This page titled PC1. Absorbance is shared under a CC BY-NC 3.0 license and was authored, remixed, and/or curated by Chris Schaller.

in which A = Absorbance, the percent of light absorbed; c = the concentration; l = the length of the light's path through the solution; ε = the "absorptivity" or "extinction coeficient" of the material, which is a measure of how easily it absorbs a photon that it encounters.

The higher the frequency, the higher the energy of the photon. The longer the wavelength, the lower the energy of the photon.

In this context it is worth noting that we also see a lot of colors that are not contained in the visible light spectrum. That is, we see colors other than violet, blue, green, yellow, orange, and red. This is because the visible light spectrum contains only colors that have one wavelength. These colors are called pure colors. However, a number of different wavelengths can mix to give rise to other new colors. That is why, the human eye also sees colors like pink that is a variation of red or magenta that is a variation of violet.

These two factors together make up part of a mathematical relationship, called Beer's Law, describing the absorption of light by a material:

Different materials absorb photons of different wavelengths because absorption of a photon is an absorption of energy. Something must be done with that energy. In the case of ultraviolet and visible light, the energy is of the right general magnitude to excite an electron to a higher energy level.

A "colour wheel" or "colour star" can help us keep track of the idea of complementary colours. When a colour is absorbed on one side of the star, we see mostly the colour on the opposite side of the star.

However, we know that energy is quantized. That means photons will be absorbed only if they have exactly the right amount of energy to promote an electron from its starting energy level to a higher one (producing an "excited state"). Just like Goldilocks, a photon with too much energy won't do the trick. Neither will a photon with too little. It has to be just right.

That white light is made up of seven different colors can be proven with this experiment. If you make white light pass through a prism, the light will be separated into its seven components—red, orange, yellow, green, blue, indigo, and violet—and pass out of the prism in a rainbow of colors. The sequence, in which these colors are present in the visible light spectrum, in their reverse order, is expressed using the popular acronym VIBGYOR. When passing through the prism, the color violet is bent more than red because the former has a shorter wavelength.

These seven colors represent the visible light spectrum. Water vapor present in the atmosphere may act like a prism. When sunlight passes through these water vapor particles, the light is separated into the seven different colors of the visible light spectrum. As a result, we see a rainbow after a spell of rain.

Ultraviolet light -- invisible to humans and with wavelengths beyond that of violet light -- is associated with damage to skin; these are the cancer-causing rays from the sun. Explain their danger in terms of their relative energy.

The electromagnetic waves in the visible light spectrum are often expressed in nanometers, 1 nanometer being equal to 0.0000001 centimeter or 10-7 centimeters. Violet, indigo, blue, green, yellow, orange, and red lights have wavelengths between 400 and 700 nanometers. The following diagram represents the electromagnetic spectrum. It is evident that human beings can perceive only a fraction of the entire electromagnetic spectrum.

Human beings can perceive only a tiny fraction of waves in the electromagnetic spectrum. These electromagnetic waves are known as visible light waves and are emitted by objects like light bulbs, stars, and fireflies. We perceive these light waves as the seven colors of the rainbow. At one end of this range is the color red with the longest wavelength. At the other end of the visible light spectrum is the color violet with the shortest wavelength. When all these light waves are mixed, we see white light.

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Remember, often a particular soda will absorb light of a particular colour. That means, only certain photons corresponding to a particular colour of light are absorbed by that particular soda.

If the absorption of a UV-visible photon is coupled to the excitation of an electron, what happens when the electron falls back down to the ground state? You might expect a photon to be released.

Absorptionspectra

Why do certain materials absorb only certain colours of light? That has to do with the properties of photons. Photons have particle-wave duality, just like electrons. They have wave properties, including a wavelength.

That last factor, ε, suggests that not all photons are absorbed easily, or that not all materials are able to absorb photons equally well. There are a couple of reasons for these differences.

Visiblelightwavelength

The electromagnetic (EM) spectrum refers to the range of all types of EM rays. These types of rays include radio waves, microwaves, infrared light waves, ultra-violet rays, X-rays, and gamma rays. These rays have different frequencies and reside at various points of the electromagnetic spectrum. Many electrical appliances that we use in our daily lives emit one of these rays.

Electromagnetic radiation can be measured and expressed using units of energy, frequency, or wavelength. The unit is frequency is cycles per second or Hertz. Energy is expressed in electron volts while wavelength is expressed in meters. The different units are used to facilitate ease of expression.

Light is composed of photons. As photons shine through the solution, some of the molecules catch the photons. They absorb the light. Generally, something in the molecule changes as a result. The molecule absorbs energy from the photon and is left in an excited state.

This phenomenon was observed during the late nineteenth century, when scientists studied the "emission spectra" of metal ions. In these studies, the metal ions would be heated in a flame, producing characteristic colours. In that event, the electron would be thermally promoted to a higher energy level, and when it relaxed, a photon would be emitted corresponding to the energy of relaxation.

Alternatively, the Planck-Einstien equation can be thought of in terms of frequency of thr photon: as a photon passes through an object, how frequently does one of its "crests" or "troughs" encounter the object? How frequently does one full wavelength of the photon pass an object? That parameter is inversely proportional to the wavelength. The equation becomes:

How does that affect what we see? If the red light is being absorbed by the material, it isn't coming back out again. The blue and yellow light still are, though. That means the light coming out is less red, and more yellowy-blue. We see green light emerging from the glass.

The above-mentioned pieces of information regarding electromagnetic radiation, the visible light spectrum, and how human beings perceive colors help to explain the common physical phenomena that take place around us every day.

We can see objects and perceive the different colors on these objects because of the presence of visible light. The sun is a natural source of visible white light while light bulbs produce visible light artificially. The cone cells in human eyes perceive these visible light waves. When white light falls on an object, one or more of the component colors are absorbed while some are reflected. The color of an object as we perceive it to be is the color or the specific light wave that has not been absorbed.

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Let's think first about the interaction of light with matter. We have all seen light shine on different objects. Some objects are shiny and some are matte or dull. Some objects are different colors. Light interacts with these objects in different ways. Sometimes, light goes straight through an object, such as a window or a piece of glass.

in which E = energy of the photon, h = Planck's constant (6.625 x 10-34 J s-1), c = speed of light (3.0 x 108 m s-1), λ = wavelength of light in m.

Imagine sunlight shining through a glass of soda. Maybe it is orange or grape soda; it is definitely coloured. We can see that as sunlight shines through the glass, colored light comes out the other side. Also, less light comes out than goes in.

As a result of this relationship, different photons have different amounts of energy, because different photons have different wavelengths.

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Alternatively, if we kept the concentration of molecules the same, but doubled the length of the vessel through which the light traveled, it would have the same effect as doubling the concentration. Twice as much light would be absorbed.

Waterabsorptioncoefficient

In this context, it is worth noting that nowadays the color indigo is not considered to be a part of the visible light spectrum. This is because it is believed that this color or its frequency cannot be easily distinguished in the light spectrum. In fact, it has been found that some people with marvelous vision cannot distinguish indigo from shades of blue and cyan. So the acronym VIBGYOR is now actually VBGYOR.

The more of these molecules there are in the solution, the more photons will be absorbed. If there are twice as many molecules in the path of the light, twice as many photons will be absorbed. If we double the concentration, we double the absorbance.

For instance, the remote control of a television set emits infrared rays. Some rays like radio waves are also emitted by astral bodies like stars while some “hot” bodies in the space, like the sun, emit ultra-violet rays. Not all rays are visible to the human eye. EM rays in a particular range of the electromagnetic spectrum emit light that human beings can see.

By passing this light through a prism or grating, scientists could separate the observed colour into separate lines of different wavelengths. This evidence led directly to the idea of Niels Bohr and others that atoms had electrons in different energy levels, whci is part of our view of electronic structure today.

So, what is the soda made of? Molecules. Some of these molecules are principally responsible for the colour of the soda. There are others, such as the ones responsible for the flavor or the fizziness of the drink, as well as plain old water molecules. The soda is a solution; it has lots of molecules (the solute) dissolved in a solvent (the water).

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The visible spectrum ranges from photons having wavelengths from about 400 nm to 700 nm. The former is the wavelength of violet light and the latter is the wavlength of red light. Which one has higher energy: a photon of blue light or a photon of red light?