What is LIGHT? Without light we couldn’t see the world around us, yet we still don’t know exactly what light is!

We now however, know that light is a form of energy. Its speed can be measured and the way it behaves is very much known to us. We also know that white light is not a special kind of light – it is a mixture of all colors. We call this “the spectrum.”

We also further know that color is not in the objects seen – it is instead, in the light by which they are seen. A piece of green paper looks green because it absorbs all the colors except green, which it reflects to the eyes. Blue glass allows only blue light to pass through it; all other colors are absorbed in the glass. Sunlight is energy. The heat in rays of sunlight, when focused with a lens, will start a fire. Light and heat are reflected from white materials and absorbed by black materials. That’s why white clothing is cooler than black clothing.

But then, what is the nature of light? The first man to make a serious effort to explain the nature of light was Sir Isaac Newton. He believed that light is made up of corpuscles, like tiny bullets that are shot from the source of the light and thus, propounded the so called “corpuscular” theory of light. But some of the things that happen to light couldn’t be explained according to this theory.

So a man called Huygens came up with another explanation of light. He developed the “wave” theory of light. His idea was that light started pulses, or waves, the way a pebble dropped into a pool makes waves.

Whether light was waves or corpuscles was argued for nearly 150 years. The wave theory seemed to be the one that most scientists accepted. Then something was discovered about the way light behaves that upset this theory. Then, a very reasonable question arises:

Where does science stand today about the actual nature of light? Well, it is now believed that light behaves both as particles and as waves. Experiments can be made to prove that it is one or the other. So there just doesn’t seem to be a single satisfactory answer to the question of what is light. Yet for the time being, it is both particulate & wave in nature although, it is more of a wave nature than particles.

What are Electromagnetic waves? All forms of light radiations, whether of the visible spectrum or IR or UV are nothing, but electromagnetic radiations in nature, capable of behaving both as a wave and particle as such.

In strict Physics terms, Electromagnetic waves are coupled periodic, electrical and magnetic disturbances created by oscillating electric charges. These waves cover a wide range of frequencies, and have different effects. However they have certain common properties such as:

All waves of the electromagnetic spectrum travel at the same velocity, a quantity what is known as the speed of light. This speed is approximately 300,000 km per second (or about 186,000 miles per second) or 3 x 10⁸ m/s.

They always show the phenomena of diffraction, interference as well as reflection and refraction.

They always obey the equation c = ƒλ; where c is the speed of light, ƒ is the frequency of the wave and λ is its wavelength. Since c is constant (for a given medium), it follows that larger the frequency of a wave, the smaller is its wavelength.

Because of their electrical origin and an ability to travel in a vacuum, they all are regarded as progressive transverse waves consisting of a combination of traveling electric and magnetic forces which vary in value and are directed at right angles to each other and to the direction of their travel.

Connecting concepts: What are X-rays?

X-rays were discovered in Germany in 1895 by Wilhelm Roentgen, and thus are sometimes called “Roentgen rays.”

They are penetrating rays similar to light rays. They differ from light rays in the length of their waves and in their energy. The shortest wave length from an X-ray tube may be one fifteen-thousand to one-millionth of the wave length of a green light. X-rays can pass through materials which light will not pass through because of their very short wave length and consequently high energy. The shorter the wave length, the more penetrating the wave becomes. 

X-rays are produced in an X-ray tube. The air is pumped from this tube until less than one hundred-millionth of the original amount is left. In the tube, which is usually made of glass, there are two electrodes. One of these is called “the cathode.” This has a negative charge. In it is a coil of tungsten wire which can be heated by an electric current so that electrons are given off. The other electrode is “the target,” or “anode.”

The electrons travel from the cathode to the target (anode) at very great speeds because of the difference between the cathode and the target. They strike the target at speeds that may vary from 60,000 to 175,000 miles per second.

The target i.e. anode is either made up of a block of tungsten or a tungsten wheel, and it stops the electrons suddenly. Most of the energy of these electrons is changed into heat, but some of it becomes X-radiation, and emerges from a window at the bottom as X-rays.

Have you ever wondered how X-ray pictures are taken of bones in your body?  The Ex-ray “picture” is a shadowgraph or shadow picture. X-rays pass through the part of the body being X-rayed and cast shadows on the film. The film is coated with a sensitive emulsion on both sides. After it is exposed, it is developed like ordinary photographic film. The bones and other objects that the X-rays do not pass through easily cast denser shadows and so show up as light areas on the film.

Today, X-rays play an important part in medicine, science and industry and are one of man’s most helpful tools…

All electromagnetic waves represent an electromagnetic spectrum. At the extreme left end or so called short end of this spectrum are present the gamma rays of extremely high frequency and short wavelength.

It is customary to describe these short wavelengths in units of Angstroms. One Angstrom is equal to 10^-8 cm. Gamma rays are shorter than 0.03 Angstroms.

Next to the gamma rays while moving right of the EM-spectrum, comes the X-rays which are being classified as hard & soft X-rays respectively. The shorter X-rays described as “hard”, fall in the range 0.03 to about 0.6 Angstroms. The longer X-rays, described as- “soft”, range from about 0.6 to about 100 Angstroms. These can penetrate the flesh but not bone. They are used in dental X-ray photography and to inspect welded joints and castings in industry.

X-rays then grade next in order and at the third place, into ultraviolet rays which extend to about 4000 Angstroms. They can be detected just beyond the violet end of the visible spectrum of sunlight or can be detected as the radiations from a filament lamp using fluorescent paper because, this paper can absorb energy from UV radiation and re-radiates them as visible light so that they can be seen to glow brightly.

Ultraviolet radiations even also cause teeth, finger nails, fluorescent paints and clothes washed in some detergents to fluoresce.

Ultimately after the UV, next in order at fourth place comes our very own visible spectrum also called as white spectrum.

For the visible light spectrum, it is customary to switch to a longer unit of length what we call as micron. One micron equals 10^-4 cm: and one micron in terms of Angstroms thus equals, ten thousand Angstroms. The term micrometer can also be used in place of micron.

The visible light portion of the spectrum begins with violet at 0.4 microns. Colors then grade successively through blue, green, yellow, orange and red, reaching at the end of the visible spectrum at about 0.7 microns.

Next in the spectrum after the visible light, comes the infrared (IR) region consisting of wavelengths starting from about 0.7 microns to about 300 microns.

Our bodies detect infrared radiation by its heating effect. Anything that is not but not glowing (i.e. below 500⁰C) emits infrared radiations alone. Special photographic films detect infrared radiation and can take pictures in the dark. Infrared lamps help to dry paint on cars, and are used in the treatment of muscular complaints. A keypad (TV-Remote) for the remote control of a television set contains a tiny infrared transmitter.

Infrared rays at 6^th.place then grade into yet still longer wavelengths as compared to IRs, called as microwaves. The microwave region is in between about 0.03 cm to about 1 cm.

 Within the microwave region is the radar region beginning at about 0.1 cm and extending to about 100 cm.

Frequencies at which radar systems operate grade into television and radio frequencies, the latter extending into wavelengths exceeding 300 m. They are radiated from aerials (antennae) and used to ‘carry’ sound, pictures and other information over long distances. Long, medium and short waves (2 km to 10m) can bend (diffract) round obstacles and so can be received even if, say a hill or a tower is in their way.

At the same time, they are also reflected by the layers of electrically charged particles in the upper atmosphere (the ionosphere), thus, making long distance reception possible, although, the signals received far off may be weak because of their slight absorption by the ionosphere. At night radio reception is better because, the ionosphere is more settled in the absence of sunlight… (Question asked in GS-pre-2011).

Connecting concepts: Electromagnetic waves in the service of mankind!

Gamma rays: In radio-diagnosis and radiotherapy

VHF (very-high-frequency) and UHF (ultra-high-frequency) waves: They have shorter wavelengths and need a clear, straight-line path to the receiver. Local radio and television use them. They pass through the ionosphere, and can be received only over a limited range. The earth’s curvature limits the range of reception. Satellites help to overcome this shortcoming.

Microwaves (with wavelengths of a few cm): They are used for radar and also for international (as well as national) telephone and television links. The international links are via geostationary communication satellites. Signals from the earth are beamed by large dish aerials to the satellite where they are amplified and sent back to a dish aerial in another part of the world.

Connecting concepts: How does a Radar Work?

Radar sets differ in design and purpose, but they all operate on the same general principles. All radars produce and transmit signals in the form of electromagnetic waves, that is, related patterns of electric and magnetic energy. Radar waves may be either radio waves or light waves. Almost all radar sets transmit radio waves. But a few called optical radars or laser radars send out light waves. When the electromagnetic waves transmitted by a radar set strike an object, they are reflected. Some of the reflected waves return to the set along the same path on which they were sent. This reflection closely resembles what happens when a person shouts in a mountain valley and hears an echo from a nearby cliff. In this case, however, sound waves are reflected instead of radio waves or light waves. The waves transmitted by radar have a definite frequency. The frequency of such a wave is measured in units called megahertz (MHz). One megahertz equals 1 million hertz (cycles per second). Radio waves have lower frequencies i.e. higher wavelengths than that of the light waves. Most radars that transmit radio waves operate at frequencies of about 5 to 36,000 MHz. Optical radars operate at much higher frequencies. Some generate light waves with frequencies up to 1 billion MHz. In many cases, radar sets designed for different purposes operate at different frequencies.  

Microwaves are also used for cooking since like all electromagnetic waves, they also have a heating effect when absorbed.

Remote sensing makes extensive use of the ultraviolet, visible, infrared, microwave and radar portion of the spectrum.

The LIGHT in everyday Physics

While it has now been established more than once that light carries a dual nature and travels in a straight line in a homogenous medium, a property what we call as the “rectilinear propagation of light”. The straight line path traveling of light can be observed in the shafts of sunlight that come through a rift in the clouds or for that matter, in the sharpness of the shadows cast by an obstacle when placed in the light coming in from a small concentrated source.

At the same time, it has also been observed that whenever a ray of light travels from one medium to another and reaches at a surface that separates these two media say, for example, glass and water or water and air, some of the light from the said ray of light may be reflected from the surface, some of it may pass through the surface into the second medium and suffers refraction and of course, some of the light from the given light ray would be absorbed by the medium molecules present at the surface as well as within the medium. This is how we say that light always shows the characteristics of reflection, refraction & absorption and thus, accounts for several natural phenomena happening in the nature; some of which, the man has even capitalized on and may be reflected in what we call as everyday Physics…

I) REFRACTION of Light:

In a homogenous medium such as glass or water, light travels in a straight line path, but when light passes from one optical medium to another, it is deviated from its original path. This bending of light when it passes from one medium to another is called refraction of light. In short, refraction is nothing but a change in the direction of travel of a light when it travels from one medium to another say for example from air to water. Refraction is essentially a surface phenomenon. When a ray of light travels from a rarer medium to an optically denser medium (air to glass, for example), the ray bends towards the normal. Conversely, a ray passing from denser to a rarer medium (glass to air), it bends away from the normal. The ray which is incident normally, does not suffer refraction just because, the incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane. Noted that the perpendicular to the very point where a ray of light strikes is called the normal and the angle thus formed between the incident ray and the normal is referred to as the angle of incidence.

Refraction can be explained in terms of change in the speed of light when it passes from one medium to another.

Examples OF REFRACTION in Live Action: Why does a pencil appear bent when placed in a glass of water?

To observe refraction, place a pencil in a glass of water and then look at the pencil from the top and one side. The pencil appears bent at the water surface. The light from the top part of the pencil comes directly to your eyes. The rays from the bottom part pass through the surface between the water and the air and thus suffer refraction and appear bent. Similarly refraction can also be seen in a pool of water whose apparent depth seems to be less than its actual or real depth and so is the case when we see something say, a coin lying at the bottom of a pool or pond that appears to be at a lesser depth than it actually does. The rationale being that the rays of light from the said coin at the bottom of a pool are refracted away from the normal at the surface of water as the same are passing from a denser (water) to a rare (air) medium and thus, when these ray of light enter the eye, they appear to come from a point little above than the actual point of depth.

Connecting concepts: Why do the Stars twinkle?

 Interestingly this phenomenon is again explained by the refraction of light that occurs in the atmosphere. Since, the atmosphere consists of a number of layers of air of different densities. The rays from the stars are continually being refracted, before reaching the eyes of the observer. Since the very fact that the layers of atmospheric air are not stationary and hence, the image of the star is always appear at different points after very shortly intervals. These different images of the star give an impression to a fixed observer that the star is twinkling. On the other hand, the planets being nearer do not twinkle because, the amount of light received from them is greater and so the variation is not very appreciable. Atmospheric refraction also accounts for the sun’s visibility for some minutes after its setting down in the horizon.

II) REFLECTION of Light: The reflection of light is governed by two most important laws of reflection:

First: That the angle of incidence (i) always equals the angle of reflection.

Second: That the incident ray (i), the reflected ray (r) and the normal all lie in the same plane meaning thereby that they all can be drawn on a flat sheet of paper.

The reflection may be a regular reflection or an irregular one. When light reflects from a smooth surface, all of its rays reflect in the same direction and thus, describe a regular reflection. On the other hand, when light reflects from a rough surface, the rays reflect in many different directions just because, the normals at all spots on the surface may be pointed in many different ways and hence, describe diffuse or irregular reflection. This is how exactly, you can see your image in a mirror, but can’t in a sheet of a writing paper!

Connecting concepts: Why do we “silver” an image viewing mirror?

This is to ensure that the ‘silver’ at the back of the glass should act as the reflecting surface to enable us to view ourselves in the said mirror. This “silvering” of a mirror is done by depositing a thin layer of metal called silver, but most commonly; this is done by an amalgam consisting of a tin foil and mercury followed by painting the surface. This does not however mean that the ordinary un-silvered glass surface can not reflect the light. It does reflect some and can be evidenced at night in a well lit room in which you might be sitting wherein, the interiors of your room can be seen reflected in the window panes. This further explains as why do the reflection of Sun in a lake is not extremely bright when the sun is overhead, but it gets too dazzling when the Sun is low in the sky. Just because, when the sun is overhead, the rays of light from it fall perpendicularly down in the lake with a very small angle of incidence such that barely 8% of the incident light is reflected. However, when the sun is low in the sky, the rays of light fall with considerably a large angle of incidence & almost entire light is reflected down in the lake.

TOTAL INTERNAL REFLECTION: Rationale behind the sparkling of diamonds & glitter of air bubbles in water:

When light passes at small angels of incidence from a denser to a less dense medium (from glass to air, for example) a strong refracted ray emerges into the less dense medium and at the same time, a weak ray is reflected back into the denser medium. Increasing the angle of incidence i.e. (the angle between the incident ray and the normal), increases the angle of refraction. At a certain angle of incidence, called the critical angle c, the angle of refraction is 90⁰. For angles of incidence greater than c, the refracted ray disappears and all the incident light is reflected inside the denser medium. The light does not cross the boundary and is said to suffer internal reflection.

Total internal reflection causes a diamond to sparkle and air bubbles in water to glitter.

Connecting concepts: How does a Mirage occur? A mirage is caused by the total internal reflection of light at layers of air of different densities. In the desert, the sand becomes very hot during the day time and it heats the layer of air which is in its immediate contact. The layer of air that is in close contact with the land surface on being heated expands and thus, its density decreases. As a result, the successive upward layers are denser than those below them because of their being cooler than the layers below. When a beam of light traveling from the top of a tree enters a rarer layer, it is refracted away from the normal. As a result, at each surface of separation of successive layers of air, the angle of incidence increases and ultimately, a state is reached when the angle of incidence becomes greater than the critical angle between the two layers. At this position the incident ray suffers total internal reflection and is directed upwards. When this reflected beam of light enters the eye of an observer, an inverted image of the tree is seen and the sand looks like a pool of water to the said observer. On hot summer days, similar mirages are seen by motorists even on the roads.

Looming is a similar phenomenon: In this case the air closer to the ground being much colder than the air above, the rays are bent in such a way that they enter the observer’s eyes above the line of sight. The objects in the circumstances seem to be floating in the air.

Where do we see the world’s best Mirages?

Imagine a wanderer in the desert, dying of thirst. He looks off into the distance and sees a vision of a lake of clear water surrounded by trees. He stumbles forward until the vision fades and there is nothing but the hot sand all around him.

The lake he saw in the distance was a mirage. What caused it? A mirage is a trick Nature plays on our eyes because of certain conditions in the atmosphere. First we must understand that we are able to see an object because rays of light are reflected from it to our eyes. Similarly, the colour of an object is actually the colour of the visible spectrum that is being reflected by it towards us. Usually, these rays reach our eye in a straight line. So if we look off into the distance, we should only see things that are above our horizon.

Now we come to the tricks, the atmosphere plays with rays of light. In a desert, there is a layer of dense air above the ground which acts as a mirror. An object may be out of sight, way below the horizon. But when rays of light from it hit this layer of dense air, they are reflected to our eyes and we see the object as if it were above the horizon and in our sight. In reality, we are really “seeing” objects which our eyes cannot see! When the distant sky is reflected by this “mirror” of air, it sometimes looks like a lake.

 Another similar mirage is created before our eyes when on a given hot day, as you approach the top of a hill, you may think the road ahead is wet. This is a mirage, too! What you are seeing is light from the sky that has been bent or so called reflected by the hot air just close to the pavement so that it seems to come from the road itself.

Mirages occur at sea, too, with visions of ships sailing across the sky! In these cases, there is cold air near the water and warm air over it. Distant ships that are beyond the horizon can be seen because the light waves coming from them are reflected by the layer of warm air and we see the ship in the sky!

One of the most famous mirages in the world takes place in Sicily (Italy), across the Strait of Messina. The city of Messina is reflected in the sky, and fairy castles seem to float in the air. The Italian people call it “Fata Morgana”, after Morgan Le Fay, who was supposed to be an evil fairy who caused this mirage.

Thus, the bottom line about the occurrence of mirages in nature is that in Physics parlance, they are nothing but the illusions created by the reflection of light…

Light pipes or optical fibres Light can be trapped by total internal reflection inside a bent glass rod and ‘piped’ along a curved path. A single, very thin fibre of very pure (optical) glass behaves in the same way. If several thousand such fibres are taped together a flexible light pipe is obtained that can be used (by a doctor or an engineer, for example) to light up some awkward spot. If necessary a second bundle of fibres carries back the image for inspection. A very recent use of optical fibres is in telecommunication, for carrying pulses of light from a laser which represent information such as telephone conversation, television pictures and computer data. An optical fibre has a much greater information-carrying capacity than a copper cable of the same thickness carrying an electric current, as well as being thinner and lighter.

III) DISPERSION of Light AND Physics of COLOURS: What is dispersion? Dispersion is simply the splitting of ordinary light (White light) or visible light in a spectrum of seven distinct colors starting from violet at one extreme end to the red at the other and may be acronymed as VIBGYOR.

An experiment was performed by Newton in 1666. He allowed a narrow beam of sunlight (which is white) to fall on a triangular glass prism. It produced a band of colours (called a spectrum) on a white screen. The effect is known as dispersion and Newton concluded that white light is a mixture of many colours of light, which the prism separates out because the refractive index of glass is different for different colours; the refractive index is greater for violet light (since it is refracted the most) and least for red light.

A pure spectrum is one in which the colours do not overlap, as they do when a prism alone is used. Therefore, a convex lens has to be used to focus each color distinctly.

Connecting concepts: The dispersion of light & RAINBOW:

What is a Rainbow?

A rainbow is one of the most beautiful sights in nature, and man has long wondered what makes it happen. Even Aristotle, the great Greek philosopher, tried to explain the rainbow. He thought it was a reflection of the sun’s rays by the rain, and he was wrong!

Sunlight or ordinary white light is really a mixture of all the colors. You’ve probably seen what happens when light strikes the beveled edge of a mirror, or a soap bubble. The white light is broken up into different colors. We see red, orange, yellow, green, blue and violet. An object that can break up light in this way is called “a prism.” The colors that emerge from a band of stripes, each color grading into the one next to it. This band is called “a spectrum.” A rainbow is simply a great curved spectrum, or band of colors, caused by the breaking-up of light which has passed through raindrops. The raindrops act as prisms here in this case.

A rainbow is seen only during showers, when rain is falling and the sun is shining at the same time. You have to be in the middle with the sun behind you and the rain just in front of you or you can’t see a rainbow! The sun shines over your shoulders into the raindrops, which break up the light into a spectrum or band of colors. The logic is that the sun, your eyes and the center of the arc of the rainbow must all be in a straight line!

If the sun is too high in the sky, it’s impossible to make such a straight line. That’s why rainbows are seen only in the early morning or late afternoons or when the sun is setting. A morning rainbow means the sun is shining in the east; showers are falling in the west. An afternoon rainbow means the sun is shining in the west and rain is falling in the east.

Therefore, in Physics terms, we can say that rainbow is one of the most commonly observed examples of dispersion of light. It is formed when sunlight passes through myriad droplets of water suspended in the air after a shower. An arc of the spectrum colors is produced as a result of refraction and total internal reflection of rays of sunlight passing through raindrops. A bright, primary bow with violet on the inside of the arc is produced as a result of one total internal reflection. A dimmer, secondary bow with violet on the outside of the arc may also be seen at times. This is caused by two internal reflections within the raindrops. Noted that a rainbow could only be seen when the sun is low in the sky and behind the observer.