Instruct: Given a question, retrieve relevant documents that answer the question.
Query: What is the frequency of light?
An important aspect of light's nature is its frequency. The frequency of a wave is its rate of oscillation and is measured in hertz, the SI unit of frequency, where one hertz is equal to one oscillation per second. Light usually has multiple frequencies that sum to form the resultant wave. Different frequencies undergo different angles of refraction, a phenomenon known as dispersion.
In a medium, light usually does not propagate at a speed equal to "c"; further, different types of light wave will travel at different speeds. The speed at which the individual crests and troughs of a plane wave (a wave filling the whole space, with only one frequency) propagate is called the phase velocity "v". An actual physical signal with a finite extent (a pulse of light) travels at a different speed. The largest part of the pulse travels at the group velocity "v", and its earliest part travels at the front velocity "v".
The phase velocity is important in determining how a light wave travels through a material or from one material to another. It is often represented in terms of a "refractive index". The refractive index of a material is defined as the ratio of "c" to the phase velocity "v" in the material: larger indices of refraction indicate lower speeds. The refractive index of a material may depend on the light's frequency, intensity, polarization, or direction of propagation; in many cases, though, it can be treated as a material-dependent constant. The refractive index of air is approximately 1.0003. Denser media, such as water, glass, and diamond, have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-"c" speeds in material substances. As an extreme example of light "slowing" in matter, two independent teams of physicists claimed to bring light to a "complete standstill" by passing it through a Bose–Einstein condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge. However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrarily later time, as stimulated by a second laser pulse. During the time it had "stopped," it had ceased to be light. This type of behaviour is generally microscopically true of all transparent media which "slow" the speed of light.
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to 740 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 430–770 THz.
A human perceives the intensity of the sinusoidal disturbance as the brightness of the light and the frequency as the color. If a light is turned on or off at a specific time or otherwise modulated, then the amplitude of the sinusoidal disturbance is also time-dependent. The time-varying amplitude does not propagate at the phase velocity but rather at the group velocity. The group velocity depends not only on the refractive index of the material, but also the way in which the refractive index changes with frequency (i.e. the derivative of refractive index with respect to frequency).
Infrared light lies between the visible and microwave portions of the electromagnetic spectrum. Infrared light has a range of wavelengths, just like visible light has wavelengths that range from red light to violet. "Near infrared" light is closest in wavelength to visible light and "far infrared" is closer to the microwave region of the electromagnetic spectrum. The longer, far infrared wavelengths are about the size of a pin head and the shorter, near infrared ones are the size of cells, or are microscopic.
The Rabi frequency is the radian frequency of the Rabi cycle undergone for a given atomic transition in a given light field. This is thus the frequency of fluctuations in the populations of the two atomic levels involved in that atomic transition in that situation. It is proportional to the strength of the coupling between the light and the atomic transition and to the amplitude ("not" intensity) of the light's electric field. Rabi flopping between the levels of a 2-level system illuminated with light exactly resonant with the transition will occur at the Rabi frequency; when the incident light is detuned from resonance then this occurs at the generalized Rabi frequency. The Rabi frequency is a semiclassical concept as it is based on a quantum atomic transition and a classical light field.
Electromagnetic radiation with a wavelength between 380 nm and 760 nm (400–790 terahertz) is detected by the human eye and perceived as visible light. Other wavelengths, especially near infrared (longer than 760 nm) and ultraviolet (shorter than 380 nm) are also sometimes referred to as light, especially when the visibility to humans is not relevant. White light is a combination of lights of different wavelengths in the visible spectrum. Passing white light through a prism splits it up into the several colors of light observed in the visible spectrum between 400 nm and 780 nm.
In 1913, Georges Sagnac showed that if a beam of light is split and sent in two opposite directions around a closed path on a revolving platform with mirrors on its perimeter, and then the beams are recombined, they will exhibit interference effects. From this result Sagnac concluded that light propagates at a speed independent of the speed of the source. The motion of the earth through space had no apparent effect on the speed of the light beam, no matter how the platform was turned. The effect had been observed earlier (by Harress in 1911), but Sagnac was the first to correctly identify the cause.
Many different units of measure are used for photometric measurements. People sometimes ask why there need to be so many different units, or ask for conversions between units that can't be converted (lumens and candelas, for example). We are familiar with the idea that the adjective "heavy" can refer to weight or density, which are fundamentally different things. Similarly, the adjective "bright" can refer to a light source which delivers a high luminous flux (measured in lumens), or to a light source which concentrates the luminous flux it has into a very narrow beam (candelas), or to a light source that is seen against a dark background. Because of the ways in which light propagates through three-dimensional space — spreading out, becoming concentrated, reflecting off shiny or matte surfaces — and because light consists of many different wavelengths, the number of fundamentally different kinds of light measurement that can be made is large, and so are the numbers of quantities and units that represent them.
Faster-than-light communication is, according to relativity, equivalent to time travel. What we measure as the speed of light in a vacuum (or near vacuum) is actually the fundamental physical constant "c". This means that all inertial observers, regardless of their relative velocity, will always measure zero-mass particles such as photons traveling at "c" in a vacuum. This result means that measurements of time and velocity in different frames are no longer related simply by constant shifts, but are instead related by Poincaré transformations. These transformations have important implications:
The light-nanosecond was popularized as a unit of distance by Grace Hopper as the distance which a photon could travel in one billionth of a second (roughly 30 cm or one foot): "The speed of light is one foot per nanosecond." In her speaking engagements, she was well known for passing out light-nanoseconds of wire to the audience, and contrasting it with light-microseconds (a coil of wire 1,000 times as long) and light-picoseconds (the size of ground black pepper). Over the course of her life, she found many uses for this visual aid, including demonstrating the waste of sub-optimal programming, illustrating advances in computer speed, and simply giving young scientists and policy makers the ability to conceptualize the magnitude of very large and small numbers.
Measurement of light or photometry is generally concerned with the amount of useful light falling on a surface and the amount of light emerging from a lamp or other source, along with the colors that can be rendered by this light. The human eye responds differently to light from different parts of the visible spectrum, therefore photometric measurements must take the luminosity function into account when measuring the amount of useful light. The basic SI unit of measurement is the candela (cd), which describes the luminous intensity, all other photometric units are derived from the candela. Luminance for instance is a measure of the density of luminous intensity in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square metre (cd/m). The CGS unit of luminance is the stilb, which is equal to one candela per square centimetre or 10 kcd/m. The amount of useful light emitted from a source or the luminous flux is measured in lumen (lm).
The candela (cd) is the SI unit of luminous intensity, that is power per unit solid angle in a given direction, weighted according to the sensitivity of the human eye to various colours of light. A typical candle produces light with about 1 candela of luminous intensity in all directions. A lamp can produce higher luminous intensity either by producing more light, or by focusing it tighter. The luminous intensity of a light depends on many factors, including the colour of the light and the eye's sensitivity to that colour, the optics involved, reflector and lens. Despite its complexity, it is a more useful measure than watts, because it defines how much usable light is shed in a given place: a dynamo headlight designed for road use and focused for seeing the road makes more efficient use of the power of the lamp than lights using rotationally symmetrical optics.
Light is measured with two main alternative sets of units: radiometry consists of measurements of light power at all wavelengths, while photometry measures light with wavelength weighted with respect to a standardised model of human brightness perception. Photometry is useful, for example, to quantify Illumination (lighting) intended for human use. The SI units for both systems are summarised in the following tables.
Blue Light is a range of the visible light spectrum, defined as having a wavelength between 400−495 nm. This short wavelength means that blue light is a type of high-energy visible light, defined as having a wavelength between 400 and 450 nm. Violet, indigo, and some blue-green light are other types of high-energy visible light.
Light or other electromagnetic radiation from many sources, such as the sun, flames, and incandescent lamps, consists of short wave trains with an equal mixture of polarizations; this is called "unpolarized light". Polarized light can be produced by passing unpolarized light through a polarizer, which allows waves of only one polarization to pass through. The most common optical materials (such as glass) are isotropic and do not affect the polarization of light passing through them; however, some materials—those that exhibit birefringence, dichroism, or optical activity—can change the polarization of light. Some of these are used to make polarizing filters. Light is also partially polarized when it reflects from a surface.
A flick is a unit of time equivalent to exactly 1/705,600,000 of a second. The figure was chosen so that frequencies of 24, 25, 30, 48, 50, 60, 90, 100 and 120 Hz, as well as 1/1000 divisions of all those, can be represented with integers. The unit was launched in January 2018 by Facebook. A flick is approximately 1.42 × 10 s, which makes it larger than a nanosecond but much smaller than a microsecond.
In photometry, luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd), an SI base unit.
Skywave modes of radio communication operate by bending (refracting) radio waves (electromagnetic radiation) through the Ionosphere. During the "peaks" of the solar cycle, the ionosphere becomes increasingly ionized by solar photons and cosmic rays. This affects the propagation of the radio wave in complex ways that can either facilitate or hinder communications. Forecasting of skywave modes is of considerable interest to commercial marine and aircraft communications, amateur radio operators and shortwave broadcasters. These users occupy frequencies within the High Frequency or 'HF' radio spectrum that are most affected by these solar and ionospheric variances. Changes in solar output affect the maximum usable frequency, a limit on the highest frequency usable for communications.
Since then a large array of treatments using controlled light have been developed. Though the popular consumer understanding of "light therapy" is associated with treating seasonal affective disorder, circadian rhythm disorders and skin conditions like psoriasis, other applications include the use of low level laser, red light, near-infrared and ultraviolet lights for pain management, hair growth, skin treatments, and accelerated wound healing.
An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization state. The independent variable is usually the wavelength of the light or a unit directly proportional to the photon energy, such as reciprocal centimeters or electron volts, which has a reciprocal relationship to wavelength.
Light and sound are similar in various ways. They both can be thought of in terms of waves, and they both come in quantum mechanical units. In the case of light we have photons while in sound we have phonons. Both sound and light can be produced as random collections of quanta (e.g. light emitted by a light bulb) or orderly waves that travel in a coordinated form (e.g. laser light). This parallelism implies that lasers should be as feasible with sound as they are with light. In the 21st century, it is easy to produce low frequency sound in the range that humans can hear (~20 kHz), in either a random or orderly form. However, at the terahertz frequencies in the regime of phonon laser applications, more difficulties arise. The problem stems from the fact that sound travels much slower than light. This means that the wavelength of sound is much shorter than light at a given frequency. Instead of resulting in orderly, coherent phonon laser structures that can produce terahertz sound, tend to emit phonons randomly. Researchers have overcome the problem of terahertz frequencies by following various approaches. Scientists in Caltech have overcome this problem by assembling a pair of microscopic cavities that only permit specific frequencies of phonons to be emitted. This system can be also tuned to emit phonons of different frequencies by changing the relative separation of the microcavities. On the other hand, the group from the University of Nottingham took a different approach. They have built their device out of electrons moving through a series of structures known as quantum wells. Briefly, as an electron hops from one quantum well to another neighbouring well it produces a phonon.
The individual frequency components of the transmitted light are spatially separated using a simple spectrometer, usually a diffraction grating. In order to achieve a highly parallel measurement of the individual frequency components transmitted through the sample and out of the cavity, a CCD camera capable of operating in the spectral range of the laser light is used. In the case of the diffraction grating, the frequency components are separated in one spatial direction and focused into the CCD camera. In order to take advantage of the other spatial direction of the CCD, the light is scanned across the perpendicular direction of the CCD at the same time that the cavity length is scanned using an actuator. This produces a grid of peaks on the CCD image corresponding to a mode matching condition between the frequency comb and optical cavity.
Another way to measure the speed of light is to independently measure the frequency "f" and wavelength "λ" of an electromagnetic wave in vacuum. The value of "c" can then be found by using the relation "c" = "fλ". One option is to measure the resonance frequency of a cavity resonator. If the dimensions of the resonance cavity are also known, these can be used to determine the wavelength of the wave. In 1946, Louis Essen and A.C. Gordon-Smith established the frequency for a variety of normal modes of microwaves of a microwave cavity of precisely known dimensions. The dimensions were established to an accuracy of about ±0.8 μm using gauges calibrated by interferometry. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light.
More recently, researchers demonstrated that indoor lights such as fluorescent lights and incandescent bulbs vary their light intensity in accordance with the voltage supplied, which in turn depends on the voltage supply frequency. As a result, the light intensity can carry the frequency fluctuation information to the visual sensor recordings in a similar way as the electromagnetic waves from the power transmission lines carry the ENF information to audio sensing mechanisms. Based on this result, researchers demonstrated that visual track from still video taken in indoor lighting environments also contain ENF traces that can be extracted by estimating the frequency at which ENF will appear in a video as low sampling frequency of video (25–30 Hz) cause significant aliasing. It was also demonstrated in the same research that the ENF signatures from the visual stream and the ENF signature from the audio stream in a given video should match. As a result, the matching between the two signals can be used to determine if the audio and visual track were recorded together or superimposed later.