The study of light and the interaction of light and matter are termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light. light usually refers to visible light, which is electromagnetic radiation that is visible to the human eye and responsible for the sense of sight. Visible light is usually defined as having a wavelength in the range of 400 nanometers (nm), or 400×10-9 m, to 700 nanometers–between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths.
The main source of light on Earth is the Sun. Sunlight provides the energy that green plants use to create sugars mostly in the form of starches, which release energy into the living things that digest them. This process of photosynthesis provides virtually all the energy used by living things. Historically, another important source of light for humans has been fire, from ancient campfires to modern kerosene lamps. With the invention of electricity, electric lighting has all but replaced firelight. Some species of animals generate their own light, called bioluminescence. For example, fireflies use light to locate mates, and vampire squids use it to hide themselves from prey.
Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum.
In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not. In this sense, gamma rays, X-rays, microwaves and radio waves are also light. like all types of light, visible light is emitted and absorbed in tiny “packets” called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics.
Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io’s orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth’s orbit. However, its size was not known at that time. If Rømer had known the diameter of the Earth’s orbit, he would have calculated a speed of 227,000,000 m/s.
On December 20, 2013, the UN General Assembly 68th Session proclaimed 2015 as the International Year of light and light-Based Technologies (IYl 2015). In proclaiming an International Year focusing on the topic of light science and its applications, the UN has recognised the importance of raising global awareness about how light-based technologies promote sustainable development and provide solutions to global challenges in energy, education, agriculture and health. light plays a vital role in our daily lives and is an imperative cross-cutting discipline of science in the 21st century. It has revolutionised medicine, opened up international communication via the Internet, and continues to be central to linking cultural, economic and political aspects of the global society. The first official event was the Opening Ceremony held at UNESCO HQ in Paris over 19-20 January 2015. This was the beginning of a year-long series of activities all over the world.
(Dr Senapati is Professor and HOD Chemistry, Trident Academy of Technology, Bhubaneswar)