Why does light interact with matter

As a result of many experiments we now know that stable matter, whether in gas or in liquid or in solid phase, is made of collections of so called atoms. Each atom in turn is made up of a very massive nucleus surrounded with less massive, but equally charged electrons. Measurements have verified that protons and neutrons make up the nucleus of the atom. These are of nearly the same mass and size particles. Their primary difference is that protons carry a positive electric charge, whereas the neutrons are, as suggested by their name, electrically neutron.

As a result, almost all of the mass of the atom is due to its number of protons and neutrons. The role of the electron is to cancel out the positive electric charge in the nucleus. Since positive and negative charges attract, the electrons bind to the nucleus and in this combination create a neutral atom. If the protons are positively charged and therefore repel each other by their electric interaction, then how do they come together to form the nucleus; and how do they hold on to neutrons, and the neutrons to each other?

The answer turns out to be in the secret of the nucleus: another much stronger force, called the hadronic force, is involved. Protons and neutrons, called nucleons, strongly attact as a result of this hadronic force when they are very close to each other. This force of attraction is much much larger than the electric force, but it falls off very quickly when the nucleons begin to separate; i. As soon as the separation distance between two protons increases, this hadronic force becomes smaller than the electric repulsion and the two protons separate from each other.

Of course, two neutrons, or a proton and a neutron, have no repulsive forces. So, they continue to attract each other, but only very weakly once they begin to separate; again because the hadronic force has a "short range".

In passing, it is worthwhile to mention that it is the "harnessing" of the hadronic force that provides for the nuclear energy. So, when people are talking about "atomic energy" they are really preferring to the energy within the nucleus of the atom and not the energy that holds the electrons of the atom bound to its nucleus. When light falls onto an atom the oscillations of the electromagnetic wave causes the electron to oscillate.

In this fashion the atom absorbs the energy of the light. If this atom now interacts with other atoms it can pass along the photon energy wave oscillations.

This can happen in several different scenarios. One of these is that the energy is passed on without any loss. In another scenario some of the light's energy could be "used" before it is transmitted to the next atom.

But what happens to light when it falls onto an isolated atom? The answer to this question, turns out, to be totally anti-intuitive and somewhat beyond the realm of our understanding of macroscopic world. It is answered by quantum physics. It turns out that very small atoms form a bound structure that can absorb energy and still remain a cohesive unit provided that the energy amount is just right.

If the energy amount is not of the "correct" value the atom does not absorb it at all. In the context of quantum physics these "correct" energy values create a set of energy quanta chunks. So, it is said that the atom's energy are quantized. This is, of course, in contrast to a macroscopic case, say a child on a swing, that could absorb any value energy - any light or any strong push.

So, if the photon's frequency wavelength is not just right, the atom totally ignores photon's presence. Because the atoms in a vapor tend to be very far away from one another, for all practical purposes they can be considered as isolated atoms. Equally, when the atomic vapor was energised say by a high voltage source or by collisions with other atoms it just emitted discrete wavelengths of light.

Below picture shows light emitted from a tube containing hydrogen atoms in a vapor. A diffraction grating is used to separate the light of different wavelength. There is not a contnium of colors, as we see being emitted from the sun, for example. This behaviour is similar to the case of a person climbing a ladder, as opposed to a person going up an inclined slope. The one who goes up a ladder changes its energy in steps chunks , while the one who is going up the slope changes energy continuously.

By jumping down two rung of the ladder, the ladder climber reduces two chunks of its energy. And so on. It turns out that atoms are "ladder climbers". They change their energies by going from one rung of their energy ladder a so called energy level to another rung another energy level. To go up their energy ladder they absorb energy equal to the difference between the two energy levels. When they jump down from a higher level to a lower one they emit energy equal to the difference between the two levels.

This energy exchange could happen by interacting with another atom, or in isolation by emitting or absorbing a photon. Pictorially, this atom light interaction is depicted as shown in the last of the three diagrams,below. The first one shows our model of energy levels.

The second one depicts energy changes in the atom. If the molecule absorbed light energy at particular frequencies, how do you think the IR spectrometer produces a spectrum like the one shown?

How is it that the spectrometer allows us to "see" that light has been absorbed? Tip: To turn text into a link, highlight the text, then click on a page or file from the list above. To edit this page, request access to the workspace.

Spectroscopy: How does light interact with matter Page history last edited by Ben Geller 8 years, 7 months ago. Spectroscopy: How does light interact with matter? You are probably familiar with the idea that molecules can attain a higher energy state by absorbing light: Because different molecules absorb light of different frequencies, one can obtain spectra that show how much light is absorbed by a particular compound as a function of the frequency or wavelength of the light.

Part 1 - How does light interact with molecules? Part 2 - What about the molecule would change which light is absorbed? Recall the natural frequency of a harmonic oscillator is defined by: In modeling the molecule as a harmonic oscillator, what changes to the molecule would change the values of k and m , and therefore the natural frequency? Spectroscopy: How does light interact with matter.

Page Tools Insert links Insert links to other pages or uploaded files. Pages Images and files. Insert a link to a new page. No images or files uploaded yet. Insert image from URL. Printable version. Join this workspace. The interaction of light and matter determines the appearance of everything around us. Light interacts with matter in ways such as emission and absorption.

The photoelectric effect is an example of how matter absorbs light. What are the properties of light? The primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarization, while its speed in a vacuum, ,, meters per second, is one of the fundamental constants of nature.

Like all types of EM radiation, visible light propagates as waves. Is light a matter? Light is a form of energy, not matter. Matter is made up of atoms. Light is actually electromagnetic radiation. Moving electric charge or moving electrons electric current cause a magnetic field, and a changing magnetic field creates an electric current or electric field.

Why are energy levels quantized? Quantized energy levels result from the relation between a particle's energy and its wavelength. For a confined particle such as an electron in an atom, the wave function has the form of standing waves. How do the properties of light determine its interaction with matter? How does light interact with matter? How do we experience light?

Do all objects absorb light? For instance, sunlight also comprises lights of various frequencies; from around to nm. Therefore, most objects selectively absorb, transmit or reflect the light.