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.