WHAT
IS LIGHT???
Light is electromagnetic radiation within a certain
portion of the electromagnetic spectrum.
The word usually refers to visible
light, which is visible to the human
eye and
is responsible for the sense of sight.[1] Visible light is
usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m,
to 700 nanometres – between the infrared (with longer
wavelengths) and the ultraviolet (with shorter
wavelengths). Often, infrared and ultraviolet are also called light.
PRIMARY PROPERTIES
Primary
properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per
second, is one of the fundamental constants of nature. Visible light, as with all
types of electromagnetic radiation (EMR), is
experimentally found to always move at this speed in vacuum.
NOTE:-
Lene Vestergaard Hau
she (born in Vejle, Denmark, on November 13, 1959) is a Danish physicist. In 1999, she led a Harvard University team who, by use of a Bose-Einstein condensate, succeeded in slowing a beam of light to about 17 metres per second, and, in 2001, was able to stop a beam completely.[1] Later work based on these experiments led to the
transfer of light to matter, then from matter back into light,[2] a process with important implications for quantum encryption and quantum computing
·
In 1999 the Danish physicist Lene Hau
managed to slow down the fastest thing we know: light.Since then she has
continued her work with light at Harvard University.
·
In
her latest experiment, she not only stopped the light but also moved it around,
manipulating it for half a minute, before she made it reappear.
”We can hold
on to the light, move it around or even save it for later. We can actually
manipulate it, “ says Hau” in an interview with ScienceNordic after her talk at the annual Hans
Christian Andersen (HCA) lecture at The University of Southern Denmark.
The speed of light takes on new
meaning
It may sound incredible that light,
which usually moves at 300,000 kilometers per second, can be stopped and packed
away for a rainy day.
After many years of research and
experiments with lasers, cooled atoms, and other instruments and techniques it
has become possible to control light.
In the laboratory Hau used
Bose–Einstein condensation. This means cooling down atoms in a gasseous state
to extremely low temperatures near the absolute zero at minus 273.15 degrees.
"The temperature is down to
about a billionth of one degree above the absolute zero," says Hau.
The light is compressed
With a coupling laser Hau shone a
beam of light -- a light wave -- through the Bose-Einstein condensate. The cold
environment of the codenstate does not only slow down the light, but also
compresses it.
Coupling laser light is special
because it couples together two energy levels of the molecule to make a
superposition.
Inside the condestate the light from
the coupling laser is compressed from being one kilometer long to only 0.02
millimeters.
“Once the lightwave is inside the
Bose-Einstein condensate, we turn off the laser. Though the light is gone, it
leaves a distinct imprint behind in the atom cloud,” says Hau.
This process creates a sort of cast of light in the actual matter -- an
imprint.
Using such an imprint, Hau has shown that it is possible to save the
light wave and even move it around for up to half a minute. This also means
moving it out of the condenstate where is was created.
“We can slow it [the light] down,
stop it, and move it around, and then create a copy of it in a new place,”
explains Hau
.
WHAT ARE NORTHERN LIGHTS?
Auroral displays appear in many colours although pale green and pink are the most common. Shades of red, yellow, green, blue, and violet have been reported.
WHAT CAUSES THE NORTHERN LIGHTS?
The Northern Lights are actually
the result of collisions between gaseous particles in the Earth's atmosphere
with charged particles released from the sun's atmosphere. Variations in colour
are due to the type of gas particles that are colliding. The most common
auroral color, a pale yellowish-green, is produced by oxygen molecules located
about 60 miles above the earth. Rare, all-red auroras are produced by
high-altitude oxygen, at heights of up to 200 miles. Nitrogen produces blue or
purplish-red aurora.
The temperature above the
surface of the sun is millions of degrees Celsius. At this temperature,
collisions between gas molecules are frequent and explosive. Free electrons and
protons are thrown from the sun's atmosphere by the rotation of the sun and
escape through holes in the magnetic field. Blown towards the earth by the
solar wind, the charged particles are largely deflected by the earth's magnetic
field. However, the earth's magnetic field is weaker at either pole and
therefore some particles enter the earth's atmosphere and collide with gas
particles. These collisions emit light that we perceive as the dancing lights
of the north (and the south).
The lights of the Aurora generally extend from 80 kilometres (50 miles) to as high as 640 kilometres (400 miles) above the earth's surface.
WHERE IS THE BEST PLACE TO WATCH THE NORTHERN LIGHTS?
Northern Lights can be seen in
the northern or southern hemisphere, in an irregularly shaped oval centred over
each magnetic pole. The lights are known as 'Aurora borealis' in the north and
'Aurora australis' in the south. Scientists have learned that in most instances
northern and southern auroras are mirror-like images that occur at the same
time, with similar shapes and colors.
Because the phenomena occurs near the magnetic poles, northern lights have been seen as far south as New Orleans in the western hemisphere, while similar locations in the east never experience the mysterious lights. However the best places to watch the lights (in North America) are in the northwestern parts of Canada, particularly the Yukon, Nunavut, Northwest Territories and Alaska. Auroral displays can also be seen over the southern tip of Greenland and Iceland, the northern coast of Norway and over the coastal waters north of Siberia. Southern auroras are not often seen as they are concentrated in a ring around Antarctica and the southern Indian Ocean.
Areas that are not subject to 'light pollution' are the best places to watch for the lights. Areas in the north, in smaller communities, tend to be best.
WHEN IS THE BEST TIME TO WATCH FOR AURORAL
DISPLAYS?
Researchers have also discovered that auroral activity is cyclic,
peaking roughly every 11 years. The next peak period is 2013.
Winter in the north is generally a good season to view lights. The long periods
of darkness and the frequency of clear nights provide many good opportunities
to watch the auroral displays. Usually the best time of night (on clear nights)
to watch for auroral displays is local midnight (adjust for differences caused
by daylight savings time).
LEGENDS OF THE LIGHTS
'Aurora borealis', the lights of
the northern hemisphere, means 'dawn of the north'. 'Aurora australis' means
'dawn of the south'. In Roman myths, Aurora was the goddess of the dawn. \par
Many cultural groups have legends about the lights. In medieval times, the
occurrences of auroral displays were seen as harbingers of war or famine. The
Maori of New Zealand shared a belief with many northern people of Europe and
North America that the lights were reflections from torches or campfires.
Visual forms and colors
·
Red: At the highest altitudes,
excited atomic oxygen emits at 630.0 nm (red); low concentration of atoms
and lower sensitivity of eyes at this wavelength make this color visible only
under more intense solar activity. The low amount of oxygen atoms and their
gradually diminishing concentration is responsible for the faint appearance of
the top parts of the "curtains". Scarlet, crimson, and carmine are
the most often-seen hues of red for the aurorae.
·
Green: At lower altitudes the
more frequent collisions suppress the 630.0 nm(red) mode: rather the
557.7 nm emission (green) dominates. Fairly high concentration of atomic
oxygen and higher eye sensitivity in green make green auroras the most common.
The excited molecular nitrogen (atomic nitrogen being rare due to high
stability of the N2molecule) plays its role here as well, as it can
transfer energy by collision to an oxygen atom, which then radiates it away at
the green wavelength. (Red and green can also mix together to produce pink or
yellow hues.) The rapid decrease of concentration of atomic oxygen below about
100 km is responsible for the abrupt-looking end of the lower edges of the
curtains.
·
Yellow and pink are a mix of red
and green or blue. Other shades of red as well as orange may be seen on rare
occasions; yellow-green is moderately common. As red, green, and blue are the
primary colours of additive synthesis of colours, in theory practically any
colour might be possible but the ones mentioned in this article comprise a
virtually exhaustive list.
·
Blue: At
yet lower altitudes, atomic oxygen is uncommon, and ionized molecular nitrogen
takes over in producing visible light emission; it radiates at a large number
of wavelengths in both red and blue parts of the spectrum, with 428 nm
(blue) being dominant. Blue and purple emissions, typically at the lower edges
of the "curtains", show up at the highest levels of solar activity.
·
Ultraviolet:
Ultraviolet light from aurorae (within the optical window but not visible to
virtually all humans) has been observed with the requisite equipment, and
otherwise invisible aurorae of this type were produced on a very small scale by
certain HAARP experiments. Ultraviolet aurorae have also been seen on
Mars.
Infrared:
Infrared light, in wavelengths that are within the optical window, is also part
of many aurorae
RAINBOW
A rainbow is an optical and meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured arc. Rainbows
caused by sunlight always appear in the section of sky directly opposite the
sun.
- Rainbows can be full circles; however, the average observer sees only an arc formed by illuminated droplets above the ground, and centred on a line from the sun to the observer's eye.
- In a primary rainbow, the arc shows red on the outer part and violet on the inner side. This rainbow is caused by light being refracted (bent) when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.
v
In a double rainbow, a
second arc is seen outside the primary arc, and has the order of its colours reversed, with red on the inner side of the arc
.
Explanation
Light rays enter a
raindrop from one direction (typically a straight line from the sun), reflect
off the back of the raindrop, and fan out as they leave the raindrop. The light
leaving the rainbow is spread over a wide angle, with a maximum intensity at
the angles 40.89–42°. (Note: Between 2 and 100% of the light is reflected at
each of the three surfaces encountered, depending on the angle of incidence.
This diagram only shows the paths relevant to the rainbow.)Variations:-
Multiple rainbows
Secondary rainbows are caused by a double reflection of sunlight inside the raindrops, and are centered on the sun itself. They are about 127° (violet) to 130° (red) wide. Since this is more than 90°, they are seen on the same side of the sky as the primary rainbow, about 10° above it at apparent angles of 50–53°. As a result of the "inside" of the secondary bow being "up" to the observer, the colors appear reversed compared to the primary bow. The secondary rainbow is fainter than the primary because more light escapes from two reflections compared to one and because the rainbow itself is spread over a greater area of the sky
. Each rainbow reflects white light inside its colored bands, but that is "down" for the primary and "up" for the secondary. The dark area of unlit sky lying between the primary and secondary bows is called Alexander's band, after Alexander of Aphrodisias who first described it.
Twinned rainbow
Unlike a double rainbow that consists of two separate and concentric rainbow arcs, the very rare twinned rainbow appears as two rainbow arcs that split from a single base. The colours in the second bow, rather than reversing as in a double rainbow, appear in the same order as the primary rainbow. It is sometimes even observed in combination with a secondary rainbow. The cause of a twinned rainbow is the combination of different sizes of water drops falling from the sky.
Full circle rainbow
In theory every rainbow is a circle, but from the ground only its upper
half can be seen. Since centre is diametrically opposed to the sun's position
in the sky, more of the circle comes into view as the sun approaches the
horizon, meaning that the largest section of the circle normally seen is about
50% during sunset or sunrise. Viewing the rainbow's lower half requires the
presence of water droplets below the observer's horizon, as
well as sunlight that is able to reach them.
These requirements are not usually met when the viewer is at ground level, either because droplets are absent in the required position, or because the sunlight is obstructed by the landscape behind the observer. From a high viewpoint such as a high building or an aircraft, however, the requirements can be met and the full circle rainbow can be seen. Like a partial rainbow, the circular rainbow can have a secondary bow or supernumerary bows as well
NOTE:-Noah was the first one to see rainbow and at that time he was with his 3 sons and wife
. NOBEL PRIZES IN THE FIELD OF PHYSICS
1) Gabriel Lippmann
Jonas Ferdinand Gabriel
Lippmann (16 August 1845 – 13 July 1921)
was a Franco-Luxembourgish physicist and inventor,
and Nobel laureate in physics for his method
of reproducing colours photographically based on the phenomenon of interference.
Contribution
·
Colour
photography
·
Integral
photography
2)Dennis Gabor
Dennis Gabor Hungarian: Gábor Dénes; 5 June 1900 – 8 February 1979) was aHungarian-British electrical
engineer and physicist, most notable for inventing holography,
for which he later received the 1971 Nobel Prize in Physics.
·
Colour photography
·
Hologram
WHAT IS COLOUR
PHOTOGRAPHY??
A method of reproducing colours by
photography is called colour photography,which is based on the interference
phenomenal
GABRIEL LIPPMANN'S COLORED PHOTOGRAPHY
How could Gabriel Lippmann make use of interference effects to achieve color photography? The primer on wave optics and interference told us that light of different wavelengths will generate standing wave patterns at corresponding period lengths. Lippmann started out with a pattern of standing waves, where a wavefield meets itself again after it is reflected in a mirror. He projected an optical image as usual onto a photographic plate, but through its glass plate with the almost transparent emulsion of extremely fine grains on the backside. Then he added the interference effect by placing a mercury mirror in contact with the emulsion. The image went through the emulsion, hit the mirror, and then returned the light back into the emulsion. A suitable thickness of this photographic layer corresponds to around ten or more wavelengths. The image projected onto the plate did not plainly expose the emulsion according to the local distribution of irradiance. Rather, the exposure was encoded when the wave field returned within the emulsion and created standing waves, whose nodes gave little exposure, whereas the bulges gave maximum effect.
FINLAND'S night sky gets struck by ‘Light Pillars’
People in
Finland not only enjoy the beautiful northern lights (aurora borealis), they
also have these entrancing “light pillars” shining over the land like fire
fountains.
Mika
Wist photographer who lives in the town of Hämeenlinna, made these images,
capturing the ‘light pillars’ from their balcony.
“These
pillars have occurred at around 8-9 pm and lasted about an hour. Outside, the
temperature was -19 degrees. I’ve never seen this phenomenon for a long time in
the south of Finland”, said the photographer.
Heavenly
rays vary in color and are strong enough to light the night sky.
After
she immortalized the event in balcony, Wist decided to leave the house and
capture them shining over a frozen lake
REFRACTION OF LIGHT
Refraction is the change in direction of propagation of a wave due to a change
in its transmission medium.
The phenomenon is explained by the conservation of energy and conservation of momentum. Due to change of
medium, the phase velocity of the wave is changed but
its frequency remains
constant. This is most commonly observed when a wave passes from onemedium to another at any angle other than
0° from the normal. Refraction of light is the most
commonly observed phenomenon, but any type of wave can refract when it
interacts with a medium, for example when sound waves pass
from one medium into another or when water waves move into water of a different
depth. Refraction is described by Snell's law,
which states that for a given pair of media and a wave with a single frequency,
the ratio of the sines of the angle of incidence θ1 and angle of refraction θ2 is
equivalent to the ratio of phase velocities (v1 / v2)
in the two media, or equivalently, to the opposite ratio of the indices of
refraction (n2 / n1) In general, the incident wave is partially
refracted and partially reflected; the details of this behavior are
described by the Fresnel equations.Dispersion of light
In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency.[1] Media
having this common property may be termed dispersive media.
Sometimes the term chromatic dispersion is
used for specificity. Although the term is used in the field of optics to
describe light and other electromagnetic
waves, dispersion in the same sense can apply to any
sort of wave motion such as acoustic
dispersion in the case of sound and seismic waves, in gravity
waves (ocean waves), and for telecommunication
signals propagating along transmission lines (such as coaxial cable) or optical
fiber.
Examples of dispersion
The most familiar example of dispersion is probably
a rainbow,
in which dispersion causes the spatial separation of a white light into
components of different wavelengths(different colors). However,
dispersion also has an effect in many other circumstances: for example, GVD
causes pulses to spread in optical fibers,
degrading signals over long distances; also, a cancellation between
group-velocity dispersion and nonlinear effects
leads to soliton waves.
Material and waveguide dispersion
Most often, chromatic dispersion refers to bulk
material dispersion, that is, the change in refractive index with
optical frequency. However in a waveguide there
is also the phenomenon of waveguide dispersion, in which case a
wave's phase velocity in a structure depends on its frequency simply due to the
structure's geometry. More generally, "waveguide" dispersion can
occur for waves propagating through any inhomogeneous structure (e.g., a photonic crystal),
whether or not the waves are confined to some regio. In a waveguide, both types
of dispersion will generally be present, although they are not strictly
additive
