What is greenhouse effect . Why is it names so?
Answers
The greenhouse effect is the process by which some molecules in the atmosphere of a planet help to keep the planet warm. There are two things to understand: 1) how heat energy escapes from warm objects; 2) how some gases stop the heat energy escaping.
1. How heat energy escapes from warm objects
Anything whose temperature is above absolute zero (-273.15 °C) 'glows' with electromagnetic radiation. That is to say, the heat energy of anything that is warm turns into electromagnetic waves that are radiated away from the object taking some of the heat energy away with them.
What is electromagnetic radiation? It is waves that travel through space and carry energy. The higher the wave's frequency (waves per second) the more energy the wave can carry. 'Radio' is electromagnetic radiation with a low frequency. Visible light is a form of electromagnetic radiation with a much higher frequency. X-rays have a higher frequency still.
Warm objects emit a range of frequencies and the range is determined by the temperature of the object. Very hot objects emit lots of visible light - that's why we can see them glow. Cooler objects do not emit visible light, but they still glow with invisible, low frequency electromagnetic radiation. Objects at about 15 °C (the average temperature of the surface of the Earth) emit electromagnetic waves called 'infra-red' whose frequency is less than red light and so isn't detected by our eyes. The surface of the sun is much hotter than the surface of Earth (about 6,000 °C) so it glows with visible light. In many ways it is similar though - anything with a temperature above absolute zero glows.
The Earth is 'glowing' with invisible infrared simply because it is warm. The infra red takes heat energy away from the Earth. It disappears into space.
2. How some gases stop the heat energy escaping
Molecules can vibrate. The atoms that make up molecules are a bit like little balls and the forces between them are bit like little springs. The springs can contract and stretch rhythmically bringing the balls close together and further apart. Getting a molecule vibrating is a way of temporarily storing energy.
If electromagnetic waves have the right frequency, when they hit a molecule the energy of the wave can be transferred into the vibrational energy of the molecule. That is, the molecule stores the energy of the electromagnetic wave. It doesn't store it for long though, it soon turns the energy back into an electromagnetic wave and stops vibrating.
The properties of the molecule determine which frequencies make it vibrate. Carbon dioxide, methane and the other greenhouse gases vibrate at the same frequencies as infrared radiation. Infrared radiation sails straight past nitrogen and oxygen molecules, because they don't vibrate at the same frequencies. On the other hand, if an infrared wave hits a carbon dioxide molecule, there is a good chance its energy will be absorbed by the molecule and temporarily stored as a vibration.
Here's the crucial thing though: the wave that a vibrating molecule eventually emits can come out in any direction: up, down, left, right, forward, backwards. It doesn't matter where the original wave was heading.
3. Putting it all together
Heat is continually escaping from the surface of the Earth in the form of electromagnetic radiation.
Infrared waves heading out into space are absorbed by carbon dioxide, methane and other molecules with just the right vibrational frequencies, and their energy is temporarily stored as a vibration.
When the vibration is turned back into an infrared wave, that wave can go in any direction, including back towards Earth. If it reaches the surface of the Earth its energy will be re-absorbed by the Earth.
Without 'greenhouse gases' the infrared would get out to space taking all the heat energy with it. Because greenhouse gas molecules get in the way, some of the infra red gets re-emitted back towards the Earth - returning some of the lost heat energy.
If we put more greenhouse gas molecules in the atmosphere more heat energy will be saved. The more heat energy there is, the hotter the surface of the Earth will be.