Why is the sky blue?

The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.

Universal Law of Gravitation

"Every object of the universe attracts other object .the force of attraction between them is directly proportional to product of their masses and is inversely proportional to the square of distance between them.this force is along the line joining the centre of both the objects"

Mathematical form of Newtons Universal Law of Gravitation

F (gravitational force acting) = G*m1*m2/r^2

where G is the constant , m1 and m2 are masses of the two bodies and r is the distance between them. so from this we can assume that

G= F*r^2/m1*m2

Now as the mathematical form of law of gravitation remains same at any place in universe and we derive gravitational constant from that law, so it is also constant all over universe .

Henry Cavendish found the value of G which is 6.67*10^-11.

what is the technical difference between "Gravity" and "Gravitation"?

They are often used interchangably, but gravitation often refers to the theory or concept of gravity in general, and is more often than not used in its adjectival form gravitational. Gravity often refers the gravitational field itself (a vector quantity or its magnitude). For example, being on earth, you are subject to the force of gravity. Your weight is a gravitational effect. Also, gravity tends to be the term of choice in Newton's (field) theory of gravity. Gravitation tends to be the term of choice in general relativity, in which there is no gravity field, per se.

What is Insulation Resistance, Dielectric Absorption, Polarization Index

INSULATION RESISTANCE
 Insulation Resistance is the resistance offered by insulator separating two electrodes. The value determines quality of insulation.

POLARIZATION INDEX
The ratio of the ten-minute to the one-minute measurement is known as the Polarization Index. In general, a high ratio is indicative of 'good' insulation.

DIELECTRIC ABSORPTION
Carrying out a 10-15 minute test will yield some meaningful information. Clean, dry insulation will show an increasing resistance reaching a final steady-state level, after 10 to 15 minutes. Wet or dirty insulation will reach a steady state value much sooner.

Units of Radiation

The roentgen was one of the earliest (1928) units used to measure exposure and measures
the amount of ionization produced in air by X-ray or gamma rays. By definition,
1 R = 2.58 × 10^-4 coulombs/kilogram at standard temperature and pressure. It is important to note that the unit of exposure is defined only for X or gamma rays and only for exposure in air. Thus, the
roentgen is a measure of the ability of photons to ionize air. One roentgen (R) is a rather large
exposure, so frequently the milliroentgen (mR) unit is used (1000 mR = 1 R). Many Geiger
counters and most ion chambers are calibrated to read out in terms of exposure rate, e.g.,
milli roentgen per hour (mR/hr). There is no international system (SI) unit for exposure.
Radiation deposits energy when absorbed by matter. This energy deposition leads to the
biological effect of ionizing radiation. By definition, the absorbed dose is the energy deposited
per unit mass. The unit of radiation absorbed dose is called the rad. One rad represents the
deposition of 100 ergs per gram of material. Unfortunately, the rad is difficult to measure and
must often be calculated from other measurements. However, for radiation protection purposes,
1 R = 1 rad, for X-rays or gamma rays. But, for beta radiations, 1 R equals some constant times
1 rad, where the constant is dependent upon the beta energy. For example: 1 R ≅ 2.6 rad for
P-32. Therefore, beta measurements with survey meters which read in R/hr or mR/hr must be
interpreted with care. The international system (SI) unit for absorbed dose is the gray (Gy) –
one Gy = 100 rad.
The ultimate aim of a dose measurement system, from a radiation safety viewpoint, is to arrive
at a quantity appropriate for predicting biological response independent of the source of the
radiation. This goal is only partially achieved with the rad. For example, the biological effect is
much greater for alpha radiation than for beta or gamma radiation for a given absorbed dose (in
rad) to the biological system. This difference in the biological effectiveness of the radiation has
been attributed primarily to the fact that alpha radiation releases more energy and ionizes more
particles per unit path length traversed than beta or gamma although the path length is shorter
than that for beta or gamma radiations. In order to account for the different biological effects of
different radiation types, a quality factor (QF) has been introduced which is used to convert the
absorbed dose to a dose equivalent. Quality factors are shown in slide 18. The unit of dose
equivalent is the rem (Roentgen - Equivalent - Mammal.).
Most biomedical research laboratory situations involve only beta, gamma, or X-ray radiations.
For these radiations, the QF is equal to 1 so the dose equivalent (in rem) is equal to the
absorbed dose (in rad). The dose to tissue in air from exposure to one Roentgen is about 0.95
rad. Therefore, for X-ray and gamma radiation exposure, the following expression
approximates the dose equivalent:

1 R ≈1 rad = 1 rem [Low LET radiation]

One rem is a large unit; so, we usually work in terms of millirem (mrem),
where 1000 mrem = 1 rem. The SI unit for rem is the sievert (Sv) – one Sv = 100 rem