Question

How is the color of a molecule depend on its size?

I know abosrobing different wavelength shows different color, so please explain with the terms in the keyword. I need the mechanism, not the phenomenon.

Keyword: electron, box, length, quantization, spacing, photon, absorption, kinetic energy

Answer 1

The color of most objects depends upon the interaction between visible light and the electrons of atoms or molecules that make up the object. It is usually the result of a dynamic process on the molecular level: the absorption of light and the resulting change of a molecule's quantized energy. The object absorbs certain wavelengths of white light, and we see what is left over. The particular wavelengths of light that a given substance absorbs determine the color we perceive and depend on the energy levels of the molecules or atoms that make up the substance. This absorption coloration mechanism is responsible for the colors of grass, blood and carrots, but not of the sky. The sky's color is due to selective scattering of different wavelengths of sunlight by the molecules of nitrogen and oxygen in the atmosphere.

Visible light is the very small portion of the electromagnetic spectrum that human eyes are sensitive to. Light can be described as oscillating electric and magnetic fields, as can radio waves, microwaves, infrared radiation, ultraviolet light, and x-rays. Visible light differs from these other types of light because it has a range of wavelengths that our eyes can detect. These visible wavelengths match the differences between quantized energy levels of the detection molecules in the retina of the human eye. Thus, the first step in the perception of color also involves the absorption of visible light by molecules, in the retina of the perceiver's eye.

The perceived color of an object has a complementary relationship with the color of the visible light absorbed. First consider white light, a mixture of all visible wavelengths, impinging on a colored object. The object absorbs some wavelengths of the light; exactly which ones depends on the component molecule's energy levels. The wavelengths of light that are not absorbed are transmitted or reflected to the observer's eye. A substance that appears blue is transmitting or reflecting blue light to the eye and absorbing other colors of the white light that are not blue. There are two ways for a material to produce the perception of a particular color. One is to absorb all wavelengths of visible light, aside from the perceived color. For our blue example, the material would absorb red, orange, yellow, and violet light. The absorption spectrum would show high absorbance of all visible wavelengths, besides blue. The transmission or reflectance spectrum would have a maximum at a wavelength corresponding to blue light. The second way to create the perception of blue is for a material to have a strong absorbance of the opposite or complementary color of light. A color wheel, shown below, illustrates the approximate complementary relationship between the wavelengths of light absorbed and the wavelengths transmitted or reflected. Your textbook has a color version of this color wheel that can help you understand this complementary relationship. In the example of a blue substance, there would be a strong absorbance of the complementary color of light, orange. For this case, the absorption spectrum of a blue solution would have a maximum absorbance at a wavelength corresponding to orange light.



Color wheel with approximate wavelength values
for different color light.

Since color results from the absorption of visible light, it is important to examine what happens to a molecule when it absorbs a photon, or quantum of light. Molecules and atoms absorb light only when the energy of an impinging photon matches the energy difference between the state in which the molecule initially finds itself and some excited state of the molecule. To change from a lower quatized energy level to a higher one, the energy of the photon must match the energy gap between the levels. In equation form we can write

E_{lower state} + E_photon = E_{upper state}

That is, in order for light absorption to take place,

Ephoton = h = Emolecule = Eupper state - Elower state

(1)

So to understand the color of an object, which arises from its absorption of light, we must know the array of possible energy levels of its molecules. In general these energy levels include states of quantized rotational, vibrational, and electron energy. These correspond to rotation of the entire molecule, vibration of the chemical bonds within the molecule, and changes in the electron configuration of the molecule. With rare exceptions, colored substances that absorb photons in the visible region of the spectrum undergo a transition that changes the electron energy levels of the molecule. The photon of visible light is absorbed and excites a molecule from its lowest-energy or ground electron configuration to a higher-energy electron configuration. Transitions between electron configurations, or electronic states, are responsible for the majority of the colors we see in the natural world.

You may be wondering what happens to the energy of the photon, after the molecules of a colored substance absorb it. Most typically, the molecule quickly returns to its ground electron configuration, with the extra energy imparted by the photon converted to vibration and rotation of the molecule. The visible light absorbed is thus converted to vibration and rotation, which are the molecular basis of heat. This heat will raise the temperature of the material. Here is a familiar example: A white material, which reflects all visible wavelengths, does not heat up in the sun as much as a black material, which absorbs all visible wavelengths. In this process, first the visible photons are absorbed, causing the electrons in the black material to be excited. Then the electrons return to their ground state configurations and the extra energy moves to vibrational and rotational motion, causing the temperature of the material to increase.