Optical properties- Emissions and Absorptions 

Optics is the branch of physics where the behavior, properties, and applications of light are studied. Light is a combination of tiny particles called photons, and interactions between these photons can reveal some enthralling effects. 

Emissions and absorptions

Emissions and absorptions are the two fundamental processes that drive the energy balance of our planet, alter the composition of the atmosphere, and play a pivotal role in climate change.  

Emissions

Let us imagine a dark room, and all of a sudden, a light bulb comes to life. In this situation, what we are witnessing is an emission. Let us discuss how the light is being produced. When atoms or molecules get excited from the lower energy level to the higher energy level, after some time they come back to the lower energy level, and the difference in energy is emitted as light. In other words, we can think of it as these particles getting a little extra energy and then releasing it in the form of photons.

There are, in general, two types of emissions: fluorescence and phosphorescence. 

If the emission light vanishes almost immediately after the removal of exciting radiation, then it is called fluorescence. 

Again, if the emission light persists for an appreciable length of time after the removal of the exciting radiation, then it is called phosphorescence.

Emissions are like the cosmic spotlight, where elements and molecules release energy in various forms, most commonly as light. Stars, for instance, are a big source of emitters. The sun, our closest star, is an immense source of helium that continuously emits light and heat through a process known as nuclear fusion of hydrogen atoms. This is the main process through which our solar system energizes itself and provides life on Earth.

It is important to mention here that the celestial bodies are not only the sources that emit energy. Our own activities on Earth, from lighting a candle to driving a car, all involve emissions.

Combustion processes release energy in the form of heat and light, as well as gases like carbon dioxide (CO₂) and water vapour (H₂O). These emissions can have significant consequences for the climate and atmosphere of our planet.

One of the most vital aspects of emissions is the role they play in the greenhouse effect. Greenhouse gases, particularly carbon dioxide (CO₂), methane (CH₄), and water vapour (H₂O), are proficient at absorbing and re-emitting certain wavelengths of energy. When we burn fossil fuels or clear forests, we release vast amounts of CO₂ into the atmosphere. This extra CO₂ acts as a sort of thermal blanket, trapping heat and causing an increase in the average temperature of the Earth. This, in turn, results in a phenomenon known as global warming.

Emissions also occur in the domain of atomic and molecular physics. At the atomic level, electrons orbiting around atomic nuclei can absorb energy and jump to higher energy levels. When these excited electrons return to their original energy levels, they emit energy in the form of photons, often as visible or ultraviolet light. This process is the basis for neon signs and fluorescent lights, as well as the dazzling colours of fireworks.

In molecular systems, emissions are equally fascinating. For instance, plants and phytoplankton emit light during photosynthesis through a process called chlorophyll fluorescence. Scientists use this natural glow to monitor the health and activity of photosynthetic organisms, shedding light on the intricate web of life on our planet.

Absorptions

In a simple way, the process of absorption can be thought of as matter soaking up light like a sponge soaking up water. Let us think about wearing a dark-coloured shirt on a sunny day. Our shirt absorbs sunlight and heats up our body because of the absorption of the light energy. 

Now, let us consider a leaf. It appears green because it absorbs most of the colours in sunlight but reflects green light. That is why it looks green to our eyes.

Absorptions are like cosmic shadows, where atoms and molecules soak up energy from their surroundings. When light or other forms of energy pass through a medium, some of it may be absorbed, altering the medium's properties. This absorption phenomenon occurs in various contexts across the universe.

In astronomy, astronomers rely on absorption spectra to study the composition of distant celestial objects. When light from a star or galaxy passes through the intervening space, it encounters different elements and molecules along the way. These substances absorb specific wavelengths of light, leaving dark lines, or absorption lines, in the spectrum. By analyzing these lines, astronomers can decipher the chemical makeup of far-off stars and galaxies, unraveling the mysteries of the cosmos.

In the earth's atmosphere, absorption processes are equally significant. Some molecules, such as ozone (O₃) and water vapor, have a remarkable ability to absorb certain wavelengths of solar and terrestrial radiation. Ozone, found in high concentration in the stratosphere, absorbs the harmful ultraviolet (UV) radiation emitted from the sun and provides a shield to the earth's surface from its damaging effects.

Water vapor, on the other hand, absorbs infrared radiation, which is the heat energy emitted by the earth's surface. The natural greenhouse effect is important for maintaining a habitable temperature on our planet. However, as the levels of greenhouse gases increase due to human activities, there is an increase in global warming and climate change.

Absorption processes also occur on a smaller scale in our everyday lives. When we feel warmth while standing in the sun or savor the rich aroma of a cup of coffee, we are experiencing the effects of absorption. Sunlight is absorbed by our skin, warming it, while the aromatic compounds in coffee absorb specific wavelengths of light and release them as a delightful scent.