Einstein’s Theory of Photoelectric Emission

In 1905, Einstein proposed a simple but revolutionary explanation for the photoelectric effect. He assumed that light consists of bundles of energy, called photons and viewed photoelectric effect as a collision between a photon and a bound electron. The energy E of a single photon is given by   E = hv

                                  Robert A. Millikan (1868-1953)

Robert Andrews Millikan was born on March 22, 1868 in U.S.A. During his undergraduate course, his favorite subjects were Greed and Mathematics. But after his graduation in 1891, he took, for two years, a teaching post in elementary physics. In this period, he developed interest in the subject. He received his Ph. D. (1895) for research on polarization of light emitted by incandescent surfaces.

Millikan spent a year (1895-1896) in germany, at the Universities of Berlin and Gottingen. He returned at the invitation of A.A. Michelson to take appointment as his assistant at the newly established Ryerson Laboratory at the University of Chicago (1896). He became professor at that University in 1910, a post which he retained till 1921. As a scientist, millikan made numerous momentous discoveries in the fields of electricity, optics, and molecular physics. His earliest major success was the accurate determination of the charge carried by an electron, using the elegant “falling-drop method”. He also proved that this quantity was a constant for all electrons demonstrating the quantized nature of charge.He also verified experimentally Einstein’s photoelectric equation, and made the first direct photoelectric determination of Plank’s constant h. thoughout his life, Millikan remained a prolific author, making numerous contributions to scientific journals. He was awarded the Noble Prize in Physics in 1923.

The Earth’s Atmosphere

The earth is surrounded by a layer of air, which is called the atmosphere. It reaches to 560 kilometres from the surface of the Earth, so we are only able to see what occurs fairly close to the ground. The atmosphere consists  about 77% nitrogen, 21% oxygen, with traces of argon, carbon dioxide and water. The atmosphere of the Earth can be divided into several distinct zones, as shown in the figure.

The troposphere is where all weather takes place; and extends from 8 to 14.5 kilometres above the surface of the Earth. The air pressure at the top of the troposphere is only 10% of that at sea level. Above the troposphere is the stratosphere and extends to 50 kilometres, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has a high concentration of ozone. This layer primarily important as it absorbs the ultraviolet radiation from sunlight. The mesosphere is above the stratosphere and extends to 85 kilometres. Above the mesosphere is the thermosphere (or ionosphere) and extends to 600 kilomettres, where many atoms are ionized. The ionosphere is  very thin, but it is where aurora takes place, and is also responsible for absorbing the most energetic photons from the sun. the exosphere starts at the top to the thermosphere and continues until it merges with interplanetary gases, or space.

TIDE

As the moon travels around the earth and as they, together, travel around the sun, the combined gravitational forces cause the world’s oceans to rise and fall, which is called tide. The above figure shows how tide develops. The areas of high water levels are high tides and the areas of low levels are low tides. Tides vary from day to day and also vary from place to place.

Moon data

Distance from earth: 384,199km.

Mean diameter: 3, 473km.

Mass: approx 1/81 of that of Earth

Surface gravity: 1/6 of the of Earth

Atmosphere: none, hence no clouds, no whether, no sound.

Diurnal range of temperature at the Equator:200⁰c

ECLIPSE

When the moon moves between the sun and the earth, the moon casts a shadow that touches the surface of the earth, either completely or partly, is called solar eclipse. This can only happen at new moon. The given figure illustrated the solar eclipse. The shadow cast by the Moon can be divided into the completely shadowed umbra(U) and partially shadowed penumbra(P). when the moon moves through the shadow caste by the earth is called lunar eclipse. The lunar eclipse only happens at full moon.

 

Total eclipses occur where the light source is totally blocked off by the eclipsing body. Partial eclipses occur at places where only part of the luminary is covered (solar eclipses), or when only part of a body is eclipsed by the shadow (lunar eclipses).

The Moon

The Moon is the only known natural satellite of the Earth. The moon rotates slowly and completes one turn on its axis in 27 days. 7hours, and 43minutes. The spots on moon surface as seen from the earth are craters and basins which are formed by meteorite’s impact. The moon does not have any atmosphere, so there is no weather as we find on the earth. The moon does not produce its own light, but looks bright because it reflects light from the sun.

He lunar phase changes as the Moon revolves around the earth and different portions of its surface are illuminated by the Sun. change in the shape of the moon arises from its changing position in relation to the Earth. The given figures show the phases of the Moon. The interval between one full Moon and the next is about 29.5 days. All the phases of the Moon have special names which indicate how much of the illuminated Moon can be seen from the Earth, and whether this part is going to grow or shrink.

Euygene Thomas Young

                                                       Euygene Thomas Young (1773-1829)

Born on 16 June, 1773, Euygene Thomas Young will always be know for his study on the human ear, the human eye, how it focuses and on stigmatism. His research on colour vision. Working on human ears and eyes, he dedicated much time to the speed of sound and light. He knew that if two sound waves of equal intensity reached the ear 180⁰ out of phase, they cancelled out each other’s effect and no sound was heard. It occurred to him that a similar interference effect should be observed with two light beams, if light consisted of waves. This led young to devise an experiment, now commonly referred to as the young’s double-slit experiment.

In his later years, young devoted most of his time deciphering the Egyptian hieroglyphics found on the Rosetta stone discovered in the Nile Delta in 1799. 

                                      

C.V. Raman

                                                                    C.V. Raman (1888-1970)

Chandra shekhar  Venkat Raman is the only Indian naational to receive Nobel prize (1930) in physics till date. His love for physics was so intensse that he resigned his job of an officer in Indian finance department and accepted the post of Palit Professor of Physocs at the Department of Physocs, Calcutta University. His main contributions are : Raman effect on scattering of light, molecular diffraction of light, mechanical theory of bowed strings, diffraction of X-rays, theory of musical instruments and physics of crystals.
As Director of Indian Institute of Science, Bangalore and later as the founder Director of Raman Research Institute, he did yeoman's to Indian science and put it on firm footings in pra-independence period.
1. Blue Colour of the Sky: we know that scattering of light by air molecules, water droplets or dust particles present in the atmosphere can be explained in accordance with Rayleigh's law. The shorter wavelengths are sscattered more than the longer wavelengths. Thus, the blue light is scattered almost six times more intensely than the red light as the wavelength of the blue light is roughly 0.7 time that of the red. the scattered light becomes rich in the shorter wavelengths of violet, blue and green colours. On further scattering , the violet light does not feach observe's eye as the eye is compartatively less sensitive to violet then blue and other wavelengths in its neighbourhood. So when we look at the sky far away from the sun, it appears blue.
2. White colour of the clouds: the clouds are formed by the assembly of small water drops whose sixe becomes more than the average wavelength of the visiible light (5000A). These droplets scatter all the wavelengths with almost equal intensity. The resultant scattered light is therefore white. So, a thin layer of clouds appears white.

3. Red colour of the sun at sunrise and Sunset: we are now able to understand the red colour of the sun at sunrise and sunset. In the morning and evening when the Sun is near the horizon, light has to travel a greater distance through the atomosphere. The violet and blue wavelengths are scattered by dust particles and air molecules at and angle of About 90⁰ . Thesunlight thus becomes devoid of shorter wavelengths and the longer wavelenght of red colour reaches the observer. so the sun appears to us as red.