A laser is different. Lasers do not occur in nature. However, we have figured ways to artificially create this special type of light. Lasers produce a narrow beam of light in which all of the light waves have very similar wavelengths. The laser’s light waves travel together with their peaks all lined up, or in phase. This is why laser beams are very narrow, very bright, and can be focused into a very tiny spot.

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The letters in the word laser stand for Light Amplification by Stimulated Emission of Radiation. A laser is an unusual light source. It is quite different from a light bulb or a flash light. Lasers produce a very narrow beam of light. This type of light is useful for lots of technologies and instruments—even some that you might use at home!

This animation shows a representation of the different wavelengths present in sunlight. When all of the different wavelengths (colors) come together, you get white light. Image credit: NASA

Because laser light stays focused and does not spread out much (like a flashlight would), laser beams can travel very long distances. They can also concentrate a lot of energy on a very small area.

Each color of light has a different wavelength. For example, blue light has a shorter wavelength than red light. Sunlight—and the typical light from a lightbulb—is made up of light with many different wavelengths. Our eyes see this mixture of wavelengths as white light.

It isn't necessarily. It could also be transmitted and continue to propagate through the new material. (This is how you can see things through a glass window)

Lasers have many uses. They are used in precision tools and can cut through diamonds or thick metal. They can also be designed to help in delicate surgeries. Lasers are used for recording and retrieving information. They are used in communications and in carrying TV and internet signals. We also find them in laser printers, bar code scanners, and DVD players. They also help to make parts for computers and other electronics.

Light is reflected if there is a difference in the index of refraction of the two materials, or if the second material is highly conductive (for example, metal).

Scientists have even measured the distance between the moon and Earth using lasers! By measuring the amount of time it takes for a laser beam to travel to the moon and back, astronomers can tell exactly how far away it is!

In the case of specular reflection, the photons just propagate according to Maxwell's equations. A high conductivity material or a boundary between materials with different index of refraction causes a boundary condition that causes the light wave to reflect. There's no absorption of the photons involved.

Lasers are also used in instruments called spectrometers. Spectrometers can help scientists figure out what things are made of. For example, the Curiosity rover uses a laser spectrometer to see what kinds of chemicals are in certain rocks on Mars.

This is a picture of Martian soil before (left) and after (right) it was zapped by the Curiosity rover’s laser instrument called ChemCam. By zapping tiny holes in Martian soil and rock, ChemCam can determine what the material is made of. Image credit: NASA/JPL-Caltech/LANL/ CNES/IRAP/LPGN/CNRS

NASA missions have used lasers to study the gases in Earth’s atmosphere. Lasers have also been used in instruments that map the surfaces of planets, moons, and asteroids.

Light is absorbed if it is propagating in a material and that material has some particles in it that are able to absorb light at that particular wavelength and be promoted into a higher energy state. Particles here could be electrons, but they could also be things like phonons (quantized lattice vibrations).

So if there's a light beam impinging on a surface, to be absorbed it should first not be (completely) reflected at the surface, and then it is absorbed as it begins to propagate into the material.

Do the atoms within a material absorb the photons and re-emit them in the direction they came back from (similar to atomic emission spectra, but slightly different because this involves hybridization of orbitals), or for some other reason?

And expanding upon that, why is light absorbed by a material if it is not reflected? Thinking about it like atomic emission spectra, the other light should pass through. I know this is not the case because these are molecules and have many more energy levels that single atoms, but in the case that all of the light is absorbed, why is only some of it reflected? What dictates whether light is absorbed or reflected?