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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. Page 1 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? Page i. ii. 2 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 Page 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 3 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. Page 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 4 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. Page 5 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 Page ● Construction paper ● Wood 6 Opaque: Page 7 ● Stainless Steel ● Silver Bracelet ● Stack of white paper