Physics

A hygrometer is an instrument used to measure relative humidity. Humidity is the measure of the amount of moisture in the air. A psychrometer is an example of a hygrometer.

A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation. The modern photon concept was developed gradually by Albert Einstein.

Absolute refractive index refers to a condition when light travels from vacuum to any other medium. As the absolute refractive index of a particular medium is supposed to be the refractive index of that medium in respect to the vacuum it will be the same for light as well. It will be greater than 1.

Infrared imaging is used extensively for military and civilian purposes. Military applications include target acquisition, surveillance, night vision, homing and tracking. Non-military uses include thermal efficiency analysis, environmental monitoring, industrial facility inspections, remote temperature sensing, short-ranged wireless communication, spectro-scopy, and weather forecasting. Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect objects such as planets, and to view highly red-shifted objects from the early days of the universe. There are two main types of remote sensing: passive remote sensing and active remote sensing. Reflected sunlight is the most common source of radiation measured by passive sensors. Examples of passive remote sensors include film photography, infrared, charge-coupled devices, and radiometers.

A concave lens always forms a virtual image.

The path that data takes from source to destination is called a Channel. A channel is a communication medium. A channel has a certain capacity for transmitting information. The Capacity of a communication channel is measured in Band with.

A star appears twinkling in the sky because of refraction of light by atmosphere.