Chapter 29 Reflection and Refraction

Reflection Most objects we see reflect light rather than emit their own light. Common Misconception: The moon is a light emitter. Reality: The moon is simply reflecting the sun’s rays.

Principle of Least Time Fermat's principle - light travels in straight lines and will take the path of least time MIRROR AB Wrong Path True Path

Law of Reflection “The angle of incidence equals the angle of reflection.” Incident ray Normal Reflected ray Angle of incidence Angle of reflection Mirror

Missing Light? The law of reflection holds for both plane mirrors and curved mirrors. Approximately 99% of the incoming light is reflected, depending on how highly polished the surface is. Where did that other 1% go?

Specular Reflection Specular Reflection - images seen on smooth surfaces (e.g. plane mirrors)

More on Virtual Images Virtual Image – where the divergent light rays appear to originate from. The word “virtual” is used b/c the light does not actually originate from that spot. Our eyes and the differentiation problem. D o = D i S o = S i

Concave vs. Convex Cave  Raaaooorrrrwww  curved in relative to the observer Vex  to annoy  curved out towards observer The Gap story from ~2001 and Fun House Mirrors

Convex Virtual image is smaller and closer to the mirror than the object is. D o > D i S o > S i You’ve seen these on driveways and in the corners at WaWa. (What the heck is a WaWa anyway?)

Concave Virtual image is larger and further from the mirror than the object is. D o < D i S o < S i You’ve seen these in makeup mirrors and spoons.

Diffuse Reflection Magazines vs. Hardback books Zoomed in view of paper

Diffuse Reflection Occurs when light is scattered in different directions. The law of reflection occurs for each ray, but it does not appear to b/c of the many different angles it encounters. SpecularDiffuse

New Car Look What makes a car look older? Explain in terms of diffuse vs. specular refelction. Rainy night driving.

Refraction Who still mows their lawn? Who gets groceries for their mom? What happens when one wheel hits a new surface before the other?

Refraction

Refraction of Sound All waves exhibit refraction. Refraction is the bending of a wave when it enters a new medium. The bending is caused by the differences in the speed of the wave in the media. Diagrams and Hewitt clip Large tube walk

Refraction of Sound Let’s explore sound. Recall that sound speeds up when entering water while light decreases in speed. What students have said: “Curves toward the region that the wave is slower in.” OR “Curves towards cool.”

Refraction of Sound

Refraction of Light Refraction is the bending of light when it passes from one transparent medium to another. This bending is caused by differences in the speed of light in the media.

Refraction-Fast to Slow WATER AIR Normal Line Fast Slow Light Beam The ray will bend towards the normal when the speed of light is slower in the new medium.

Refraction-Slow to Fast WATER AIR Normal Line Fast Slow Light Beam The ray will bend away from the normal when the speed of light is faster in the new medium.

What has Changed? If light enters water from air, the color of the light is the same! What must that mean about the frequency? It remains constant! But the ray decreased in speed, so what must have occurred? The wavelength decreased!

Wavelength Change

The Key to Refraction The key to refraction is the fact that a part of the wave front encounters the new medium before another part. What if the whole wave front encounters the new medium at the same time? That is, what if the light ray approaches the new medium in a perpendicular fashion? No refraction occurs! Eagles Gone Wild!

Penny in the Cup

Refraction Examples Light slows down when it goes from air into water and bends toward the normal. An Analogy: A car slows down when it goes from pavement onto gravel and turns toward the normal. An Illusion : Fish in the water appear closer and nearer the surface. (See Figure 27.27)

Refraction WATER AIR Observer True Fish False Fish

Index of Refraction The greater the density of the media that light enters, the greater the degree of bending that occurs. Example: Air to water will produce a small degree of bending. Air to diamond will produce a larger degree of bending.

Index of Refraction The ratio of the speed of light in a vacuum to the speed in a given material is called the index of refraction. n =speed of light in a vacuum speed of light in material n =c v

Sample Indices Mediumn = c/v Vacuum1 Air Water1.33 Ethyl Alcohol1.36 Quartz Glass1.46 Crown Glass1.52 Flint Glass1.58 Plexiglass1.51 Sodium Chloride1.53 Diamond2.42

Atmospheric Refraction Our atmosphere can bend light and create distorted images called mirages.

Diagrams of Mirages Desert Mirage Urban Mirage Extra 5 minutes of Sunlight???

Pumpkin Sun The bottom of the sun or moon will flare out more than normal b/c that part is going through more atmosphere. More atmosphere = more refraction.

The Moon Illusion Distorted, but the overall size is the same. The appearance that the moon is larger when it is coming up on the horizon. For future reference, the moon stays low on the horizon during the month with the summer solstice. Ponzo Illusion – the main cause of the illusion: When our brain interprets things as being bigger if we think it is far away.

The Moon Illusion So when the Moon is on the horizon, your brain thinks it's farther away than when it's overhead. The Ponzo illusion kicks in, and your brain gets fooled into thinking the Moon is HUGE. As it gets higher, the illusion vanishes. If you actually observe the Moon with binoculars or with a 'scope, you can see it is no bigger on the horizon. In fact, it should look smaller because it's a few thousand miles farther away than when it's overhead.

The Ponzo Illusion

Strongest…..Ponzo….Ever….

More Illusions

Dispersion... …is the separation of white light into pure colors (ROYGBV). Dispersion Examples: Prisms Diffraction Gratings CD’s Raindrops

Rainbows Raindrops refract, reflect and disperse sunlight. Rainbows will always appear opposite of the Sun in the sky. You cannot run from or run to a rainbow!

Total Internal Reflection... …is the total reflection of light traveling in a medium when it strikes a surface of a less dense medium

WATER AIR Light Source Critical Angle Total Internal Reflection

Total Internal Reflection occurs when the angle of incidence is greater than the critical angle. Fish can see everything above the water’s surface in a 96 o cone of vision. Demo - Laser and light pipe