Problem # 55 Why objects have different colors[Solved]
Question: Why a given object is colored orange, i.e. absorbs the blue light, whereas another object is red, i.e. absorbs the green light, while another reflects the whole white light to appear white, another black, another is transparent but colorless?
Answer:
A/ Absorption, Transmission, Reflection of light
As we have seen previously, visible light is located in the region of light spectrum where all visible colors are located: λ= 700-400 nm. The lights outside that region, such as UV and IR, cannot be detected by our eyes.
We have also seen that the color of an object depends upon the light absorbed, reflected, or emitted by the object.
When the white light hits an object, a light of a given wavelength or frequency can match the energy needed by an electron to be excited. In this case that light (with a specific color) is absorbed by the object. The remaining other lights/colors that do not match the energy of excitation of the electrons are not absorbed.
Since the excitation energy differs from one chemical species to another due to their different electron structures, this explains why they absorb lights of different wavelength to be excited.
Transparent objects: a transparent object let pass light (whole white light or some lights); it can be colored or colorless.
- A colorless transparent object lets pass and transmit the whole white light (made of all colors), without absorption of any light color; ex: Water.
- A colored transparent object absorbs a light/color that matches the excitation energy of the electrons in the object, and let pass all the remaining lights/colors; the color perceived by our eyes is the color corresponding to the remaining or transmitted colors: the complementary color of the absorbed one; ex: KMnO4 solution is red-purple because it absorbs yellow-green color.
Opaque objects: an opaque object is an object that is not transparent to light, this applies mainly to solids.
- A white object reflects the whole white light without absorbing any light color; ex: White paper.
- A colored but opaque object reflects a particular light color, and absorbs the remaining light colors of the white light; ex: The red colored car.
- A black and opaque object absorbs all light colors of the white light; ex: The black bag in which you keep your computer.
B/ Emission of light
Electrons in some chemical species such as metals and metal ions can be excited to higher energy levels by absorbing a light in the visible region, creating an excited state of the species. The excited state is not stable and tends to return to the ground state or non-excited state.
In the process, the energy absorbed for excitation is emitted back in form of corresponding light with the corresponding color (you can revisit Problem 5)
Emission flame: the principle described above is used to test and identify some metals and their ions by a technique called Flame test. In that technique, a sample is put in flame; the energy of the flame excites electrons in the sample to higher energy levels, excited state; but that excited state returns immediately to the ground state by emitting the energy previously absorbed. The color of the flame allows to identify the presence or not of a given metal (see Figure7 of Problem 5)
Electronic excitation can be caused by other means of energy such as electricity: the same principle is applied except that another kind of energy, such as electricity, is used. For example a sodium-vapor lamp is gas-discharge lamp where electrons of the gas metal (or any other gas such as He, Ar) are excited to higher energy levels, and when they come back to the ground state, they emit light of a characteristic color. Ex; Colors of the Traffic lights: the bright Yellow color is emitted by Sodium, Na.
Traffic Lights
(Pngtree)