In transverse waves, particle of the medium vibrate in a direction perpendicular to the direction of wave motion.

Without an atmosphere the sky appears black, as evidenced by the lunar sky in pictures taken from the moon. But even a black sky has some lightness. At night, the sky always has a faint color, called "skyglow" by astronomers.

'Farad' is the unit of capacitance.

In the case of bodies falling under gravity acceleration remains constant at a given place.

The instrument used to see the distant objects on the Earth is Terrestrial telescope.

In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum was independent of the motion of all observers. This was the theory of special relativity.

Earth's gravity is 6 times moon's gravity. That means moon attracts an object with less force than earth. Hence a person can jump higher on moon than on earth.

The velocity required to place a vehicle in orbit around the moon is less than required to place it in orbit round the earth.

Density is not a vector quantity.

Speed is not a vector quantity. It has only magnitude and no direction and hence it is a scalar quantity.

The Moon has been bombarded with large quantities of Helium-3 by the solar wind.

The period of revolution of a certain planet in an orbit of radius R is T. Its period of revolution in an orbit of radius 4R will be 8T.

A flight data recorder (FDR) is an electronic device employed to record any instructions sent to any electronic systems on an aircraft. Another kind of flight recorder is the cockpit voice recorder which records conversation in the cockpit, radio communications between the cockpit crew and others (including conversation with air traffic control personnel), as well as ambient sounds. Popularly referred to as a “black box” by the media, the data recorded by the FDR is used for accident investigation as well as for analyzing air safety issues, material degradation and engine performance.

A gradual increase in loudness is known as crescendo.

Loudness is a subjective characteristic of a sound (as opposed to the sound-pressure level in decibels, which is objective and directly measurable). Perceived loudness of sound is a psycho-acoustic quantity that depends on sound pressure level, the frequency spectrum, and the time behavior of the sound in question. The sone is a unit of perceived loudness that was proposed by Stanley Smith Stevens in 1936.Being a unit of how loud a sound is perceived, the sone scale of loudness is based on data obtained from subjects who were asked to judge the loudness of pure tones and noise. One sone is arbitrarily set equal to the loudness of a 1,000-hertz tone at a sound level of 40 decibels (Encyclopedia Britannica).

The component used for tuning a radio is basically a variable condenser.

The SI unit of work is the Newton-metre or joule (J). A Newton meter is also the SI unit of torque (also called “moment” or “moment of force”).

The hertz (Hz) is defined as one cycle per second and is a SI unit of frequency. Here kilohertz is also a unit of frequency of waves and is defined as 1000 cycles per second. Radio waves are measured in kilohertz (KHz).

Spectrum of an oil frame is an example for continuous emission spectrum.

A perfect sphere of spherically uniform density (density varies solely with distance from centre) would produce a gravitational field of uniform magnitude at all points on its surface, always pointing directly towards the sphere’s centre. However, the Earth deviates slightly from this ideal, and there are consequently slight deviations in both the magnitude and direction of gravity across its surface. Furthermore, the net force exerted on an object due to the Earth, called “effective gravity” or “apparent gravity”, varies due to the presence of other factors, such as inertial response to the Earth’s rotation. A scale or plumb bob measures only this effective gravity. The major reason for the difference in gravity at different latitudes is that the Earth’s equatorial bulge (itself also caused by inertia) causes objects at the Equator to be farther from the planet’s centre than objects at the poles. Because the force due to gravitational attraction between two bodies (the Earth and the object being weighed) varies inversely with the square of the distance between them, an object at the Equator experiences a weaker gravitational pull than an object at the poles.

1 Micron is equal to 10^-6 m.

The minimum velocity required for a rocket to overcome earth's gravity and travel into space is 11.2 km/sec.

Red light has a wavelength of 70 A°. In "nm" it is 700 nm.

Lux, unit of illumination (see luminous intensity) in the International System of Units (SI). One lux (Latin for “light”) is the amount of illumination provided when one lumen is evenly distributed over an area of one square metre.

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

The lux (symbol: lx) is the SI unit of illuminance and luminous emittance. It is used in photometry as a measure of the intensity of light, with wavelengths weighted according to the luminosity function, a standardized model of human brightness perception.

We measure sound intensity (also referred to as sound power or sound pressure) in units called decibels. Decibels (dB) are named in honor of Alexander Graham Bell, the inventor of both the telephone and the audiometer.

Light year - Time is an incorrect pair.

Radian is used to measure angle.

Stephen Hawking gave black hole theory.

The capacity of a water heater is measured in litres and metres.

8 bottles of water makes a gallon.

The siemens (symbol: S) is the derived unit of electric conductance, electric susceptance, and electric admittance in the International System of Units (SI).

Hydroscope is an optical device for enabling a person to see an object at a considerable distance below the surface of water by means of a series of mirrors enclosed in a steel tube. Hydrophone is an instrument used for recording sound under water. It is the underwater equivalent of a microphone that measures pressure fluctuations, and these are usually converted to sound pressure level (SPL), a logarithmic measure of the mean square acoustic pressure.

The unit of impedance, like that of resistance, is the ohm.

Weight is a measure of force that is equal to the gravitational pull on an object. Mass is independent of location, while weight depends on location.

Max Planck is known as the 'father of quantum theory'.

Stress is defined as resisting force per unit area of an element, on which load is applied. Stress is always perpendicular to a cross-section in the element. It is neither scalar (it cannot be numerically added), nor is it a vector (it does not follow the triangle rule of vector addition). It is a tensor quantity because it describes things happening in two directions simultaneously.

The Cartesian Coordinates can be retrieved from spherical coordinates (l,q,a) by:
X = r.sinq.cosa
Y = r.sinq.sina
Z = r.sinq
Using the same we get for P,
Y = P.sinq.sina

Guglielmo Marconi sent out the first wireless signals. In the early summer of 1895 and despite an intervening hill, Marconi achieved signal transmission and reception over a distance of about 2km. Success was indicated initially by the waving of a handkerchief and progressed to the need to fire a gun. The theory of relativity transformed theoretical physics and astronomy during the 20th century. When first published, relativity superseded a 200-year-old theory of mechanics stated by Isaac Newton. In 1900 Max Planck made a profound discovery in modern physics / Quantum Theory. He showed (from purely formal / mathematical foundations) that light must be emitted and absorbed in discrete amounts if it was to correctly describe observed phenomena (i.e. Blackbody radiation). The Wright brothers, Orville (August 19, 1871 – January 30, 1948) and Wilbur (April 16, 1867 – May 30, 1912), were two American brothers, inventors, and aviation pioneers who were credited with inventing and building the world’s first successful airplane and making the first controlled, powered and sustained heavier-than-air human flight, on December 17, 1903.

Sound waves cannot travel in the vacuum of space because there is no medium to transmit these mechanical waves.

In physics, the dynamo theory proposes a mechanism by which a celestial body such as Earth or a star generates a magnetic field.
Earth's crust core contains a large amount of iron and nickel is the evidence of the dynamo theory.

The color of a star mostly indicates a star's temperature, and it can also suggest the star's age. Class O stars, which are blue in color, are the hottest, and class M stars, which are red in color, are the coldest.

The wavelength of sound changes with temperature. This is because the speed of sound changes with the temperature. Since the speed of sound is different at different temperature, this means the wavelength of sound at a given frequency is a variable depending on the speed of sound. For example, the wavelength of a 100 cycle tone in air at 68ºF would be 11.27 feet while the same tone in fresh water at 68ºF would have a wavelength of 48.05 feet.

A wave with a large amplitude will be particularly tall, and a wave with a small aptitude will be particularly short. It turns out that the amplitude tells you how much energy is in the wave. A high amplitude wave is a high-energy wave, and a low-amplitude wave is a low-energy wave.

In a bounded medium, standing waves occur when a wave with the correct wavelength meets its reflection. The interference of these two waves produces a resultant wave that does not appear to move. Standing waves don't form under just any circumstances.

Henry is the unit of the physical quantity "Inductance".

A gas pycnometer is a laboratory device used for measuring the density — or more accurately the volume — of solids, be they regularly shaped, porous or non-porous, monolithic, powdered, granular or in some way comminuted, employing some method of gas displacement and the volume-pressure relationship known as Boyle’s Law. A gas pycnometer is also sometimes referred to as a helium pycnometer. While pycnometer (of any type) is recognized as density measuring devices they are in fact devices for measuring volume only.

A micron is equal to 0.001 mm.

Fathometer is depth finder that uses sound waves to determine the depth of water.