Light Absorption, Reflection, and Transmission
We have previously learned
that visible light waves consist of a continuous range of wavelengths or
frequencies. When a light wave with a single frequency strikes an object, a
number of things could happen. The light wave could be absorbed by the object,
in which case its energy is converted to heat. The light wave could be
reflected by the object. And the light wave could be transmitted by the object.
Rarely however does just a single frequency of light strike an object. While it
does happen, it is more usual that visible light of many frequencies or even
all frequencies is incident towards the surface of objects. When this occurs,
objects have a tendency to selectively absorb, reflect or transmit light
certain frequencies. That is, one object might reflect green light while absorbing
all other frequencies of visible light. Another object might selectively
transmit blue light while absorbing all other frequencies of visible light. The
manner in which visible light interacts with an object is dependent upon the
frequency of the light and the nature of the atoms of the object. In this
section of Lesson 2 we will discuss how and why light of certain frequencies
can be selectively absorbed, reflected or transmitted.
Visible
Light Absorption
Atoms and molecules contain electrons. It is often useful to
think of these electrons as being attached to the atoms by springs. The
electrons and their attached springs have a tendency to vibrate at specific
frequencies. Similar to a tuning fork or even a musical instrument, the
electrons of atoms have a natural frequency at which
they tend to vibrate. When a light wave with that same natural frequency
impinges upon an atom, then the electrons of that atom will be set into vibrational
motion. (This is merely another example of the resonance
principle introduced in Unit 11 of The Physics Classroom Tutorial.) If a light
wave of a given frequency strikes a material with electrons having the same
vibrational frequencies, then those electrons will absorb the energy of the
light wave and transform it into vibrational motion. During its vibration, the
electrons interact with neighboring atoms
in such a manner as to convert its vibrational energy into thermal energy.
Subsequently, the light wave with that given frequency is absorbed by the
object, never again to be released in the form of light. So the selective
absorption of light by a particular material occurs because the selected
frequency of the light wave matches the frequency at which electrons in the
atoms of that material vibrate. Since different atoms and molecules have
different natural frequencies of vibration, they will selectively absorb
different frequencies of visible light.
Visible
Light Reflection and Transmission
Reflection and transmission of light waves occur because the
frequencies of the light waves do not match the natural frequencies of
vibration of the objects. When light waves of these frequencies strike an
object, the electrons in the atoms of the object begin vibrating. But instead
of vibrating in resonance at a large amplitude, the electrons vibrate for brief
periods of time with small amplitudes of vibration; then the energy is
reemitted as a light wave. If the object is transparent, then the vibrations of
the electrons are passed on to neighboringatoms
through the bulk of the material and reemitted on the opposite side of the
object. Such frequencies of light waves are said to be transmitted. If the object is opaque, then the vibrations of the electrons are not
passed from atom to atom through the bulk of the material. Rather the electrons
of atoms on the material's surface vibrate for short periods of time and then
reemit the energy as a reflected light wave. Such frequencies of light are said
to be reflected.
Where Does Color Come
From?
The color of the objects
that we see is largely due to the way those objects interact with light and
ultimately reflect
or transmit it to our eyes. The color of an
object is not actually within the object itself. Rather, the color is in the light that shines upon it and is
ultimately reflected or transmitted to our eyes. We know that the visible light
spectrum consists of a range of frequencies, each of which corresponds to a
specific color. When visible light strikes an
object and a specific frequency becomes absorbed, that frequency of light will
never make it to our eyes. Any visible light that strikes the object and
becomes reflected or transmitted to our eyes will contribute to the color appearance of that object. So the color is not in the object itself, but in the light
that strikes the object and ultimately reaches our eye. The only role that the
object plays is that it might contain atoms capable of selectively absorbing
one or more frequencies of the visible light that shine upon it. So if an
object absorbs all of the frequencies of visible light except for the frequency
associated with green light, then the object will appear green in the presence
of ROYGBIV. And if an object absorbs all of the
frequencies of visible light except for the frequency associated with blue
light, then the object will appear blue in the presence of ROYGBIV.
Consider the two diagrams below. The diagrams
depict a sheet of paper being illuminated with white light (ROYGBIV). The papers are impregnated with a chemical
capable of absorbing one or more of the colors of
white light. Such chemicals that are capable of selectively absorbing one or
more frequency of white light are known as pigments. In Example A, the pigment in the sheet of paper is capable of
absorbing red, orange, yellow, blue, indigo and
violet. In Example B, the pigment in the sheet of paper is capable of absorbing
orange, yellow, green, blue, indigo and violet. In each case, whatever color is not absorbed is reflected.
Check your understanding of these principles by determining which color(s) of light are reflected by the paper and what color the paper will appear to an observer.
Example A: Green will be reflected and so the paper appears
green to an observer.
Example B: Red
will be reflected and so the paper appears red to an observer.
Transparent materials are materials that allow one or more of
the frequencies of visible light to be transmitted through them; whatever color(s) is/are not transmitted by such objects, are
typically absorbed by them. The appearance of a transparent object is dependent
upon what color(s) of light is/are incident upon
the object and what color(s) of light is/are
transmitted through the object.
Express your understanding of this principle by filling in the blanks in
the following diagrams.
Example A: Green will be transmitted and so the object appears
green to an observer.
Example B: Both
green and blue will be transmitted and so the object appears greenish-blue to
an observer.
The colors perceived of
objects are the results of interactions between the various frequencies of
visible light waves and the atoms of the materials that objects are made of.
Many objects contain atoms capable of either selectively absorbing, reflecting
or transmitting one or more frequencies of light. The frequencies of light that
become transmitted or reflected to our eyes will contribute to the color that we perceive.
Check Your Understanding
1. Natural philosophers have long pondered the underlying
reasons for color in nature. One common
historical belief was that colored objects
in nature produce small particles (perhaps light particles) that subsequently
reach our eyes. Different objects produce different coloredparticles,
thus contributing to their different appearance. Is this belief accurate or
not? __________________ Justify your answer.
Answer: Not accurate
This view
presumes that the appearance of an object is independent of the colors of light which illuminate the object. We
observe that the same object appears different colors when
viewed under different light. So the secret to an object's appearance is not
strictly due to its ability to produce a color.
In fact the object's only role in determining its appearance is in its ability
to absorb certain wavelengths of light which shine upon it.
2. What color does a red
shirt appear when the room lights are turned off and the room is entirely dark?
____________ What about a blue shirt? ____________ ... a green shirt?
____________
Answer: Black
When the room
lights are turned off (there is no light), any object present in the room
appears black. The color appearance of an
object depends upon the light which that objects reflects to the observer's eye.
Without any incident light, there can be no reflected light. Such an object
appears black - the absence of light.
close
3. The diagrams depict a sheet of paper being illuminated
with white light (ROYGBIV). The papers are impregnated with a chemical capable
of absorbing one or more of the colors of
white light. In each case, determine which color(s)
of light are reflected by the paper and whatcolor the
paper will appear to an observer.
Practice A: No light will be reflected; it is all absorbed.
Thus, the paper would appear black to an observer.
Practice B: Red
and orange will be reflected and so the paper appears reddish-orange to an
observer.
4. The appearance of a transparent object is dependent upon
which color(s) of light is/are incident upon the
object and which color(s) of light is/are
transmitted through the object. Express your understanding of this principle by
determining which color(s) of light will be
transmitted and the color that the paper
will appear to an observer.
Practice A: Green and blue light will be transmitted and so the
object would appear greenish-blue to an observer.
Practice B: Red
and orange light will be transmitted and so the object would appear reddish-orange to an observer.
Practice C: Red and blue light will be transmitted and so the
object would appear reddish-blue to an observer.
Practice D: Only
red light will be transmitted and so the object would appear red to an observer.