The Michelson-Morely Experiment

The Nature of Light
As a result of Michelson’s efforts in 1879, the speed of light was known to be 186,350 miles per second with a likely error of around 30 miles per second. This measurement, made by timing a flash of light travelling between mirrors in Annapolis, agreed well with less direct measurements based on astronomical observations. Still, this did not really clarify the nature of light. Two hundred years earlier, Newton had suggested that light consists of tiny particles generated in a hot object, which spray out at very high speed, bounce off other objects, and are detected by our eyes. Newton’s arch-enemy Robert Hooke, on the other hand, thought that light must be a kind of wave motion, like sound. To appreciate his point of view, let us briefly review the nature of sound.

The Wavelike Nature of Sound
Actually, sound was already quite well understood by the ancient Greeks. The essential point they had realized is that sound is generated by a vibrating material object, such as a bell, a string or a drumhead. Their explanation was that the vibrating drumhead, for example, alternately pushes and pulls on the air directly above it, sending out waves of compression and decompression (known as rarefaction), like the expanding circles of ripples from a disturbance on the surface of a pond. On reaching the ear, these waves push and pull on the eardrum with the same frequency (that is to say, the same number of pushes per second) as the original source was vibrating at, and nerves transmit from the ear to the brain both the intensity (loudness) and frequency (pitch) of the sound.
There are a couple of special properties of sound waves (actually any waves) worth mentioning at this point. The first is called interference. This is most simply demonstrated with water waves. If you put two fingers in a tub of water, just touching the surface a foot or so apart, and vibrate them at the same rate to get two expanding circles of ripples, you will notice that where the ripples overlap there are quite complicated patterns of waves formed. The essential point is that at those places where the wave-crests from the two sources arrive at the same time, the waves will work together and the water will be very disturbed, but at points where the crest from one source arrives at the same time as the wave trough from
the other source, the waves will cancel each other out, and the water will hardly move. You can hear this effect for sound waves by playing a constant note through stereo speakers. As you move around a room, you will hear quite large variations in the intensity of sound. Of course, reflections from walls complicate the pattern. This large variation in volume is not very noticeable when the stereo is playing music, because music is made up of many frequencies, and they change all the time. The different frequencies, or notes, have their quiet spots in the room in different places. The other point that should be mentioned is that high frequency tweeter-like sound is much more directional than low frequency woofer-like sound. It really doesn’t matter where in the room you put a low-frequency woofer—the sound seems to be all around you anyway. On the other hand, it is quite difficult to get a speaker to spread the high notes in all directions. If you listen to a cheap speaker, the high notes are loudest if the speaker is pointing right at you. A lot of effort has gone into designing tweeters, which are small speakers especially designed to broadcast high notes over a wide angle of directions.

Is Light a Wave?
Bearing in mind the above mini review of the properties of waves, let us now reconsider the question of whether light consists of a stream of particles or is some kind of wave. The strongest argument for a particle picture is that light travels in straight lines. You can hear around a corner, at least to some extent, but you certainly can’t see. Furthermore, no wave-like interference effects are very evident for light. Finally, it was long known, as we have mentioned, that sound waves were compressional waves in air. If light is a wave, just what is waving? It clearly isn’t just air, because light reaches us from the sun, and indeed from stars, and we know the air doesn’t stretch that far, or the planets would long ago have been slowed down by air resistance.
Despite all these objections, it was established around 1800 that light is in fact some kind of wave. The reason this fact had gone undetected for so long was that the wavelength is really short, about one fifty-thousandth of an inch. In contrast, the shortest wavelength sound detectable by humans has a wavelength of about half an inch. The fact that light travels in straight lines is in accord with observations on sound that the higher the frequency (and shorter the wavelength) the greater the tendency to go in straight lines. Similarly, the interference patterns
mentioned above for sound waves or ripples on a pond vary over distances of the same sort of size as the wavelengths involved. Patterns like that would not normally be noticeable for light because they would be on such a tiny scale. In fact, it turns out, there are ways to see interference effects with light. A familiar example is the many colors often visible in a soap bubble. These come about because looking at a soap bubble you see light reflected from both sides of a very thin film of water—a thickness that turns out to be comparable to the wavelength of light. The light reflected from the lower layer has to go a little further to reach your eye, so that light wave must wave an extra time or two before getting to your eye compared with the light reflected from the top layer. What you actually see is the sum of the light reflected from the top layer and that reflected from the bottom layer. Thinking of this now as the sum of two sets of waves, the light will be bright if the crests of the two waves arrive together, dim if the crests of waves reflected from the top layer arrive simultaneously with the troughs of waves reflected from the bottom layer. Which of these two possibilities actually occurs for reflection from a particular bit of the soap film depends on just how much further the light reflected from the lower surface has to travel to reach your eye compared with light from the upper surface, and that depends on the angle of reflection
and the thickness of the film. Suppose now we shine white light on the bubble. White light is made up of all the colors of the rainbow, and these different colors have different wavelengths, so we see colors reflected, because for a particular film, at a particular angle, some colors will be reflected brightly (the crests will arrive together), some dimly, and we will see the ones that win.

If Light is a Wave, What is Waving?
Having established that light is a wave, though, we still haven’t answered one of the major objections raised above. Just what is waving? We discussed sound waves as waves of compression in air. Actually, that is only one case—sound will also travel through liquids, like water, and solids, like a steel bar. It is found experimentally that, other things being equal, sound travels faster through a medium that is harder to compress: the material just springs back faster and the wave moves through more rapidly. For media of equal springiness, the sound goes faster through the less heavy medium, essentially because the same amount of springiness can push things along faster in a lighter material. So when a sound wave passes, the material—air, water or solid—waves as it goes through. Taking this as a hint, it was natural to suppose that light must be just waves in some mysterious material, which was called the aether, surrounding and permeating everything. This aether must also fill all of space, out to the stars, because we can see them, so the medium must be there to carry the light. (We could never hear an explosion on the moon, however loud, because there is no air to carry the sound to us.) Let us think a bit about what properties this aether must have. Since light travels so fast, it must be very light, and very hard to compress. Yet, as mentioned above, it must allow solid bodies to pass through it freely, without aether resistance, or the planets would be slowing down. Thus we can picture it as a kind of ghostly wind blowing through the earth. But how can we prove any of this? Can we detect it?

Detecting the Aither Wind: the Michelson-Morley Experiment
Detecting the aether wind was the next challenge Michelson set himself after his triumph in measuring the speed of light so accurately. Naturally, something that allows solid bodies to pass through it freely is a little hard to get a grip on. But Michelson realized that, just as the speed of sound is relative to the air, so the speed of light must be relative to the aether. This must mean, if you could measure the speed of light accurately enough, you could measure the speed of light travelling upwind, and compare it with the speed of light travelling downwind,
and the difference of the two measurements should be twice the wind speed. Unfortunately, it wasn’t that easy. All the recent accurate measurements had used light travelling to a distant mirror and coming back, so if there was an aether wind along the direction between the mirrors, it would have opposite effects on the two parts of the measurement, leaving a very small overall effect. There was no technically feasible way to do a one-way determination of the speed of light.
At this point, Michelson had a very clever idea for detecting the aether wind. As he explained to his children (according to his daughter), it was based on the following puzzle:
Suppose we have a river of width w (say, 100 feet), and two swimmers who both swim at the same speed v feet per second (say, 5 feet per second). The river is flowing at a steady rate, say 3 feet per second. The swimmers race in the following way: they both start at the same point on one bank. One swims directly across the river to the closest point on the opposite bank, then turns around and swims back. The other stays on one side of the river, swimming upstream a distance (measured along the bank) exactly equal to the width of the river, then swims back to the start. Who wins?
Let’s consider first the swimmer going upstream and back. Going 100 feet upstream, the speed relative to the bank is only 2 feet per second, so that takes 50 seconds. Coming back, the speed is 8 feet per second, so it takes 12.5 seconds, for a total time of 62.5 seconds.
The swimmer going across the flow is trickier. It won’t do simply to aim directly for the opposite bank-the flow will carry the swimmer downstream. To succeed in going directly across, the swimmer must actually aim upstream at the correct angle (of course, a real swimmer would do this automatically). Thus, the swimmer is going at 5 feet per second, at an angle, relative to the river, and being carried downstream at a rate of 3 feet per second. If the angle is correctly chosen so that the net movement is directly across, in one second the swimmer must have moved four feet across: the distances covered in one second will form a 3,4,5 triangle. So, at a crossing rate of 4 feet per second, the swimmer gets across in 25 seconds, and back in the same time, for a total time of 50 seconds. The cross-stream swimmer wins. This turns out to true whatever their swimming speed. (Of course, the race is only possible if they can swim faster than the current!)
Michelson’s great idea was to construct an exactly similar race for pulses of light, with the aether wind playing the part of the river. The scheme of the experiment is as follows: a pulse of light is directed at an angle of 45 degrees at a half-silvered, half transparent mirror, so that half the pulse goes on through the glass, half is reflected. These two half-pulses are the two swimmers. They both go on to distant mirrors which reflect them back to the half-silvered mirror. At this point, they are again half reflected and half transmitted, but a telescope is placed behind the half-silvered mirror as shown in the figure so that half of each half-pulse will arrive in this telescope. Now, if there is an aether wind blowing, someone looking through the telescope should see the halves of the two half-pulses to arrive at slightly different times, since one would have gone more upstream and back, one more across stream in general. To maximize the effect, the whole apparatus, including the distant mirrors, was placed on a large turntable so it could be swung around.
Let us think about what kind of time delay we expect to find between the arrival of the two half-pulses of
light. Taking the speed of light to be c miles per second relative to the aether, and the aether to be flowing at v miles per second through the laboratory, to go a distance w miles upstream will take w/(c-v) seconds, then to come back will take w/(c+v) seconds. The total roundtrip time upstream and downstream is the sum of these, which works out to be 2wc/(c²-v²), which can also be written (2w/c)×1/(1-v²/c²). Now, we can safely assume the speed of the aether is much less than the speed of light, otherwise it would have been noticed long ago, for example in timing of eclipses of Jupiter’s satellites. This means v²/c² is a very small number, and we can use some handy mathematical facts to make the algebra a bit easier. First, if x is very small compared to 1, 1/(1-x) is very close to 1+x. (You can check it with your calculator.) Another fact we shall need in a minute is that for small x, the square root of 1+x is very close to 1+x/2.
Putting all this together, the upstream–downstream roundtrip time
Now, what about the cross-stream time? The actual cross-stream speed must be figured out as in the example above using a right-angled triangle, with the hypoteneuse equal to the speed c, the shortest side the aether flow speed v, and the other side the cross-stream speed we need to find the time to get across. From Pythagoras’ theorem, then, the cross-stream speed is the square root of (c²-v²).
Since this will be the same both ways, the roundtrip cross-stream time will be
This can be written in the form
using the following approximations valid for low velocities,
Therefore the across-stream roundtrip time, assuming the aether velocity is much less than that of light, is
Looking at the two roundtrip times at the ends of the two paragraphs above, we see that they differ by an amount (2w/c) × v²/2c². Now, 2w/c is just the time the light would
take if there were no aether wind at all, say, a few millionths of a second. If we take the aether wind speed to be equal to the earth’s speed in orbit, for example, v/c is about 1/10,000, so v²/c² is about 1/100,000,000. This means the time delay between the pulses reflected from the different mirrors reaching the telescope is about one-hundred-millionth of a few millionths of a second. It seems completely hopeless that such a short time delay could be detected. However, this turns out not to be the case, and Michelson was the first to figure out how to do it. The trick is to use the interference properties of the light waves. Instead of sending pulses of light, as we discussed above, Michelson sent in a steady beam of light of a single color. This can be visualized as a sequence of ingoing waves, with a wavelength one fifty-thousandth of an inch or so. Now this sequence of waves is split into two, and reflected as previously described. One set of waves goes upstream and downstream, the other goes across stream and back. Finally, they come together into the telescope and the eye. If the one that took longer is half a wavelength behind, its troughs will be on top of the crests of the first wave, they will cancel, and nothing will be seen. If the delay is less than that, there will still be some dimming. However, slight errors in the placement of the mirrors would have the same effect. This is one reason why the apparatus is built to be rotated. On
turning it through 90 degrees, the upstream-downstream and the cross-stream waves change places. Now the other one should be behind. Thus, if there is an aether wind, if you watch through the telescope while you rotate the turntable, you should expect to see variations in the brightness of the incoming light.
To magnify the time difference between the two paths, in the actual experiment the light was reflected backwards and forwards several times, like a several lap race.
Michelson calculated that an aether windspeed of only one or two miles a second would have observable effects in this experiment, so if the aether windspeed was comparable to the earth’s speed in orbit around the sun, it would be easy to see. In fact, nothing was observed. The light intensity did not vary at all. Some time later, the experiment was redesigned so that an aether wind caused by the earth’s daily rotation could be detected. Again, nothing was seen. Finally, Michelson wondered if the aether was somehow getting stuck to the earth, like the air in a below-decks cabin on a ship, so he redid the experiment on top of a high mountain in California. Again, no aether wind was observed. It was difficult to believe that the aether in the immediate vicinity of the earth was stuck to it and moving with it,
because light rays from stars would deflect as they went from the moving faraway aether to the local stuck aether.
The only possible conclusion from this series of very difficult experiments was that the whole concept of an all-pervading aether was wrong from the start. Michelson was very reluctant to think along these lines. In fact, new theoretical insight into the nature of light had arisen in the 1860’s from the brilliant theoretical work of Maxwell, who had written down a set of equations describing how electric and magnetic fields can give rise to each other. He had discovered that his equations predicted there could be waves made up of electric and magnetic fields, and the speed of these waves, deduced from experiments on how these fields link together, would be 186,300 miles per second. This is, of course, the speed of light, so it is natural to assume that light is made up of fast-varying electric and magnetic fields. But this leads to a big problem: Maxwell’s equations predict a definite speed for light, and it is the speed found by measurements. But what is the speed to be measured relative to? The whole point of bringing in the aether was to give a picture for light resembling the one we understand for sound, compressional waves in a medium. The speed of sound through air is measured relative to air. If the wind is blowing towards you from the source of sound, you will
hear the sound sooner. If there isn’t an aether, though, this analogy doesn’t hold up. So what does light travel at 186,300 miles per second relative to?
There is another obvious possibility, which is called the emitter theory: the light travels at 186,300 miles per second relative to the source of the light. The analogy here is between light emitted by a source and bullets emitted by a machine gun. The bullets come out at a definite speed (called the muzzle velocity) relative to the barrel of the gun. If the gun is mounted on the front of a tank, which is moving forward, and the gun is pointing forward, then relative to the ground the bullets are moving faster than they would if shot from a tank at rest. The simplest way to test the emitter theory of light, then, is to measure the speed of light emitted in the forward direction by a flashlight moving in the forward direction, and see if it exceeds the known speed of light by an amount equal to the speed of the flashlight. Actually, this kind of direct test of the emitter theory only became experimentally feasible in the nineteen-sixties. It is now possible to produce particles, called neutral pions, which decay each one in a little explosion, emitting a flash of light. It is also possible to have these pions moving forward at 185,000 miles per second when they self destruct, and to catch the light emitted in the forward
direction, and clock its speed. It is found that, despite the expected boost from being emitted by a very fast source, the light from the little explosions is going forward at the usual speed of 186,300 miles per second. In the last century, the emitter theory was rejected because it was thought the appearance of certain astronomical phenomena, such as double stars, where two stars rotate around each other, would be affected. Those arguments have since been criticized, but the pion test is unambiguous.

তড়িত তাণ্ডব(পর্ব ৩)

তড়িত তাণ্ডব(পর্ব ৩)

This entry is part 3 of 4 in the series তড়িত তাণ্ডব

হ্যালাে বন্ধুরা, আশাকরি ভালাে আছাে সবাই! তড়িত তাণ্ডবের আরাে একটি পর্বে তােমাদের জানাই স্বাগতম । তড়িত এর পৃথিবী অনেক ব্যাপক ও বিস্তৃত এতে সন্দেহ নাই। আমরা এর সবটাই জানার চেষ্টা করবাে,তবে ধীরে ধীরে। গতপর্বে তােমরা রােধ,বিভব আর প্রবাহ তথা রেজিস্ট্যান্স,ভােল্টেজ এবং কারেন্ট নিয়ে মােটামুটি ধারণা পেয়েছাে ! আজ তােমাদের বিদ্যুতের দুই ধরণ নিয়ে কথা বলব।

ততা গতপর্বে তােমরা দেখেছাে যে ভােল্টেজ এবং কারেন্ট কী। এখন বন্ধুরা তড়িতের দুইয়ার সবকিছুই এই ভােল্টেজ আর কারেন্টময়। এরাই তড়িতের কী প্লেয়ার ! কারণ এরাই বহন করে পাওয়ার বা শক্তি। তাে চলাে চট করে দেখে নিই প্রবাহ বা ভােল্টেজ কত প্রকারের
(1) পরিবর্তী প্রবাহ
(২) অপরিবর্তী প্রবাহ

তােমরা অনেকেই হয়ত ভাবছাে,এ দুই ই তাে প্রবাহের প্রকারভেদ হইলাে ! ভােল্টেজ টা কোথায়। আসলে বন্ধুরা ভােল্টেজ আর কারেন্ট এ দুই ই আসলে পরিবর্তী আর অপরিবর্তী এ দুই ক্যাটাগরীর ! কিন্তু কোনাে এক অজানা কারণে কেন জানি একে ইংরেজীতে AC আর DC নাম দেওয়া হয়েছে ! আর AC মানে যেহেতু Alternating Current আর DC মানে Direct Current তাই বাংলাতেও এ দুই এর নাম হয়ে গেছে পরিবর্তী আর অপরিবর্তী প্রবাহ! এখন এজন্য যদি বিভব বা ভােল্টেজ হরতাল শুরু করে দেয় যে আমার নাম কেনাে দেওয়া হলাে না ! তােখন কিন্তু আমায় দায়ী করতে পারবে না!

তাে চলাে এবার আসল বিষয়ে প্রবেশ করি। অর্থাৎ পরিবর্তী আর অপরিবর্তী প্রবাহ।
প্রথমেই আসি অপরিবর্তী প্রবাহ বা ডিসি তে। সােজা সাপ্টা সংগা হলাে এই যে, যে প্রবাহ বা ভােল্টেজ সময়ের সাথে সাথে পরিবর্তীত হয় না বা অপরিবর্তীত থাকে, তাকেই বলে ডিসি বা অপরিবর্তী প্রবাহ। এখন মনে প্রশ্ন আসবে একটু সময় পর পর আবার প্রবাহ চেঞ্জ হয় কীভাবে! হয় বন্ধুরা, অবশ্যই হয়। তুমি জানাে, আমরা বাসাবাড়িতে যে বিদ্যত ব্যবহার করি তা সেকেণ্ডে ৫০ বার দিক পরিবর্তন করে! কী অনেক অবাক হলে ? অবাক হবার মতােনই আসলে ব্যাপারটা! তবে সে ব্যাপারে আমরা এসি বা পরিবর্তী প্রবাহে জানব। আপাততাে চলাে আগে অপরিবর্তী প্রবাহ বা ডিসির ব্যাপারটা শেষ করে ফেলি। তাে ডিসি কে আমরা বলছি অপরিবর্তী প্রবাহ। নামের মাঝেই রয়েছে পরিচয় এবং আমরা সংগাতেও সেটাই দেখলাম। তাে সময়েরব সাথে দিক পরিবর্তন করে না কথাটার অর্থ কী। তার আগে চলাে একটা ছবি দেখে আসি…

তাে দেখতেই পাচ্ছ বন্ধুরা যে এখানে একটি লাইট এবং ব্যাটারীর বৈদ্যুতিক ব্যবস্থা দেখানাে হয়েছে। তােমরা দেখতে পাচ্ছ যে এখানে ইলেক্ট্রন ব্যাটারীর নেগেটিভ প্রান্ত হতে লাইটের মধ্য দিয়ে পজিটিভ প্রান্তে যাচ্ছে ! যদিও ইলেকট্রনের প্রবাহই বিদ্যুত প্রবাহ! কিন্তু জানাে তাে মানুষ ইলেকট্রন আবিষ্কারের অনেক অনেক দিন আগে বিদ্যুত আবিষ্কার করে ! তাে তখন তারা মনে করত যে চার্জ পজেটিভ থেকে নেগেটিভে ফ্লো হয়! যদিও পরে দেখা গেলাে ঘটনাটা হয় উলটো, তবুও ঐতিহ্যগত কারণে বিদ্যুত । প্রবাহের দিক ধরা হলাে পজেটিভ টু নেগেটিভের দিকে! অর্থাৎ ইলেকট্রন প্রবাহের দিকের বিপরীত। এখন লক্ষ্য করাে যে ছবিটায় যে দিক দেখানাে হয়েছে তা ইলেকট্রন প্রবাহের দিক, আসলে সকল সার্কিটে বিদ্যুত প্রবাহের দিক হিসেবে ঐতিহ্যগত দিক টাই ধরা হয় অর্থাৎ পজেটিভ টু নেগেটিভ। 

তাে দেখাে ব্যাটারীর এই যে একপাশ হতে ইলেকট্রন প্রবাহিত হয়ে অন্যদিকে যাচ্ছে, এটি কিন্তু এক অবরাম ঘটনা। যতক্ষন অবদি ব্যাটারীর চার্জ শেষ না হবে অথবা ব্যাটারীর দুই প্রান্তের বিভব সমান না হবে ততক্ষন অবদি এ প্রবাহ চলবে! এতে প্রবাহের দিকের কোন পরিবর্তন নেই! একটা গ্রাফ পেপার নিয়ে তাতে X অক্ষে সময় আর Y অক্ষে প্রবাহ নিয়ে যদি আমরা গ্রাফ আকি তবে ডিসির জন্য তা দেখা যাবে নিচের ছবির মতাে •••

দেখতেই পাচ্ছ যে এখানে প্রবাহ সময়ের ওপর নির্ভরশীল নয়! সময় গেলেও প্রবাহ ঠিকই আছে এবং কোনরূপ পরিবর্তন এক্ষেত্রে ঘটে নি ! আর এজন্যই একে বলা হচ্ছে অপরিবর্তী প্রবাহ!

আশাকরি সবাই বুঝেছ বিষয়টা ! তাে বন্ধুরা আজ এটুকুই। আগামী পর্বে আমরা আলােচনা করব পরিবর্তী প্রবাহ বা AC নিয়ে। ততদিন অবদি, “হ্যাপি লার্নিং”…

Light emitting diode

Light emitting diode

Light emitting diode, generally known as LED has become one of the essential electronic components in our daily life. LEDs are everywhere! From traffic lamp to reading light, from vehicles to remote! Actually, we are surrounded by LEDs. So, why LEDs have become so much popular! There are of course some great reasons. To describe LED, first, we should take a look at the history of electrical light. We know the first commercially light was invented by Thomas Alva Edison in 1878. He tried almost everything for a perfect filament and succeeded by using tungsten. Although it was a very important invention, in the case of efficiency, it was not so good. The tungsten bulbs convert only 5% of their total given energy to light, in case of practical use, it remains mostly at 2.2%, most of the given energy is converted to heat. So, to work on the problem, scientists pay attention to invent efficient light bulbs. To fulfill this objective, the researched and invent Fluorescent bulbs. Although fluorescent bulbs were more efficient than tungsten bulbs and did not produce too much heat and were also capable of working in low energy, its efficiency was not also more than 10%. So, scientists further look forward to inventing a more efficient light source. In the middle of the nineteen century, a German scientist named Ferdinand Braun invented the first semiconductor diode. As we are talking about light emitting diodes, so we have to go through the diode first. So, after the invention of the diode, there comes

a revolution in the field of electronic and science. Soon after the invention of the diode, the legendary transistor was invented by joining two back to back diodes. The invention of the transistor was really a milestone in the field of science. Later it was followed by JFET & MOSFETS. So, we were talking about LEDs. So, after doing so much research and collaboration on diode, the first LED was invented in 1961 by James R. Biard & Gary E. Pittman and commonly was used in various remote control & other devices cause it was an infrared LED. So there was still no scope of visible light. Being inspired by the infrared LEDs an American Engineer Nick Holonyak first successfully invented an LED emitting visible light in 1962. It was a shiny bright semiconductor diode emitting Red light! Later it was followed by George Crawford and Shuji Nakamura by inventing yellow & blue light gradually in 1972 & 1993. Though various kinds of visible lights were invented, still white light was a quest for scientists. As we know, blue LED was invented late (in 1993), before that, it was not possible to make white light because as we know the white light is a complex light and is a mixture of various colors. It was really a tough challenge for the scientists to make white light as blue LEDs were yet to invent. Creating a blue LED was a difficult challenge, after researching for several years, the Japanese Engineer Shuji Nakamura invented a bright blue LED by using gallium nitrate. So, after this long path, we are now belonging in the era of LED. Can we imagine that the tungsten light bulbs which consumed average 30 watts were replaced by a component which consumes only 2.5 watts on average! Really it was a

miracle. LEDs are replacing almost every light sources around us and it would not? LEDs are cheap, LEDs consume less power, LEDs are more efficient. Various kinds of LEDs are now being used for various purposes. In medical science, space science security and everywhere! Now we click photo by using LED flash and also see them in the LED monitor screen. Doctors use LED enabled sensors to find diseases. Police are using LED lights to keep documents of the criminals. Optical mark readers are being used hugely to count marks and give results. Not only that, fingerprint sensors, smoke sensors and almost every kind of optical devices have been and replaced and the process is going. So, we cannot imagine a life without LEDs now. In street light, in rooms, in decoration everywhere is LEDs. Let me finish by a quote from the 2014 Noble Prize committee, “Incandescent light bulbs lit the 20th century, the 21st century will be lit by LED bulbs”. So, the next time you switch on your room lamp, try to recount the awesome contribution of different scientists who gave their time and knowledge to provide you with this great facility.

Chemical Bond

Chemical Bond

A chemical bond is a lasting attraction between atoms, ions or molecules that enables the formation of chemical compounds. In other word, Chemical compounds are formed by the joining of two or more atoms. A stable compound occurs when the total energy of the combination has lower energy than the separated atoms. The bound state implies a net attractive force between the atoms known as a chemical bond.

We know, a chemical bond is an attraction between atoms. This attraction may be seen as the result of different behaviors of the outermost or valence electrons of atoms. These behaviors merge into each other seamlessly in various circumstances, so that there is no clear line to be drawn between them. However, it remains useful and customary to differentiate between different types of bond, which result in different properties of condensed matter.

There are mainly three types of chemical bonding. They are

  1. Ionic Bonding 2. Covalent Bonding & 3. Metallic Bonding.

There are also other chemical bonding like

  1. Hydrogen Bonding
  2. Van der Waals Bonding
  3. Co-ordinate covalent bond

Different types of chemical bonding are discussed below…

Ionic Bonds

In chemical bonds, atoms can either transfer or share their valence electrons. In the extreme case where one or more atoms lose electrons and other atoms gain them in order to produce a noble gas electron configuration, the bond is called an ionic bond.

Typical of ionic bonds are those in the alkali halides such as sodium chloride, NaCl.

Covalent Bonds

Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms, in contrast to the transfer of electrons in ionic bonds. Such bonds lead to stable molecules if they share electrons in such a way as to create a noble gas configuration for each atom.

Hydrogen gas forms the simplest covalent bond in the diatomic hydrogen molecule. The halogens such as chlorine also exist as diatomic gases by forming covalent bonds. The nitrogen and oxygen which makes up the bulk of the atmosphere also exhibits covalent bonding in forming diatomic molecules.

There are mainly three types of covalent bond depending upon the number of shared electron pairs…

They are …




Metallic Bonds

The properties of metals suggest that their atoms possess strong bonds, yet the ease of conduction of heat and electricity suggest that electrons can move freely in all directions in a metal. The general observations give rise to a picture of “positive ions in a sea of electrons” to describe metallic bonding.

Now we shall discuss the other types of chemical bonding…

Hydrogen Bonding

Hydrogen bonding differs from other uses of the word “bond” since it is a force of attraction between a hydrogen atom in one molecule and a small atom of high electronegativity in another molecule. That is, it is

an intermolecular force, not an intra-molecular force as in the common use of the word bond.

When hydrogen atoms are joined in a polar covalent bond with a small atom of high electronegativity such as O, F or N, the partial positive charge on the hydrogen is highly concentrated because of its small size. If the hydrogen is close to another oxygen, fluorine or nitrogen in another molecule, then there is a force of attraction termed a dipole-dipole interaction. This attraction or “hydrogen bond” can have about 5% to 10% of the strength of a covalent bond.

Hydrogen bonding has a very important effect on the properties of water and ice. Hydrogen bonding is also very important in proteins and nucleic acids and therefore in life processes. The “unzipping” of DNA is a breaking of hydrogen bonds which help hold the two strands of the double helix together.

Van der Waals Bonding

Water molecules in liquid water are attracted to each other by electrostatic forces, and these forces have been described as van der Waals forces or van der Waals bonds. Even though the water molecule as a whole is electrically neutral, the distribution of charge in the molecule is not symmetrical and leads to a dipole moment – a microscopic separation of the positive and negative charge centers. This leads to a net attraction between such polar molecules which finds expression in the cohesion of water molecules and contributes to viscosity and surface tension. Perhaps it is fair to say that van der Waals forces are what holds water in the liquid state until thermal agitation becomes violent enough to break those van der Waal bonds at 100°C. With cooling, residual electrostatic forces between molecules cause

most substances to liquify and eventually solidify (with the exception of helium, which never becomes a solid at atmospheric pressure).

Even nonpolar molecules experience some van der Waals bonding, which can be attributed to their being polarizable. Even though the molecules don’t have permanent dipole moments, they can have instantaneous dipole moments which change or oscillate with time. These fluctuations of molecular dipole moments lead to a net attraction between molecules which allow nonpolar substances like carbon tetrachloride to form liquids. Examination of the dipole electric field shows that the electric field from one instantaneous dipole will tend to polarize a neighboring molecule such that it will be attracted – sort of the electrical analog to a bar magnet magnetizing a paper clip so that it will be attracted to the magnet. (This happens regardless of which pole of the magnet is brought close to the paper clip.) The weaker van der Waals forces in nonpolar liquids may be manifested in low surface tension and low boiling points.

Coordinate covalent bond

A covalent bond is formed by two atoms sharing a pair of electrons. The atoms are held together because the electron pair is attracted by both of the nuclei.

In the formation of a simple covalent bond, each atom supplies one electron to the bond – but that doesn’t have to be the case. A coordinate bond (also called a dative covalent bond) is a covalent bond (a shared pair of electrons) in which both electrons come from the same atom.

So , there you go… We have come to the conclusion of today’s lesson. Hope you have enjoyed it & understand chemical bonding!

Hope to see you next time. Till then, Happy Learning…