Download EVERYDAY ENGINEERING EXAMPLES FOR SIMPLE CONCEPTS

EVERYDAY
ENGINEERING
EXAMPLES FOR SIMPLE
CONCEPTS
ENGR 3350 - Materials Science
Dr. Nedim Vardar
Copyright © 2015
Optical
Properties of
Materials
MSEIP – Engineering
Everyday Engineering Examples
Light: Transparent, Translucent, Opaque
Engage:
The student will investigate and understand basic characteristics of light and
optical properties of materials. Key concepts include;
 photon,
 refraction, reflection, adsorption,
 opaque, transparent, and translucent.
Issues to address;
1. Why are some materials transparent and others are translucent or
opaque?
2. What phenomena occur when light is shined on a material?
3. Understand how the thickness of an object can change whether it is
transparent, translucent or opaque
4. Why can't you see your reflection in a brick wall?
Transparent
Translucent
Figure 1. Optical classification of materials.
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Opaque
Explore:
Provide students the following worksheet (Table 1) and give them the materials
indicated in the worksheet. Ask students to make predictions on their worksheets
and to sort out the materials into transparent, translucent and opaque without
using any light source. Ask for volunteers to share their predictions with the class.
Table 1. Worksheet for Light Properties of Materials
Material Name
White Light
Translucent
Transparent
Laser
Opaque
Translucent
Transparent
Opaque
1 Aluminum paper
2 Clear glass
3 Wax paper
4 Polyurethane foam
5 Granite
6 Acrylic sheet
7 Piece of a paper
After the prediction phase, provide each student a flashlight and laser light. Ask
students to hold the flashlight to each material and record their observations
and explanations for what happens. Ask students to hold the other light source
to each materials and record their observations.
Which materials allowed the light to pass through?
Did you find any materials that let some light through?
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i.
ii.
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Ask students if they had to change their original predication. After the
exploration, invite volunteers to share their results with the class. Ask questions,
such as:
iii.
In which material was light absorbed the most? How do they know?
Explain:
When materials are exposed to electromagnetic radiation, it is sometimes
important to be able to predict and alter their responses. This is possible when
we are familiar with their optical properties and understand the mechanisms
responsible for their optical behaviors.
When light proceeds from one medium into another (e.g., from air into a solid
substance), several things happen. Some of the light radiation may be
transmitted through the medium, some will be absorbed, and some will be
reflected at the interface between the two media (Figure 1). The intensity I0 of
the beam incident to the surface of the solid medium must equal the sum of the
intensities of the transmitted, absorbed, and reflected beams, denoted as IT, IA
and IR, and respectively, or
I 0= I T + I A + I R
Figure 1 Light radiation from one medium into another
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i) Transparent materials, such as glass or water, transmit (let through) most light
without it being scattered or absorbed. They still reflect some light otherwise we
could not see them. Very clear glass might appear to be invisible because
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Materials can be classified as transparent, translucent, and opaque according
to what they do to light:
virtually all light is transmitted. Because most light is transmitted through
transparent materials, we can see objects through them.
ii) Translucent materials, such as wax paper and frosted glass, transmit less light
than transparent materials. Some light is scattered or absorbed and some is
reflected. We can see objects through translucent materials but because some
of the light is scattered or absorbed we cannot see them clearly.
iii) Opaque materials, such as cardboard or wood, scatter, absorb or reflect
most light and transmit almost no light. Thus, we cannot see through them. The
light which is reflected does so in a scattered way, that is, in all directions.
Depending on the composition, opaque materials absorb some colours of
visible light more than others. The material will appear the colour that it reflects
the best.
Transparent, translucent and opaque surfaces all reflect some light otherwise we
could not see them. However, they reflect light in differing ways and amounts.
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The optical phenomena that occur within solid materials involve interactions
between the electromagnetic radiation and atoms, ions, and/or electrons.
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. 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
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Elaborate:
different natural frequencies of vibration, they will selectively absorb different
frequencies of visible light.
The amount of light absorbed by a material is calculated using Beer’s Law.
I T I 0 e  
where;
 = absorption coefficient, cm-1
 = sample thickness, cm
I T = incident light intensity
I 0
= transmitted light intensity
When light radiation passes from one medium into another having a different
index of refraction, some of the light is scattered at the interface between the
two media even if both are transparent. The reflectivity R represents the fraction
of the incident light that is reflected at the interface, or
 
R I R 
 
 I0 
where I0 and IR are the intensities of the incident and reflected beams,
respectively. If the light is normal (or perpendicular) to the interface, then
 n2  n1 
R

 n2  n1 
2
where n1and n2 are the indices of refraction of the two media. If the incident
light is not normal to the interface, R will depend on the angle of incidence.
When light is transmitted from a vacuum or air into a solid s, then
 n 1
R

 n 1
2
After these theory part, ask students why one sheet of paper is translucent but a
stack of papers is opaque.
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The answer to that question is: Each sheet absorbs some light. When light has to
pass through many sheets, all the light is absorbed.
Deep-sea divers know it is dark deep below the ocean’s surface. Just as a
paper absorbs some light, so does water. As light passes through a lot of water,
a little bit of light is absorbed at a time. After a while, very little light is left. In the
ocean, sunlight passes only through the top layers of water. It is very dark deep
in the ocean.
For this reason, many kinds of sea creatures that live deep down in the ocean
have to create their own light as opposed to depending on the light of the sun.
This light that they make is called bioluminescent light.
What did you learn?
How light absorbed and transmitted in opaque, transparent and translucent
materials. Water pressure increases with water depth. Therefore, water pressure
is higher at the bottom of the bottle than at the top. As result, the lower jet,
which has higher water pressure, squirts out further.
Evaluate:
Give students following materials and ask them to sort out the materials into
transparent, translucent and opaque;
Paper towel, wood, stack of white paper, laminating material, frosted light bulb,
water, stainless steel, silver bracelet, construction paper, plastic wrap.
Answer:
Transparent items:
● Plastic wrap
● Laminating material
● Water
Translucent items:
● Paper towel
● Frosted light bulb
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● Construction paper
● Wood
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Opaque:
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● Stainless Steel
● Silver Bracelet
● Stack of white paper