Light Energy

Light Energy - Uses and Properties of Light Energy

Light energy is a kind of kinetic energy. It helps us see the world around us.  Light can be defined as a form of electromagnetic radiation emitted by hot objects like burning objects, bulbs, lasers, sun and other stars.

The primary source of light on our planet is the Sun. Sunlight provides the energy that is utilized by green plants to create sugars mostly in the form of starches. The sugars release energy into the living things that digest them. This process of photosynthesis provides all the energy required by living being to survive. Historically, another important source of light for humans has been fire, from ancient campfires to modern day kerosene lamps. Over the years with the development of electric lights and power systems, electric lighting has effectively replaced firelight.

Some species of animals generate their own light in a process called bioluminescence. For example, fireflies use light generated by themselves to locate mates, and vampire squids use it to hide themselves from prey.

Light contains minute packets of energy called photons. When the atoms of an object are heated up, it results in the production of photons. The electrons find excitement from the heat and results in earning extra energy. The energy is released in the form of a photon and more photons are emitted as the substance gets hotter.

When light travels, it travels in the form of a wave. However, light is not dependent on any material medium to travel. This is the exact reason why light can travel through free space where there is no material medium. On the contrary, this is not so for sound waves, which require a material medium for propagation. Light energy is the fastest form of energy.

The speed of light in vacuum is calculated to be 299,792,458 m/s i.e. approximately 186,282 miles per second. The fixed value of the speed of light in SI units is since the metre is now defined in terms of the speed of light. All forms of electromagnetic radiations travel at exactly this same speed in vacuum.

Over the centuries different physicists have attempted to measure the speed of light throughout history. In the seventeenth century, Galileo attempted to measure the speed of light. An early experiment to measure the speed of light was conducted by a popular Danish physicist Ole Rømer, in 1676. With the help of a telescope, Rømer successfully observed the motions of Jupiter and one of its moons “Io”. Observing the differences in the period of Io's orbit, he calculated that light takes approximately 22 minutes to traverse the diameter of Earth's orbit. However, the diameter size was not known at that time. If Rømer had known the exact diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s.

Another more accurate attempt to measure the speed of light was performed in Europe by Hippolyte Fizeau in the year 1849. Fizeau directed a beam of light at a mirror placed several kilometers away. A rotating wheel was placed in the path of the light beam as it traveled from the source to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Since he was aware of the distance to the mirror, the number of teeth on the wheel and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.

Léon Foucault carried out an experiment in 1862 which used rotating mirrors to obtain a value of 298,000,000 m/s. Later in 1877, Albert A. Michelson conducted experiments on the speed of light until his death in 1931. He further revamped Foucault's process in 1926. He used improved rotating mirrors to measure the time it took light to complete a round trip from Mount Wilson to Mount San Antonio in California. The precise measurements yielded a velocity of 299,796,000 m/s.

The effective speed of light in various transparent substances containing ordinary matter, is less than that in vacuum. For example, the speed of light in water is about 3/4 of that in vacuum.

Two independent teams of physicists, one at Harvard University and the Rowland Institute for Science in Cambridge, Massachusetts, and the other at the Harvard–Smithsonian Center for Astrophysics, also in Cambridge were said to bring light to a complete standstill by passing it through a Bose–Einstein condensate of rubidium. However, the state of light being halted in these experiments implies only to light being temporarily stored in the excited states of atoms and then re-emitted at an arbitrary later time, while stimulated by another laser pulse. During the period it had stopped, it had ceased to be light.

Human eyes contain cells that can identify different wavelengths. Light waves include several different wavelengths. Red has a wavelength of 700 nanometers, while violet is much shorter at 380 nanometers. These wavelengths are perceived as different colors by the human eye. Hotter objects produce shorter wavelengths, as a result hotter objects appear blue. Certain forms of light are invisible to the eye as their wavelengths are too long or too short to be perceived by the human eye.
Uses of Light Energy

Light is the single source of food generation for all living organisms. All organisms dependent on light for their energy and food 
except for a few chemotrophic bacteria.

Natural light by itself is very useful. Any organism is capable to view the objects around them due to presence of eyes. However, eyes might be useless without light. The human eye uses these light waves to see. When light enters the cornea (cornea is the front surface of the eye), it bends the light waves through the pupil. The iris (a thin circular structure in the eye which is primarily responsible for controlling the diameter and the size of the pupil) opens and lets the light in through the retina and optic nerve. Devices such as cameras utilize these processes that the eye uses to capture light and transform it into images.

However, light energy can be harvested for multiple purposes. It can be reflected, refracted or harvested to see objects. Mirrors and glasses are basic forms of this application, but the same principle is also used by engineers to build devices such as telescopes which enable us view objects that are very far away. Lasers which contain very bright lights have a vast variety of purposes including medicine, industrial production and computer technology.

Light energy from the sun can also be harvested to create electricity. Solar panels contain smaller cells that are dominantly made of minerals that absorb light energy from the sun. As solar panels absorb the energy and heat up, electrons in the molecules start becoming loose due to their interactions with the photons from the sun's rays. This results in an electric charge. The wiring in panels picks up the electric charge and lets it flow through an electrical system or into a grid.

Forms of Light Energy

Light energy can be classified as follows:

Visible light: It is the only form of light that can be seen through the naked eye. It is a type of electromagnetic energy. The main source of visible light is the sun however it can also be emitted by artificial sources such as lanterns, flashlight, light bulbs etc.

Infrared Light: Infrared Light is also a type of electromagnetic energy which emits heat. Since it has longer wavelengths as compared to visible light, it cannot be viewed by the naked eye. One of the most popular uses of infrared light is to operate your television set with a ‘TV remote’. Infrared waves travel from the ‘remote’ to the TV. It is also used in night vision equipment where vision is impaired due to insufficient supply of visible light to see. Infrared light is also used to remotely determine the temperature of objects. This process is termed as thermography.

X-rays and Ultraviolet light: X-rays and Ultraviolet light are shorter light waves. Doctors take photographs of internal organs of our body to find out fractures on our bone and other internal damage using x-rays. Dentists also use x-rays to check the depth of tooth decay and damage.

Ultraviolet wavelengths measure less than 200 nanometers. Ultraviolet radiations are used for tanning. Longer exposure to Ultraviolet light can cause skin cancer because it pierce the skin and causes destruction to cells. Owing to the destructive power of ultraviolet light, it can be used to kill germs.

Key properties of light energy

Intensity: The intensity of light is the rate at which the light energy is emitted from the source through unit area. The intensity is often expressed in units of watts. It can also be defined as brightness measured at the rate at which light is emitted in a unit of surface or energy per unit time per unit area.

Frequency: Light waves also come in multiple frequencies. The Frequency of light is defined as the number of waves that passes through a point in the space during a particular time interval, usually considered as one second. Frequency is measured either in the units of cycles or waves per second or hertz (Hz).
Wavelength: The wavelength of light is defined as the distance between the two consecutive crests or troughs. In simple terms it is the distance over which the wave's shape repeats. The wavelength depends on the medium through which the light passes.

Polarization: Polarization of light is a process in which unpolarized light is converted polarized light. In general, light waves vibrate in more than one plane and hence called as unpolarized light but in polarized light waves all particles vibrate only along a single plane.

Phase: Phase is a point in the time period during the cyclic waveform. It is observed that the intensity of light increases when the waves are in phase.

facts about Light Energy:

1. The sunlight visible to us on Earth is emitted by the Sun 8.3 minutes before. Thus, it takes only 8.3 mins for sun light to travel 149,597,870,700 meters between the Earth and the Sun.
2. During photosynthesis process, plants use light energy to create chemical energy. This chemical energy is stored in carbohydrate molecules such as sugars which are a source of energy for plants.
3. UV lights are often used by forensic scientists to see details that are not seen by the naked eye.
4. Although humans cannot see UV light, some insects can see it.
5. Optics is the study of light.
6. The speed of light changes when traveling through different objects.
7. Light is a type of electromagnetic energy.
8. Light is made of tiny photons which are in an excited state since they contain lots of energy.
9. The intensity of photons is dependent upon the amount of energy they constitute.
10. Space is dark because light is only visible when it has an object of which it can bounce off.
11. Light energy is always moving and can therefore not be stored or held.
12. Light is also called electromagnetic radiation when speaking of other forms of light other than visible light.
13. Of all the forms of radiation and light on the electromagnetic spectrum, humans can only visibly see a very small amount of light through their naked eye.
14. The reason we see different colors of light is because each color has a different wavelength that the light wave is made of.

Units and measures of Light energy

Light energy is measured with two main sets of units namely “radiometry” which consists of measurements of light power at all wavelengths and “photometry” which measures light with wavelength weighted with reference to human brightness perception. Photometry is useful, for example, to quantify lighting intended for human use. The photometry measurement units are different from most systems of physical units since they consider how the human eye responds to light.