicates that more
exposure to sunshine early in a person’s life relates to less risk from multiple sclerosis (MS)
later in life.
References
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