LUNAR ECLIPSE AND RED MOON

Edited-30-september-2015

Total Lunar Eclipse

During a total lunar eclipse, the Sun, Earth and Moon form a straight line. The Earth blocks any direct sunlight from reaching the Moon. The Sun is behind the Earth, so the Sun's light casts the Earth's shadow on the Moon. This shadow covers the entire Moon and causes a total lunar eclipse.

The figure shown above shows the shadow of the earth on the moon . Umbra a conical shaped shadow made by earth along with it a different visualization of shadow shows penumbra . Don't be confuse with these terminology as you can observe the rays which are tangent to both sun and earth on same side creating Umbra shadow . And those rays tangent to both sun and earth but in opposite sides creating  Penumbra shadow.  

The Moon does not have any light of it's own – it shines because its surface reflects sunlight. During a total lunar eclipse, the Earth moves between the Sun and the Moon and cuts off the Moon's light supply. When this happens, the surface of the Moon takes on a reddish glow instead of going completely dark. Also the Moon appears dark until earth's shadow covers it completely .

The red color of a totally eclipsed Moon has prompted many people in recent years to refer to total lunar eclipses as Blood Moons.

Why Red?

The reason why the Moon takes on a reddish color during totality is a phenomenon called Rayleigh scattering. It is the same mechanism responsible for causing colorful sunrises and sunsets and the sky to look blue. Rayleigh law is based on the fact that if white light is falling on a particle whose particle size is much much less than the wavelength of incident light so the light will scattered in all direction in splitting pattern into its constituents VIBGYOR . As Rayleigh says the intensity of scattered light is inversely proportional to fourth power of wavelength of different lights . Let say wavelength of RED color is large so it will scattered least . And for violet its wavelength in small so it will scattered high . Means visibility of red color for longer distance is more . That's why light pass through the earth atmosphere it scattered according to the Rayleigh when light reach us after reflection from the moon only red color reach to us . 

Colorful Sunlight

We all know from our school science textbooks that even though sunlight may look white to human eyes, it is actually composed of different colors. These colors are visible through a prism or in a rainbow. Colors towards the red spectrum have longer wavelengths and lower frequencies compared to colors towards the violet spectrum which have shorter wavelengths and higher frequencies.

Earth's Atmosphere

The next piece in the puzzle of why the sky looks blue and a totally eclipsed Moon turns red is the Earth's atmosphere. The atmosphere is about 300 miles (480 kilometers) thick and is made up of different gases, water droplets and dust particles.

When sunlight entering the Earth's atmosphere strikes the particles that are smaller than the light's wavelength, it gets scattered. Not all colors in the light spectrum, however, get equally scattered. Colors with shorter wavelengths, for example those towards the violet spectrum, are scattered more strongly than those with longer wavelengths like red and orange, which pass through the atmosphere. This light then gets bent or refracted back to the surface of the Moon and gives it the reddish-orange glow that total lunar eclipses are famous for.

Look for a Blue Band

Veteran eclipse watchers will tell you that if you look really hard right at the beginning and just before the end of totality, you may detect a light blue band on the Moon's face. This happens because the Earth's Ozone Layer scatters red light and lets through some of the blue light that gets refracted to the Moon.

We must have to understand the reason why some time moon appears blue around its ring along with red during lunar eclipse. As you can observe in the figure the rays those refracted from the outer atmosphere means 12 kilometer above the sea level results blue appearance . In figure you can see two rays both are white but after refraction they are showing different color . These are just representation . The ray which refract from 12 km above sea level will bring you the blue appearance of the moon reason is that the Ozone layer absorbs the orange , red light and offers least scattering because at height the density of atmosphere is less , so when that ray reflected from the surface of the moon and reached our eye it's Blue color intensity retained  , which results in blue appearance.

Sunbeams change their color as they pass through the earth atmosphere and refract into the umbra. The upper beam turns blue because orange light is absorbed by ozone when passes through the stratosphere. The lower beam, which refracts more due to denser earth atmosphere, turns red because short wavelength are scattered more thoroughly as the beam passes through denser troposphere. (source: Stanley David Gedzelman)

Please observe the image above carefully and see the blue  appearance on the moon.  

Many Shades of Red

The Moon can take on different shades of red, orange or gold during a total lunar eclipse, depending on the conditions of the Earth's atmosphere at the time of the Eclipse. The amount of dust particles, water droplets, clouds and mist can all have an effect on the shade of red. Volcanic ash and dust in the atmosphere can also lead to the Moon turning dark during an eclipse. In December 1992, not long after the eruption of Mount Pinatubo in the Philippines, there was so much dust in Earth’s atmosphere that the totally eclipsed moon could barely be seen.

What happened if there is no atmosphere on the earth?

If Earth didn’t have an atmosphere, then, when the moon was entirely eclipsed within Earth’s shadow, the moon would would appear dark …

Choke in Fluoroscent tubes

Why we use choke in tube lights | why we use choke in Fluorescent tube | working of Tube light | working of starter | why we use starter in tube light | Mercury vapours in tube light | inductor in flourescent tube

What is Fluorescent Tubes?
Ans:- It is a long Tube containing filaments at the ends . Filled with Mercury vapours with Argon. Basically when mercury excites it produce UV radiation , to filter that , A coating of phosphorous used inside the tube as white powder.

What’s Inside a Fluorescent Tube Light?

• A fluorescent lamp basically consists of a long glass gas discharge tube. Its inner surface is coated with phosphorous and is filled with an inert gas, generally argon, with a trace of mercury.

• The tube is then finally sealed at low pressure with two filament electrodes each at its both ends.

• These electrode filaments are used to preheat the tube and initiate a rapid conduction of electrons between the two end electrodes. The process initially requires a relatively high amount of power.
• The energy also converts some of the mercury from a liquid to a glass. Electrons then collide with the gaseous mercury atoms, increasing the amount of energy. As electrons return to their original energy level, they begin to release light. However, the light they emit is ultraviolet, and not visible to the naked eye, so another step needs to take place before we can see the light.
• This is why the tube was coated with phosphorous. Phosphors will give off light when exposed to light. When exposed to the ultraviolet light, the particles emit a white light which we can see.

Image showing UV radiation


• Once the conduction of electrons between the electrodes is complete, no more heating of the filaments is required and whole system works at a much lower current.

As you see above . You see two names:- Ballast and starter

Ballast :-An electrical ballast is a device intended to limit the amount of current in an electric circuit. A familiar and widely used example is the inductive ballast used in fluorescent lamps, to limit the current through the tube, which would otherwise rise to destructive levels due to the tube's negative resistance characteristic.

CHOKE

We also called it Choke :-

• The choke is in fact a large inductor. It consists of a long copper winding over iron laminations.
• An inductor by nature always has a tendency to throw back the stored current in it, every time the power through it is switched OFF. This principle of the choke is exploited in lighting a fluorescent tube light. 

Internal elements of tube light

Starter:- It is a type of capacitor with two metal strips . When Current tries to pass through the choke coil  it blocked by the back EMF of the coil . Generally choke produces 1700 volts . Due to that high voltage its strips starts bending . At a moment when strips touch each other a large amount of current flows in the circuit . which results induction of  back EMF in the choke coil . That EMF and current in combination ionise the mercury vapour . 

Starter

READ below for more explaination :-

• When an AC voltage is applied to a tube light fixture, the voltage passes through the choke, the  starter, and the filaments of the tube.                                                                                                                                        

• The filaments light up and instantly warm up the tube. The starter is made up of a discharbe bulb with two electrodes next to it. When electricity passes through it an electrical arc is created between the two electrodes. This creates light, however the heat from the bulb causes one of the electrodes (a bimetallic strip) to bend, making contact with the other electrode. This stops the charged particles from creating the electrical arc that created light. However, now that the heat from the light is gone, the bimetallic strip cools and bends away from the electrode, opening the circuit again.                                                                                                                                                      

• At this point, the ballast or choke "kick's back" it’s stored current, which again passes through the filaments and ignites the tube light once again.                                                                                                    

• If the tube does not sufficiently charge up, subsequent kicks are delivered by the choke due to rapid switching of the starter, so that finally the tube strikes.                                                                            

• After this the choke only acts like a low impedance current limiter to the tube as long as the light is kept illuminated.

A common problem associated with these types of fixtures is humming or buzzing. The reason for this lies in the loosely fitted choke on to the fixture which vibrates in accordance with the 50 or 60 hertz frequency of our AC mains and creates a humming sort of noise. Tightening the choke's screws may instantly eliminate the problem.

The working principle of today’s modern electronic ballasts is to avoid the use of starters for the preheating purpose. They are also very light in weight. These inhibit the initial flickering of the tube light as normally seen in the ordinary tube fixtures by changing the frequency of the mains power to a much higher 20,000 hertz or more. Moreover, electronic ballasts are very energy efficient.

Why tube light without choke coil is more efficient?

Ans:-Many fluorescent lamps now use electronic ballasts, which convert AC to DC and then back to AC at a higher frequency to operate the lamp. The electronics also generate the high voltage needed to start the lamp. Despite the complexity these are more efficient, partly because the fluorescent tube itself is more efficient at higher frequencies. They also eliminate any noticeable flickering since the frequency is higher.

That circuit can perform the task without the choke coil.

For more information put your comments .

Principle of DC Generator

There are two types of generators, one is ac generator and other is dc generator. Whatever may be the types of generators, it always converts mechanical power to electrical power. An ac generator produces alternating power. A DC generator produces direct power. Both of these generators produce electrical power, based on same fundamental principle of Faraday's law of electromagnetic induction. According to these law, when an conductor moves in a magnetic field it cuts magnetic lines force, due to which an emf is induced in the conductor. The magnitude of this induced emf depends upon the rate of change of flux (magnetic line force) linkage with the conductor. This emf will cause an current to flow if the conductor circuit is closed.
Hence the most basic tow essential parts of a generator are
a) a magnetic field and
b) conductors which move inside that magnetic field.
Now we will go through working principle of dc generator. As, the working principle of ac generator is not in scope of our discussion in this section.

Single Loop DC Generator

In the figure above, a single loop of conductor of rectangular shape is placed between two opposite poles of magnet. Let's us consider, the rectangular loop of conductor is ABCD which rotates inside the magnetic field about its own axis ab. When the loop rotates from its vertical position to its horizontal position, it cuts the flux lines of the field. As during this movement two sides, i.e. AB and CD of the loop cut the flux lines there will be an emf induced in these both of the sides (AB & BC) of the loop.

As the loop is closed there will be a current circulating through the loop. The direction of the current can be determined by Flemming's right hand Rule. This rule says that is you stretch thumb, index finger and middle
finger of your right hand perpendicular to each other, then thumbs indicates the direction of motion of the 
conductor, index finger indicates the direction of magnetic field i.e. N - pole to S - pole, and middle finger
indicates the direction of flow of current through the conductor.

Now if we apply this right hand rule, we will see at this horizontal position of the loop, current will flow 
from point A to B and on the other side of the loop current will flow from point C to D.

Now if we allow the loop to move further, it will come again to its vertical position, but now upper side of 
the loop will be CD and lower side will be AB (just opposite of the previous vertical position). At this 
position the tangential motion of the sides of the loop is parallel to the flux lines of the field. Hence there will 
be no question of flux cutting and consequently there will be no current in the loop.

If the loop rotates further, it comes to again in horizontal position. But now, said AB side of the loop 
comes in front of N pole and CD comes in front of S pole, i.e. just opposite to the previous horizontal 
position as shown in the figure beside.

Here the tangential motion of the side of the loop is perpendicular to the flux lines, hence rate of flux cutting 
is maximum here and according to Flemming's right hand rule, at this position current flows from B to A and on 
other side from D to C.

Now if the loop is continued to rotate about its axis, every time the side AB comes in front of S pole, the current flows from A to B and when it comes in front of N pole, the current flows from B to A. Similarly, every time the 
side CD comes in front of S pole the current flows from C to D and when it comes in front of N pole the current flows from D to C.

If we observe this phenomena in different way, it can be concluded, that each side of the loop comes in front of N pole, the current will flow through that side in same direction i.e. downward to the reference plane and similarly 
each side of the loop comes in front of S pole, current through it flows in same direction i.e. upwards from 
reference plane. From this, we will come to the topic of principle of dc generator.

Now the loop is opened and connect it with a split ring as shown in the figure below. Split ring are made out of a conducting cylinder which cuts into two halves or segments insulated from each other. The external load terminals 
are connected with two carbon brushes which are rest on these split slip ring segments.

Working Principle of DC Generator

It is seen that in the first half of the revolution current flows always along ABLMCD i.e. brush no 1 in contact with segment a. In the next half revolution, in the figure the direction of the induced current in the coil is reversed. But at the same time the position of the segments a and b are also reversed which results that brush no 1 comes in touch with that segment b. Hence, the current in the load resistance again flows from L to M. The wave from of the current through the load circuit is as shown in the figure. This current is unidirectional.

This is basic working principle of DC generator, explained by single loop generator model.

The position of the brushes of DC generator is so arranged that the change over of the segments a and b from one brush to other takes place when the plane of rotating coil is at right angle to the plane of the lines of force. It is so become in that position, the induced emf in the coil is zero.

To know about AC generator please click here