SNC2D PHYSICS 5/19/2013. LIGHT & GEOMETRIC OPTICS L The Refraction of Light (P ) Activity: Observing Refraction (Part 1)

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1 SNC2D PHYSICS LIGHT & GEOMETRIC OPTICS L The Refraction of Light (P ) Activity: Observing Refraction (Part 1) INSTRUCTIONS A. Take a pencil and place it into a glass of water. B. Observe the glass from the side at various angles. May 19, DPHYS - The Refraction of Light 1 Activity: Observing Refraction (Part 1) QUESTIONS 1. What do you notice? the pencil appears to be bent at the air/water boundary May 19, DPHYS - The Refraction of Light 2 1

2 Activity: Observing Refraction (Part 2) INSTRUCTIONS C. Place a jar lid on a desk and put a coin close to the edge. (No lid? Use a Petri dish with some masking tape along the edge.) D. Keep watching the coin while you lower the height of your head until the coin just disappears from view behind the rim of the lid. E. Keeping your head at the same level, pour water into the lid, on top of the coin. Record your observations. May 19, DPHYS - The Refraction of Light 3 Activity: Observing Refraction (Part 2) QUESTIONS 2. Why are you able to see the coin after the water was added? the water caused the light rays to be bend May 19, DPHYS - The Refraction of Light 4 What is Refraction? Light travels in straight lines through air. But what happens when it travels from one material into another? You likely have noticed that a spoon or stir stick, when placed in a glass of water, looks somewhat disconnected at the surface of the water. The spoon is not really disconnected. It is made of a solid material. So what is happening? May 19, DPHYS - The Refraction of Light 5 2

3 What is Refraction? When light crosses a boundary, for example, from water to air, the light is refracted. Refraction is the bending of light at the boundary of two transparent substances. The transparent substances are sometimes called media. While refraction causes some very interesting visual effects, it is more than just an optical curiosity. Refraction is used in designing and building camera lenses, eyeglasses, and telescopes. May 19, DPHYS - The Refraction of Light 6 What Causes Refraction? When a light ray moves from one medium into another, its speed changes. For example, the speed of a light ray in water is less than the speed of a light ray in air (see chart). It is the change in the speed of light that causes the refraction of light. NOTE! Since light travels faster in air than it does in water the air is the fast medium while the water is the slow medium. Medium Speed (km/s) air ice liquid water vegetable oil glass ruby diamond May 19, DPHYS - The Refraction of Light 7 What Causes Refraction? LIGHT travels at different speeds through different media media are referred to as fast and slow Medium Speed (km/s) air ice liquid water vegetable oil glass ruby diamond May 19, DPHYS - The Refraction of Light 8 3

4 What Causes Refraction? PRACTICE 1. The speed of light in glass is 197,000 km/s while the speed of light in diamond is 124,000 km/s. Which is the fast medium? since 197,000 > 124,000 the glass is the fast medium May 19, DPHYS - The Refraction of Light 9 What Causes Refraction? PRACTICE 2. As stated earlier, it is the change in the speed of light that causes the refraction of light. But why is there a change in the speed? May 19, DPHYS - The Refraction of Light 10 What Causes Refraction? To help visualize this phenomena, consider a child s wagon being pulled at an angle from pavement (a fast medium) onto sand (a slow medium). When the right front wheel meets the sand, it slows down. The left front wheel does not slow down because it is still on the pavement. As a result, the wagon pivots about the slower right wheel. May 19, DPHYS - The Refraction of Light 11 4

5 What Causes Refraction? And the same way that the wagon changes direction when it travels from one surface to another, light rays change direction when they pass from one medium to another. May 19, DPHYS - The Refraction of Light 12 What Causes Refraction? REFRACTION bending of light as it moves from one medium into another caused by a change in the speed of light the more the light slows down the more it bends (refracts) toward the normal May 19, DPHYS - The Refraction of Light 13 Rules of Refraction NOTE! Because the speed of light changes depending on the medium through which it is travelling, it is possible to make two general statements or rules about refraction and the angle of refraction(r) (i.e. the angle between the refracted light ray and the normal). May 19, DPHYS - The Refraction of Light 14 5

6 Rules of Refraction 1. When travelling from a fast medium into a slow medium (i.e. such as air to glass) the light slows down and bends toward the normal. In this case, the angle of incidence (i) is larger than the angle of refraction (R). fast û slow i>r May 19, DPHYS - The Refraction of Light 15 Rules of Refraction 2. When travelling from a slow medium into a fast medium (i.e. such as glass to air) the light speeds up and bends away from the normal. In this case, the angle of incidence (i) is smaller than the angle of refraction (R). slow û fast i<r May 19, DPHYS - The Refraction of Light 16 Rules of Refraction RULES OF REFRACTION Î When light travels from a fast medium into a slow medium it: slows down and bends toward the normal (i>r) Ï When light travels from a slow medium into a fast medium it: speeds up and bends away from the normal (i<r) May 19, DPHYS - The Refraction of Light 17 6

7 Rules of Refraction NOTE! Whenever a ray is normal to a boundary (i.e. i = 0E), the transmitted or refracted ray continues in the same direction as the incident ray without bending (i.e. R = 0E). However, partial reflection at the boundary still occurs. We can t see the partially reflected rays because the reflected ray lies exactly on the incident ray. May 19, DPHYS - The Refraction of Light 18 Ray Diagrams of Refraction PRACTICE 3. According to the diagram, which is the fast medium the air or liquid? May 19, DPHYS - The Refraction of Light 19 Dispersion A special kind of refraction occurs in both a diamond and raindrops. A diamond can appear completely colourless and yet glitter in all colours of the rainbow because the amount of refraction is different for each color. Since white light contains many colours, a single beam of white light can enter a diamond and be split into a whole rainbow of colours. This kind of refraction is called dispersion. Dispersion is the separation of white light into its individual spectral colours. May 19, DPHYS - The Refraction of Light 20 7

8 Dispersion The most common type of dispersion is in the formation of a rainbow. When sunlight passes through a raindrop, some of the light is reflected internally while some of the light is refracted twice, once on entering the raindrop and once on leaving. Both refractions cause the separation of the white sunlight into the colours of the rainbow. May 19, DPHYS - The Refraction of Light 21 Dispersion DISPERSION separation of white light into its spectral colours rainbows, prisms, May 19, DPHYS - The Refraction of Light 22 Dispersion PRACTICE 4. How or why does refraction cause the separation of white light into its spectral colours? not only does light have a different speed in different media, each colour also has a slightly different speed in slower media such as water, diamond, which results in the separation of the white light into its spectral colours May 19, DPHYS - The Refraction of Light 23 8

9 Refraction & Optical Illusions The refraction of light can cause optical illusions. An optical illusion is something that tricks the eye. May 19, DPHYS - The Refraction of Light 24 9

10 Refraction & Optical Illusions Apparent Depth Objects under water always appear to be nearer to the surface than they actually are. Apparent depth is an optical illusion. This is what makes fish or a paddle in water appear to be closer to the surface than they actually are. It also explains why the legs of someone standing in water appear to be shorter. May 19, DPHYS - The Refraction of Light 27 Refraction & Optical Illusions Apparent Depth PRACTICE 5. An oar partly under water looks bent when viewed from above. Explain this phenomenon with the aid of a labelled ray diagram. May 19, DPHYS - The Refraction of Light 28 Refraction & Optical Illusions Flattened Sun Sunsets offer a unique opportunity to see an unusual image due to refraction. People notice that when the Sun is near the horizon during sunset, it appears to be flattened. The Sun, of course, is not really flattened. May 19, DPHYS - The Refraction of Light 29 10

11 Refraction & Optical Illusions Flattened Sun When the Sun is close to the horizon, light from the bottom of the Sun is refracted more than light from the top of the Sun. Since air is more dense near Earth s surface than higher up in the atmosphere the increased density of air closer to Earth results in greater bending of the Sun s rays. May 19, DPHYS - The Refraction of Light 30 Refraction & Optical Illusions Flattened Sun In addition, the light rays from the bottom of the Sun have a greater angle of incidence than the light rays from the top of the Sun. This results in the Sun having a flattened appearance. May 19, DPHYS - The Refraction of Light 31 Refraction & Optical Illusions Shimmering Another application of refraction occurs when light passes through warm air, as it does above a hot stove. The refraction is not uniform, however, since the warm air is not uniform. As a result, light from objects seen through the warm air is distorted and the objects appear to shimmer. You can see the same shimmering effect over hot pavement in the summeror as jets go by. May 19, DPHYS - The Refraction of Light 32 11

12 Refraction & Optical Illusions OPTICAL ILLUSIONS refraction can cause various optical illusions including: apparent depth flattened sun shimmering May 19, DPHYS - The Refraction of Light 33 Refraction & Optical Illusions DYK? Researchers at the University of Costa Rica believe the metallic appearance of some species of beetles may help them go unnoticed by potential predators (their surface reflects light much like a water droplet so they appear as bright spots). The beetles have a cuticle which contains 70 separate layers of chitin. Each of the chitin layers have different refractive indices. Light reflected through a sequence of these chitin layers creates the beetles' metallic look. This is similar to the way in which a prism breaks white light into the colors of the rainbow by refraction. May 19, DPHYS - The Refraction of Light 34 U Check Your Learning 1. The diagram shows a ray travelling between two transparent media. (a) In which medium is light travelling faster? (a) medium A May 19, DPHYS - The Refraction of Light 35 12

13 U Check Your Learning 1. The diagram shows a ray travelling between two transparent media. (b) How do you know this? (b) pa > pb May 19, DPHYS - The Refraction of Light 36 U Check Your Learning 1. The diagram shows a ray travelling between two transparent media. (c) Can you tell which way the light is moving? Explain. (c) no there are no arrows May 19, DPHYS - The Refraction of Light 37 U Check Your Learning 2. Why do your legs appear to be shorter when you are standing in water? optical illusion (apparent depth) objects under water always appear to be nearer to the surface than they actually are May 19, DPHYS - The Refraction of Light 38 13

14 U Check Your Learning 3. Imagine you are looking at a fish in a pond. Would the fish appear to be closer to the surface or farther from the surface than it really is? Use a diagram to help explain your answer. May 19, DPHYS - The Refraction of Light 39 U Check Your Learning 4. What causes a diamond to sparkle? when light rays enter a diamond, they experience several total internal reflections before travelling back into the air the sparkle is caused by these total internal reflections May 19, DPHYS - The Refraction of Light 40 U Check Your Learning 5. A scuba diver looks up and sees a seagull flying above the surface of the water. Is this a real or virtual image? Where is the seagull actually located? May 19, DPHYS - The Refraction of Light 41 14

15 Activity: The Path of Light PURPOSE To observe the path of light as it travels from one transparent medium into another. INSTRUCTIONS A. In your notebook, create a table similar to the one below. Type of Angle Size of angle angle of incidence 0E 10E 20E 30E 40E 50E 60E 70E angle of refraction in water (1.33) angle of refraction in glass (1.50) angle of refraction in custom (1.60) May 19, DPHYS - The Refraction of Light 42 Activity: The Path of Light INSTRUCTIONS B. Open up the PhET Physics Bending Light simulation. Ensure that the incident (1 st ) material is air (n=1.00) and the refractive (2 nd ) material is water (n=1.33). C. Place the protractor so the 90E marks line up with the surface of the boundary between the two media and the 0E marks line up with the dotted line. D. Turn the laser light on and change its orientation so that the angle of incidence is 0E. Measure and record the angle of refraction. E. Repeat step D for the remaining angles of incidence. F. Change the index of refraction (n) for the 2 nd material to (a) 1.50 and (b) 1.60 and repeat steps D and E. May 19, DPHYS - The Refraction of Light 43 Activity: The Path of Light QUESTIONS 1. When the angle of incidence in air was 0E, why was the angle of refraction 0E? 2. Describe, with the aid of a diagram, how a refracted light ray in water (n=1.33) bent in relation to the normal. 3. How did the results change when the 2 nd material was changed to glass (n=1.50)? custom (n=1.60)? 4. Which material refracted the light most? Explain how you made this decision. 5. In which material does light travel the fastest? Explain how you made this decision? 6. Explain how could you use the method in this activity to show that two transparent media, that appear to be identical, are actually different? May 19, DPHYS - The Refraction of Light 44 15

16 U Check Your Learning WIKI(PHYSICS) O... 2DPHYS - WS4 (Refraction) May 19, DPHYS - The Refraction of Light 45 16