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Light waves. Can light bounce?. Yes! We call this a reflection. Humans have been looking at reflections for thousands of years, however they’re not always clear. Reflections. Why is the reflection blurred? How could you get a clear reflection?. The Egyptians.
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Can light bounce? • Yes! We call this a reflection. • Humans have been looking at reflections for thousands of years, however they’re not always clear.
Reflections • Why is the reflection blurred? • How could you get a clear reflection?
The Egyptians • Almost 4,000 years ago the Egyptians realized that only smooth ponds produce clear reflections. • However it wasn’t until 1835 when Justus von Liebig, a German scientist, developed a method of coating glass with silver to produce a sharp, well-defined, reflected image. • Before then no one really knew what they looked like!
The Law of Reflection • Consider what happens when I bounce pass a basketball. • How can I change the angle of the bounce pass?
The Law of Reflection • Light reflects the same way the basketball does. • Laser Demo • The Law of Reflection: The angle that a reflected ray makes as measured from the normal to a reflective surface equals the angle that the incident ray makes as measured from the same normal.
The Law of Reflection • is the angle of the incident ray • is the angle of the reflected ray • The normal line is perpendicular the surface
The Law of Reflection • Smooth vs rough surfaces • Which will be a better reflector? • Flashlight demo
The Law of Reflection • Specular Reflection is when parallel light rays are reflected in parallel • Diffuse Reflection is when parallel light rays are not reflected in parallel but instead they are scattered. • This is what allows us to read text or see images from various angles.
Bellringer – 2 mins to hand in • What are the two types of reflection and what is the difference? • Feel free to draw two diagrams instead of using words. DO WORK STOP
Objectives • Complete problems using you knowledge of the law of reflection. • Learn and use the law a refraction to understand and solve real life problems.
Missing Labs – Due Today • Mapping Magnets • Caleb • Tuning Fork Lab • Matt O • Jon • Annika
The Law of Reflection • The law of reflection still applies to both types of reflection. • No matter how much light reflects off a wall or a sheet of paper, you will never be able to use them as mirrors. • The light that reflects off the wall or paper is how we see the wall or paper.
Changing Reflection Types • How could you make a diffuse surface a specular surface? • Polish it, or coat it with something to smooth it over.
Flipped Images • Why is the reflected image of this pond upside down?
Law of Reflection • What is the angle of incidence of the highest leaf if the tree is 10 meters tall and its reflection is 20 meters long? • If the observer is 1.83 meters tall, how far away from the base of the tree are they?
Practice Problems - 467 • Page 467 numbers 1-6 • 12 minutes • Pass back papers
Practice Problem Answers • Water fills in the rough areas and makes the surface smoother. The surface normals are then parallel. • 35 • 42, 48, 84 • 51 • 30 • 90
Light and Boundaries • What happens when light encounters a boundary like a window? • Some light is transmitted through the window and some light is reflected back. • Laser out the window demo • Is this a property of a wave or a particle?
Thirsty?? • What is wrong with this pencil?? • Have you ever looked into a pool and it seemed shallower than it was?
Bellringer – 2 mins to hand in • How tall is a mountain if its reflection is 500m long and the angle of incidence of the light from the highest point of the mountain is 30 degrees? • 866m DO WORK STOP
Objectives • Use Snell’s Law to solve problems. • Understand real life applications of refraction. • Begin collecting data about refraction.
Refraction • These phenomena happen because the light rays bend as they enter a new medium. • Refraction: The change in direction of waves at the boundary between two different mediums.
Refraction • The incident angle in all three cases is the same. What is different? • The medium, and the angle of refraction.
Refraction • The change in direction of a light beam depends on the properties of the mediums that the light rays are traveling from and into. • Notes • on the reference table is the angle of reflection not the angle of refraction! • The angles are measured from the normal line.
Willebrord Snell • Snell came up with a mathematical law to relate the angle of incidence with the angle of refraction. • He did this using a light source and a piece of glass. • He shot the light at various angles and measured the results. • Then he plotted his data and found a mathematical relationship that fit and could correctly predict the outcome. • Let’s try it! • Watch your eyes…laser beam
Snell’s Findings • He found that the sines of the angles were related! • Woo trig! • He also found that each material had its own “index of refraction” (n) which determines the angle of refraction of light as it crosses the boundary between two mediums.
Snell’s Law of Refraction • This is found in the wave section of your reference table.
Index of Refraction • By doing experiments with other combinations of mediums Snell found the index of refraction of many different materials. • A table of the different indices of refraction can be found in the reference table.
Beaker Demo • Will light bend as it goes from air to corn oil? • Why? • Will light bend as it goes from corn oil to corn oil? • Why? • Will light bend as it goes from corn oil to another medium that has the same n as corn oil? • Why? • Can we make something invisible?
Example • Find the angle of refraction is a light ray is shot through air at an angle of 25 degrees into glycerol. • (1) • 0.287 degrees
Practice • Complete practice problems 1-5 on page 494 of the textbook. • Answers • 26.3 degrees • 34.2 degrees • 17.0 degrees • 1.5 degrees • The medium is float glass
Where does “n” come from? • The index of refraction (n) describes how much light bends as it enters a medium, but why does it bend? • The light bends because it is changing its speed!
What is the speed of light? • The speed of light is in a vacuum. • Its symbol is “c”. • It is found on the front page of the reference table.
The speed of light • The speed of light can never be faster than when it is traveling through a vacuum, but it can be slower when traveling through a different medium. • The index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the medium.
The Index of Refraction • n is the index of refraction • c is the speed of light in a vacuum • v is the speed of light in a medium
Critical Angle • As the light travels from a medium of higher n to lower n, the angle of refraction is larger than the angle of incidence. • Critical Angle: A certain angle of incidence where the refracted light ray lies along the boundary of the two mediums.
Total Internal Reflection • Total Internal Reflection occurs when light traveling from a region of higher n to a region of lower n strikes the boundary at an angle greater than the critical angle such that all light reflects back into the region of higher n. • is the medium of the incident ray (higher n value)
Total Internal Reflection • If you’re under water in a pool and looking up at the surface you may not be able to see what’s above the surface. • Fiber optic cables use this phenomena to transmit data very efficiently and at the speed of light.
Mirages • As the ground heats up from the sun it creates a thin layer of much warmer air near the ground. • This layer of hot air has a smaller n value than the air at eye level. • This difference in n makes the light rays of the sky and usually an object reflect of the cool and hot air boundary.
Index of Refraction Lab • Take a moment to read the procedure.
Bellringer • When is my birthday? • When is Gunnar’s birthday?
Objectives • Count your lab minutes • Finish collecting data for the index of refraction lab