Light 5

Q.1. What is meant by Refraction? Why does this take place ?
A. The bending of light rays, as it travels from one medium to another having different refractive indices, is called refraction of light.
The phenomenon of refraction takes place because of difference in the speed of light in different media.
For eg, Light travels with 3x10^8 m/s in vacuum or air , 2x10^8 m/s in glass and 2.25x10^8 m/s in water.

Q.2. Is optical density and density same ? Explain.
A. No. Optical density and density are not the same physical quantities.
Optical density is measured by the speed of light in that medium, whereas, density is the mass contained per unit volume.
For eg, Kerosene floats above water suggesting that it is lighter than water, i.e. its density is lower than that of water. But the refractive index of kerosene is 1.44 whereas that of water is 1.33, suggesting that kerosene is optically denser than water.

Q.3. What are the factors on which refractive index of a medium depends ?
A. The factors on which refractive index of a medium depends are :
i. Nature of the medium, in other words its optical density. More is the optical density, more is its refractive index, and hence, less is the speed of light in it.
ii. Colour of the light used or the wavelength. Red light, having largest wavelength in the visible region, shows lowest refractive index and hence, bends less. Violet, having shortest wavelength, shows highest refractive index, and hence, bends more.
iii. Temperature. Due to increase in temperature the density of the medium decreases, so light moves fast through it. As a result decreases.

Light 4

Characteristics of images formed in a plane mirror :
1. Size of the image is always equal to the size of the object. Hence, its magnification is unity.
2. The image formed is laterally inverted. When you stand in front of the mirror, your right hand appears left and your left hand appears right.
3. The image formed is as far behind the mirror as the object is in front of it. The image distance is equal to the object distance.

If the mirrors are not plane surfaces then they are called spherical mirrors. There can be two types of spherical mirrors : Concave and Convex.
Few definitions related to spherical mirrors are :
1. Centre of curvature: The centre of an imaginary sphere of which the mirror is a part.
2. Radius of curvature : The radius of the hollow imaginary sphere of which the mirror is a part. It is denoted by R.
3.Pole : The central point of the spherical mirror.
4. Focus : When parallel beam of light is incident on a spherical mirror, then after reflection the rays meet at this point in case of concave mirror, and appear to diverge out from the point in case of a convex mirror, then that point is called the Focus.
5. Focal Length : The distance between the pole and the focus is called focal length. Generally, it is denoted by f , and the relation f = R/2 always holds true.
6. Magnification : The ratio of the size of the image to the size of the object is called magnification. It is denoted by m.

Light 3 - Reflection

The phenomenon of rebouncing of light rays into the same mediumon striking a surface is called Reflection of light.

Laws of Reflection
Following are the two laws of reflection of light:
1. The incident ray, the reflected ray and the normal at the point of incidence all lies in the same plane.
2. The angle of incidence is always equal to the angle of reflection at the point of incidence.

Angle of incidence : It is the angle which the incident ray makes with the normal at the point of incidence.
Angle of Reflection : It is the angle which the reflected ray makes with the normal at the point of incidence.

Types of Reflection :
1. Regular Reflection : When a parallel beam of reflected light rays is obtained when parallel light rays are incident on a smooth reflecting surface, then that is called Regular Reflection.
Example: Light rays falling on a mirror gets reflected back.
2. Irregular or Diffused Reflection : When a non-parallel beam of reflected rays are obtained when parallel incident light rays fall on an uneven surface , then that is called Irregular or Diffused Reflection.
Example: Light rays falling on a wall.

Note: The Laws of Reflection holds true for both regular and irregular reflection. Incase of irregular reflection, however, the laws hold true for each point of the incident beam of light.

Light 2

Objects that emit light of their own are called Luminous objects. For example - Sun, stars etc. Whereas objects that do not emit light but are visible because light from other sources fall on them are called Non-luminous objects. For example - moon, chair, table, wall etc.
If light rays coming from an object meet or appear to meet at a point after reflection or refraction, then that point is called image of the object. If the rays actually meet then that is called a real image, and if the rays appear to meet then that is a virtual image.

Light 1 : Introduction

Light is an electromagnetic radiation that helps in the sensation of sight.The different properties of light can be understood only if we consider its dual character - wave and particle.
Many optical phenomena can be understood if we consider light as waves. Some characteristics of light waves are -
1. They are electromagnetic in nature. They consist of oscillating electric and magnetic fields at mutually perpendicular planes in same frequency.
2. They do not require any material medium for their propagation. Light from Sun reaches Earth travelling vast expanse of space where nothing exists.
3. Light waves travel with a speed of 30,00,00,000 m/s in vacuum or free space.
But, the speed of light decreases when it travels through a medium of higher density.
A light wave propagates from source to destination in one single straight line, which we generally refer to as a 'Ray'. A collection of such rays make a beam of light

Attempts to watch the heavens..

The night sky, with its large number of twinkling stars, have always fascinated mankind. Since the dawn of civilization, attempts were made and further improvements followed for making that perfect instrument for observing the stars, the planets and other celestial objects. What followed was a series of developments in the area of Optics, to achieve that perfect instrument, which now has lead to building of telescopes and satellites.

In 1300, Greek philosophers and astronomers already knew about the properties of lenses. They even tried to manufacture lenses, without bubbles, that could be used for reading glasses. What followed was arrival of eyeglasses in the cities like Florence and Venice. But they were not for both eyes, they were small lenses mounted on small cylindrical cases, that nowadays you can see used by jewelry-makers.

In 1608, the govt. of Netherlands asked the manufacturers to make an instrument that would help in seeing far-away objects clearly. The govt. would take patent for the instrument. What followed, was a man named Jacob Metins provided the basic mechanism, using only two lenses. Hans Lipperhey designed the instrument, but the mechanism was so simple that the govt. didn't give any patent for this instrument - the instrument that we call Binocular these days. Till now the stars were heavenly objects whose patterns and brilliance was believed to be ruled by Gods. But this instrument shifted the authority of what we observe, from their hands!

Galileo's Telescope

By that time, in around 1543, a brilliant man named Nicholus Copernicus had already told the concept of heliocentricity - the fact that it was sun at the centre and Earth rotates around it. The theory challenged the words of the Bible and was controversial in its time. Galileo Galilei, in 1609, made his first refractive telescope for his personal use. He used a concave lens as the eyepiece, and a convex lens as the objective. He allowed his patrons to use this, but was not satisfied as the glass used in the lenses had green tint and bubbles. As a result, the images obtained were blurred and had colourful haloes. But the positive aspect was that he achieved 9 times magnification.

Galileo's model was modified by Johannes Kepler in 1611. Instead of two different lenses ; he used two convex lenses. The result was high magnification with better clarity of the image. Though the image was upside down, it didn't matter for observing celestial objects. This was the first telescope.

Spherical and chromatic aberration still posed a problem. The craftsmen tried to get better image quality by decreasing the curvature of the lenses, thereby, increasing the focal length. As a result, the telescopes became longer in size.Gradually telescopes of length 15-20 ft became very common. The most prominent work done; was by Havelius in 1673.

Havelius telescope

Huygen and his brother Constantine achieved 100 times magnification through their telescope measuring 23 ft, in 1656. They got to observe the great Orion nebula.

Shaping up of Modern World

About hundreds of millions of years ago, when the Earth was formed, the Earth was a hot ball of molten substances containing everything that we see around us today. But gradually, with time, the upper surfaces began to cool and solidify. This process of cooling continued for millions of years forming the upper surface of the Earth - the Crust. The crust, thus, floats on semi-molten layers of elements and rocks of the mantle. This layer is also called Asthenosphere where the radioactive processes still continue and the heat generated here comes towards the surface of the Earth in Convection currents.

Convection currents within Earth

It is believed that there were two large blocks of land formed from a single large block the Pangea - the Laurasia plate(Sometimes called Angaraland) and the Gondwanaland. While the former was in the Northern hemisphere the Gondwanaland was in the southern hemisphere. The convection currents broke and drifted these large chunks of land according to the Continental Drift Theory. These gave rise to formation of different tectonic plates. These tectonic plates further collides and slide against each other giving rise to major earthquakes and large sea waves. The evidence of a single large landmass is provided by the fact that the soil types and kind of fossil that are found on both the sides of the Atlantic Ocean.

Pangea

Different tectonic plates

In this way, the modern world map came into existence, along with the 7 major tectonic plates - the North American, the South American, the Eurasian, the African, the Indo-Australian, the Pacific and the Antarctic plates.

Organic Evolution

Variation is important and occurs naturally for a species. As was put forward by Darwin's theory, the complex forms of life, that we observe around us today, were actually the result of continuous change and development of simpler forms of life, obviously the process is time consuming and is endless. Speaking lucidly, the gradual change of life forms from simple to complex is known as Organic Evolution.

Embryology is the study of development of an organism from egg to adult. Strikingly it is observed that for different vertebrates, the embryos resemble one another; though they may belong to different species altogether. The fact that the embryos of a fish, a frog, a turtle or a man resemble each other is a direct proof of evolution. We may infer that they have evolved from some common ancestor, over time.

Embryos of different vertebrates

Starting of Life from meteorites

The Andes range in South America is well known as a place for finding meteorites. It is known, that the meteorites were formed actually during the time of formation of solar system about 4.5 billion years ago. They are believed to have remained unchanged since then. So a close study of their constituents and properties actually revealed the nature of the substances that made up the different planets, moons, comets etc. In other words, their nature reveals the form of our planet; in which it existed; when it was first formed.

The Andes mountain range

These meteorites are actually pieces of rock, of different sizes and shapes, that travel through the solar system, and sometimes; they may even actually fall on Earth. Studies reveal that they contain a vital ingredient called amino acids, which are the essential building blocks for proteins. Proteins are vital substances found in all living creatures. The Carbon - another essential ingredient for all living things, is also produced in the heart of a star (as was described in a previous article). 

Section of a meteorite : the yellow points are amino acids

So, everything that we see around us, is actually built somewhere in the space in their most elementary forms. The universe is believed to have been created from the Big Bang, in which matters just burst out into space. Subatomic particles acquired mass in the process - amongst them were the quarks. Gradually as the temperature of the universe decreased, these particles joined to form larger, more complex structures like protons and neutrons. These protons and neutrons again came together; to form the building blocks of all elements - the nucleus. The basic difference in matters, that we see around, is due to difference in the structure of this nucleus. The difference in structure of these nuclei results in 92 different elements that can be found.

Oh ! Dear Gold !

The cosmos never fail in conspiring different ways to amaze us and make us wonder about its different beautiful aspects.

A nebula - the blue lights are the stars that are forming

About 13.5 billion years ago, when our universe didn't take shape that we observe today, there was only gaseous Hydrogen of a nebula from which it was formed. Hydrogen is lightest of all the elements that can be found anywhere. These atoms of hydrogen, or the tiny mass-points, were pulled together by gravity - a force that manifests itself fully with increasing mass. These clusters of mass-points produced hydrogen-clouds. Gradually with increasing conglomeration of more Hydrogen atoms; the gravity increased followed by rise in temperature. The process repeats itself time and again until a massive central core is formed with high density and temperature. The energy of the particles that are pushed towards the centre of the nebula gets converted into heat energy, thereby, increasing the temperature of the core of the nebula. Finally, a temperature of about 10million degrees is reached; when accidentally by the process of nuclear fusion, the Hydrogen at the core gets converted into the next heavier element - Helium. This nuclear reaction is followed by further rise in temperature and the energy is radiated in the form of heat and light. This is how a star is born. This is a thousand times bigger than our sun and is deep blue in colour.

The star that is born

Helium being heavier than Hydrogen sinks down to the centre of the star, the layers of Hydrogen outside the core provides the fuel required to continue the nuclear fusion process at the centre. These hydrogen-clouds are  further used up by the core to produce more Helium and the process is repeated time and again. The mass of the core increases, gravitational pull increases, further increasing the temperature. At such a high temperature, the Helium atoms fuse together forming heavier elements like Carbon(a vital element found in every living thing) and Oxygen. This process of fusion goes on repeating to produce different elements of heavier mass and finally Iron is formed. Now, this iron cannot undergo any fusion reaction further to produce heavier elements. The star which was increasing in size and temperature becomes gigantic and glows with a red colour. This is then called a Red Giant.

Red giant

While the gravitational forces tend to contract the star, the energy released by nuclear fusion taking place at the centre; pushes the matter out. An equilibrium is reached when these two opposing forces balance each other. Then the size of the star remains constant for almost a few billion years.

At a time when all the surrounding Hydrogen  are used up in the core and the star is mostly made of Iron, the nuclear furnace at the centre stops and the equilibrium is lost. Gravity now takes over - squashing all the Iron and other elements that were produced at the centre. This process is accompanied by increase in density as well as rise in temperature to about 100 times the core of our sun! Finally the star collapses and explodes, giving out enormous amount of light - a process called 'death of the star'. The exploding star throwing out large amount of matter and light in the space is a visual treat and is called a Supernova. 

                      Hubble Space Telescope image showing Supernova 1994D in galaxy NGC 4526

From the remnants of this explosion some of the most valued elements are formed! This explosion, for a brief microsecond, forces the Iron to fuse to produce heavier elements - like Gold, Platinum, Lead. So in a way, they are forged in the heart of an exploding star at its time of death!

A chunk of Gold

So, if you have a gold ring; make sure you appreciate it. The metal was built in a blinding flash of light billions of years ago!

Attempts to measure the skies and heavens

The universe contains everything  - that which are known as well as the things that are unknown to humankind. Celestial bodies like the planets, the moon, the stars are all part of the universe. The universe is vast, much larger than we can imagine. During the day, we see the sun, whereas at night we see the moon, stars and some planets ,comets, meteors etc. When we look into the sky, we actually look into the past - for light, being a radiation, has a finite definite speed (299,792,458 m/sec in vacuum if you are interested!). But even at such a high speed it takes time for the light rays to reach our eyes on this planet, after traversing vast expanse of space in between the stars and our planet.

The standard unit of length is metre. But while dealing with huge distances and vast expanse of spaces outside our solar system , metre becomes too small a scale for measurement. To handle the enormous distances involved, a more handy and practical unit of length is chosen. This is called a Light Year (symbol -ly).A light year is the distance travelled by light in one year.Grasp the enormity of this scale - 1 light year = 9.46 trillion kilometres. 1 trillion is obtained by adding 12 zeroes after putting 1 !!

Light always held fascination since the days of earliest forms of humans. Various attempts were made by people belonging to ancient civilizations to track the main source of light in their lives - the Sun , so much so that almost all ancient civilizations have given the status of God to the sun! Their beliefs may have gone with their existence but the earliest attempts and the means to track down the path of the celestial events and heavenly Gods still exits - only if you know where to look for it .

Almost 3500 years ago ( i.e going back to 1500 B.C ) the pharaohs of Egypt built a temple complex called the Karnak Hall to worship the God of all Gods -Amen Ra - the Sun God. The architecture of the temple was  built in a way so that the sun rose from the different doors at different times of the year.

Temple of Amen Ra

The worship reached its peak when the sun rises absolutely in a straight line alignment to the central passage-way, casting no shadow of its pillars in the path of the sun, enlightening the main temple. This happens once in a year (approximately around 21st of December).

Karnak Hall passage overcast by shadows of pillars during anytime of the year

Sunrise at winter solstice in the main axis of the temple

Just imagine the grandiose of purpose of making such a temple architecture.

Another such marvel exists at Chankillo - north western coast of Peru, S.America. About 2500 years ago a fortified temple, consisting of 13 towers, was built in a desert by a civilization of which nothing is known. Its walls were once brilliant white and were adorned with different figures and motifs. 

13 towers of  the temple , Chankillo

The 13 towers form an ancient solar calender. At different times of the year the sun rose in between the gaps of these towers and may be the people of the civilization gathered in the valley to welcome the Sun God and bow before the laws of nature. On 21st of December sun rises from the extreme right tower, and in June the sun rises from the extreme left of the leftmost tower. 

Sunrise at June solstice

The time taken by the sun to rise from a particular tower gave them the idea of measurement of the exact time of the year. All traces of such a cultured civilization is lost with time, but the true purpose of such a structure can be felt even today . It still serves as a calender as it did when it was first built and its stones were first laid down. What a magnificent achievement probably one of the earliest attempts to measure the heavens!

A desire to measure the sky, to predict the course of the path trodden by their God - laid down the foundation of modern astronomy. They understood the fact that it takes some definite span of time for the sun to rise from their familiar place which repeats itself after definite periods of time. These familiar timescales correctly measured the time of passage of their lives and it still can measure the age of modern man.

Space Exploration

Why are we here? Where do we come from? These are some of the most intriguing questions that has always fascinated human mind. It's an essential part of human nature to find answers to such questions. Though we can trace our ancestry back to hundreds of thousands of years - the dawn of humankind, but in reality our story extends far - further back in time - to the beginning of the universe.

The story of creation of this universe is the story of creation of man.

So, its story , is our story .

                                       Our planet - Earth , as it looks from the outer space

The Earth was formed about 4.5 billion years ago along with our solar system.In space randomly drifting molecules of gases (mostly Hydrogen) and dust collect together to form a cloud of colourful splashes in the darkness of the space.This is called a Nebula. The gravitational forces of attraction (due to their masses) helps in contraction of these particles - thereby increasing the temperature and density at the core. When the temperature of the core reaches 10∧7 Kelvin, nuclear fusion (the process of joining together of lighter molecules of elements , releasing vast amount of energy) begins.In this fusion reaction, Hydrogen gets converted into Helium. The large amount of energy released continuously by the starting of the nuclear furnace appears as light. This is called a star.

So, in this way stars are formed from the Nebulae.. and our Sun is one such star.

                                      The Rosette Nebula : Each tiny spot of light is a star

A component of the universe that has a huge group of stars and other celestial bodies bound togther by gravity is called a galaxy. There are over 100 billion galaxies and our sun is one of the ordinary-sized star belonging to the Milky Way galaxy.

                                                                The Milky Way galaxy

The structure of the Earth is deduced from the study of Seismic waves. It was found that our Earth was formed along with other objects of the solar system, some 4.5 billion years ago. The solar system was formed from a cloud of gases and dust particles that contracted to form a spinning disk at whose centre the sun was formed. In the solar system, the sun is the most massive object and around it the dust and gases got cooled forming particles that collected to form the planets, asteroids, moons etc.

                                                                       Our Solar system

Due to the kinetic energy of the constituent particles, the energy released in radioactive decays and their contraction under gravity, the Earth got heated and melted about 800 million years after its formation.

The heavier elements like Iron and Nickel sank to form the liquid core . The outer layer got cooled over time and solidified to form the crust. In between them, a semiliquid mantle was formed. What we see coming out from volcanoes are these inner core materials that come out through some opening on the Earth's crust.

                                                               Different layers of Earth

But how does the gravity comes into existence?
How life started on this planet?

These are some of the questions that fascinates the human mind - the answers are still to be found out exactly. There are many theories , controversies and loopholes in explaining these fundamental questions. Some of these will be dealt in my subsequent articles.

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