Chapter 21 Lecture Reflection and Refraction 2014 Pearson

Chapter 21 Lecture Reflection and Refraction © 2014 Pearson Education, Inc.

What Is Light? • We develop three models of light. Each model successfully explains the behavior of light within a certain domain. © 2015 Pearson Education, Inc.

What Is Light? (Three Models) The Ray Model Chapter 21 -22 An equally well-known “fact” is that light travels in a straight line. These straight-line paths are called light rays. The properties of prisms, mirrors, lenses, and optical instruments such as telescopes and microscopes are best understood in terms of light rays. The ray model of light, the basis of ray optics, is the subject of the next chapter. © 2015 Pearson Education, Inc.

What Is Light? (Three Models) The Wave Model The wave model of light is the most widely applicable model, responsible for the widely known “fact” that light is a wave. The study of light as a wave is called wave optics Chapter 23 -24. © 2015 Pearson Education, Inc.

What Is Light? (Three Models) The Photon Model Modern technology is increasingly reliant on quantum physics. In the quantum world, light consists of photons that have both wave-like and particle-like properties. Photons are the quanta of light. Much of the quantum theory of light is beyond the scope of this textbook, but we will take a peek at the important ideas in Chapters 25 and 28 of this text. © 2015 Pearson Education, Inc.

We Are Going To Discuss the Ray Model in This Chapter of the Book 21. 1 Light Sources, Light Propagation and Shadows © 2014 Pearson Education, Inc.

Light sources, light propagation, and shadows • To see something, we need a source of light and an object off which the light bounces (reflects) and then reaches the eyes of the observer. © 2014 Pearson Education, Inc.

Observational experiment © 2014 Pearson Education, Inc.

Observational experiment © 2014 Pearson Education, Inc.

Ray diagrams • Represent the travel of light from one location to another with a light ray, drawn as a straight line and an arrow. • Diagrams that include light rays are called ray diagrams. © 2014 Pearson Education, Inc.

Testing experiment © 2014 Pearson Education, Inc.

Testing experiment © 2014 Pearson Education, Inc.

Testing experiment © 2014 Pearson Education, Inc.

Ray model of light • Testing experiments show that model 1 is inconsistent with experimental evidence. • Model 2 is supported: – Each point on an extended light source emits light in many different directions. – This light can be represented by multiple rays diverging from that point. © 2014 Pearson Education, Inc.

Sources of Light Rays: Self-Luminous Objects A point source is also an idealized source of light. It is infinitely small and emits light rays in every direction. The tiny filaments of these bulbs approximate point sources. © 2015 Pearson Education, Inc. Slide 18 -15

Sources of Light Rays: Self-Luminous Objects This is the most common light source. The entire surface of an extended source is luminous, so that every point of an extended source acts as a point source. Lightbulbs, flames, and the sun are extended sources. © 2015 Pearson Education, Inc. Slide 18 -16

Shadows and semi-shadows • A sharp shadow is called an umbra. – A shadow is a region behind the object where no light reaches. • A semi-shadow is called a penumbra. – A semi-shadow is a region where some light reaches and some does not. It appears as a fuzzy shadow. © 2014 Pearson Education, Inc.

Example 21. 1 • On a sunny day, a streetlight pole casts a 9. 6 -m-long shadow on the ground. You have a meter stick that, when held vertical, casts a 0. 70 -m shadow. Use this information to determine the height of the pole. © 2014 Pearson Education, Inc.

Conceptual Exercise 21. 2 • You place a lit candle several meters from the wall in an otherwise dark room. Between the candle and the wall (and close to the candle), you place a piece of stiff paper (or cardboard) with a small hole in it. Use the ray model of light propagation to predict what you will see on the wall. © 2014 Pearson Education, Inc.

Quick. Check • The dark screen has a small hole, 2 mm in diameter. The lightbulb is the only source of light. What do you see on the viewing screen? © 2015 Pearson Education, Inc.

Quick. Check 18. 1 • The dark screen has a small hole, 2 mm in diameter. The lightbulb is the only source of light. What do you see on the viewing screen? B. © 2015 Pearson Education, Inc.

21. 2 Reflection Of Light © 2014 Pearson Education, Inc.

Reflection of light • Light from a laser pointer shines on a mirror. © 2014 Pearson Education, Inc.

Reflection of light • Incident light: light striking the mirror • Normal line: a line perpendicular to the surface where the incident light hits the mirror • Angle of incidence: the angle between the incident beam and the normal line • Angle of reflection: the angle between the reflected beam and the normal line © 2014 Pearson Education, Inc.

Reflection of light • The angle of reflection = the angle of incidence. © 2014 Pearson Education, Inc.

Law of reflection © 2014 Pearson Education, Inc.

Conceptual Exercise 21. 3 • Two mirrors stand on a table, with their faces forming an angle greater than 90 o. Place a target on the table in front of mirror 2. Use the rule of reflection to predict how to aim a laser beam so that it hits first mirror 1 and then mirror 2, and finally passes directly over the target. © 2014 Pearson Education, Inc.

Specular and diffuse reflection Specular reflection is the reflection from a smooth, shiny surface such as a mirror or a piece of polished metal. © 2014 Pearson Education, Inc.

Conceptual Exercise 21. 4 • On a sunny day, if you look at a house with its lights off, the uncovered windows look almost black but the outside walls do not. How can we explain this difference? © 2014 Pearson Education, Inc.

Red eye effect • When a camera flash illuminates the open iris, light reflects from the red blood vessels in the retina on the back of the eye. • Some of this reflected light passes back out of the pupil and makes the pupil appear red. © 2014 Pearson Education, Inc.

Quick. Check • You are looking at the image of a pencil in a mirror. What do you see in the mirror if the top half of the mirror is covered with a piece of dark paper? – The full image of the pencil – The top half only of the pencil – The bottom half only of the pencil – No pencil, only the paper © 2015 Pearson Education, Inc.

Quick. Check • You are looking at the image of a pencil in a mirror. What do you see in the mirror if the top half of the mirror is covered with a piece of dark paper? – The full image of the pencil – The top half only of the pencil – The bottom half only of the pencil – No pencil, only the paper © 2015 Pearson Education, Inc.

21. 3 Refraction of Light © 2014 Pearson Education, Inc.

Refraction of light • At the shore of a lake, you see sunlight reflecting off the water's surface. – You also see rocks and sea plants under the surface. – To see them, light must have entered the water, reflected off the rocks and plants, returned to the water surface, and then traveled from the surface to your eyes. © 2014 Pearson Education, Inc.

Refraction Two things happen when a light ray crosses the boundary between the air and the glass: 1. Part of the light reflects from the boundary, obeying the law of reflection. 2. This is how you see the reflections from pools of water or storefront windows, even though water and glass are transparent. © 2015 Pearson Education, Inc.

Refraction • Part of the light continues into the second medium. – It is transmitted rather than reflected, – The transmitted ray changes direction as it crosses the boundary. – The transmission of light from one medium to another, but with a change in direction, is called refraction. © 2015 Pearson Education, Inc.

Refraction • The angle between the incident ray and the normal is the angle of incidence. • The angle on the transmitted side, measured from the normal, is called the angle of refraction. © 2015 Pearson Education, Inc.

Refraction • The angles are the same for the ray entering Medium 2 in the first figure and the ray exiting Medium 2 in the second figure. [Insert Figure 18. 15 (c ). ] © 2015 Pearson Education, Inc.

Angle of refraction • Refraction is the change in the path of light when light travels from one medium to another. © 2014 Pearson Education, Inc.

Snell's law © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Quick. Check 18. 5 • A laser beam passing from medium 1 to medium 2 is refracted as shown. Which is true? – n 1 < n 2 – n 1 > n 2 – There’s not enough information to compare n 1 and n 2 © 2015 Pearson Education, Inc.

Quick. Check 18. 5 • A laser beam passing from medium 1 to medium 2 is refracted as shown. Which is true? – n 1 < n 2 – n 1 > n 2 – There’s not enough information to compare n 1 and n 2 © 2015 Pearson Education, Inc.

Refraction © 2015 Pearson Education, Inc. Slide 18 -47

© 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

Quick. Check • A light ray enters a glass prism as shown. Which is a possible path for the ray through the prism? © 2015 Pearson Education, Inc. A B C D

Quick. Check • A light ray enters a glass prism as shown. Which is a possible path for the ray through the prism? © 2015 Pearson Education, Inc. A B C D

21. 4 Total Internal Reflection © 2014 Pearson Education, Inc.

Total Internal Reflection • Total internal reflection (TIR) occurs when a light ray is unable to refract through a boundary. Instead, 100% of the light reflects from the boundary. © 2015 Pearson Education, Inc.

Total Internal Reflection • Crossing a boundary into a material with a lower index of refraction causes the ray to bend away from the normal. • As angle θ 1 increases, the refraction angle θ 2 approaches 90°. • The fraction of light energy that is transmitted decreases while the fraction reflected increases. © 2015 Pearson Education, Inc.

Total Internal Reflection • A critical angle θc is reached when θ 2 = 90°. • The refracted light vanishes at the critical angle; there is only reflected light. • There is no critical angle and no total internal reflection if n 2 > n 1.

Quick. Check • A laser beam undergoes two refractions plus total internal reflection at the interface between medium 2 and medium 3. Which is true? – n 1 < n 3 – n 1 > n 3 – There’s not enough information to compare n 1 and n 3 © 2015 Pearson Education, Inc.

Quick. Check • A laser beam undergoes two refractions plus total internal reflection at the interface between medium 2 and medium 3. Which is true? – n 1 < n 3 – n 1 > n 3 – There’s not enough information to compare n 1 and n 3 © 2015 Pearson Education, Inc.

Quick. Check • A fish in an aquarium with flat sides looks out at a hungry cat. To the fish, the distance to the cat appears to be A. Less than the actual distance. B. Equal to the actual distance. C. More than the actual distance. © 2015 Pearson Education, Inc.

Quick. Check • A fish in an aquarium with flat sides looks out at a hungry cat. To the fish, the distance to the cat appears to be A. Less than the actual distance. B. Equal to the actual distance. C. More than the actual distance. © 2015 Pearson Education, Inc.

21. 5 Skills to Solve Refractive and Reflective Problems © 2014 Pearson Education, Inc.

Skills for analyzing reflective and refractive processes • When you solve problems involving light, use ray diagrams to help in your reasoning and quantitative work. • The diagrams will help you evaluate the final answer. © 2014 Pearson Education, Inc.

Tip © 2014 Pearson Education, Inc.

Skills for analyzing reflective and refractive processes • When problem solving: – Draw a ray diagram showing all relevant paths the light travels. Consider the light rays originating from the object and eventually reaching the observer. You can represent the observer by an eye. – Use the sketch and ray diagram to help construct a mathematical description using the law of reflection, or Snell's law for refraction, or the application of Snell's law for total internal reflection. © 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc.

21. 6 Prisms and Other Light Things © 2014 Pearson Education, Inc.

Fiber optics • We can understand fiber optics by using total internal reflection. – Fiber optic filaments are used in telecommunications to transmit high-speed light-based data and in medicine to see inside the human body during surgery. © 2014 Pearson Education, Inc.

Fiber optics • Imagine that you have a long glass block of refractive index 1. 56 surrounded by air. Light traveling inside the block hits the top horizontal surface at a 41 o angle. What happens next? © 2014 Pearson Education, Inc.

Prisms • The refractive index of prism glass is greater for violet light and smaller for red light. © 2014 Pearson Education, Inc.

Prisms for reflection • Prisms reflect almost 100% of the light incident on them, whereas mirrors reflect somewhat less than 100%. • Prisms do not tarnish like mirrors. • Prisms can invert an image—that is, make it appear upside down. © 2014 Pearson Education, Inc.

Mirages • On a hot day, hot air may hover just above the pavement. This hot air is less dense and has a lower index of refraction than the cooler air above it. – When light from the sky passes through air with a gradually changing index of refraction, its path gradually bends, leading us to perceive that the source of light is at a different location than it actually is. © 2014 Pearson Education, Inc.

Mirages © 2014 Pearson Education, Inc.

Color of the sky • Due to their sizes, atmospheric particles reflect blue light more efficiently than other colors. © 2014 Pearson Education, Inc.

21. 7 Explanation of Light: Two Models © 2014 Pearson Education, Inc.

Explanation of Light: Two Models • So far we have used the ray model of light do describe how light behaves • What other models do we have? © 2014 Pearson Education, Inc.

Particle model of light © 2014 Pearson Education, Inc. Newton law tells us for light to bend (change speed) we must have a force in this case an attractive force from the denser material….

Wave model of light Mr Huygen © 2014 Pearson Education, Inc.

Wave model and refraction • Imagine a light wave moving in a less optically dense medium 1 and reaching an interface with a denser medium 2 at a nonzero angle of incidence. © 2014 Pearson Education, Inc.

Questions • What model is better Ray/particle or wave model • Do not know whether light propagates slower or faster in water. • We do not know how objects radiate light • Why do some stars look red and other yellow © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.

Summary © 2014 Pearson Education, Inc.