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Optics: Reflection and Refraction Shadows – total shadow, light is blocked Penumbra – only part of the light is blocked Umbra Shadows Solar eclipse The Moon passes between the Sun and the Earth and casts a shadow During what phase of the moon are eclipses possible? Solar eclipse from space Solar Eclipses If a solar eclipse is caused by the Moon passing in between the Sun and the Earth, why don’t we see an eclipse every ~28 days? Solar Eclipses Total solar eclipse visible from the US: August 21st 2017 Lunar Eclipses Lunar eclipses occur when the Moon passes through Earth’s shadow. Blood Moon Indirect sunlight passes through Earth’s atmosphere, where blue light is scattered, this filters out the blue light and illuminates the Moon in a red glow Reflection and Refraction Reflection: light is returned to the medium from which it came Refractions: light crosses from one transparent material to another Principle of Least Time Principle of Least Time Principle of Least Time: Out of all the possible paths light might take to get from one point to another, light always takes the path that results in arriving in the least amount of time. Ants follow this principle too Reflection Law of Reflection Angle of Incidence EQUALS the Angle of Reflection Reflection Straight/Plane Mirror Image behind the mirror is called a VIRTUAL IMAGE Examples (Billiards and Bank Shots…) Straight/plane mirror Virtual As image is… far behind the mirror and the real object is in front of the mirror. Same size as object Straight plane mirror Bankshot in Billiards Warm-up 1. If you stand 3 ft in front of a mirror, how far away is your image from you? 2. If you walk toward a mirror at 4m/s, how fast is your image approaching you? 3. If you are 4 ft tall, how tall does your mirror have to be to see your whole body? Reflection (cont’d) To see more of her head in the mirror, she a) should hold the mirror closer b) should hold the mirror father away c) needs a bigger mirror Reflection (cont’d) Concave Mirror When up-close, images are magnified and upright Example: Vanity mirrors When the object is far away, images are minified and inverted Convex Mirror Images are always minified and upright Example: “7-Eleven” mirror Object behind focus Object in front of focus Rules for Concave Mirrors 1. Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection. 2. Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection. Rules for Convex Mirrors 1. Any incident ray traveling parallel to the principal axis on the way to a convex mirror will reflect in such a manner that its extension will pass through the focal point. 2. Any incident ray traveling towards a convex mirror such that its extension passes through the focal point will reflect and travel parallel to the principal axis. Refraction Cause of Refraction The change in speed of a light wave Refraction Refraction Snell’s Law of Refraction When a light ray slows down as it enters a new medium, the ray will be bent towards the perpendicular. When a light ray speeds up as it enters a new medium, the ray will be bent away from the perpendicular. n1 sin 1 = n 2 sin 2 n = c / v , n is called the index of refraction, v is the speed of light in the medium n = 0 / Snell’s Law n1 * sin θ1 = n2 * sin θ2 Snell's law gives the relationship between angles of incidence and refraction for a wave impinging on an interface between two media with different indices of refraction. Snell’s law practice Light travels from air into an optical fiber with an index of refraction of 1.44. (a) In which direction does the light bend? (b) If the angle of incidence is 22o, what is the angle of refraction inside the fiber? (c) Sketch the path of light as it changes media. Snell’s law Using the information given in the following diagram, calculate the optical index of refraction for medium B Index of refraction The amount by which light slows in a given material is described by the index of refraction, n. The index of refraction of a material is defined by the speed of light in vacuum c divided by the speed of light through the material v: n = c/v How would the fish see you? Refraction (con’t) A coin lies submerged at the bottom of a pan of water. Does refraction of light from the coin make it appear deeper, or make it appear shallower than it really is? Bruno wishes to "spear" a fish with a laser. Should he aim the laser beam above, below, or directly at the observed fish to make a direct hit? Total Internal Reflection When light goes from a more dense medium to a less dense medium at a great enough angle it will be reflected back into the dense medium Total Internal Reflection Requirements: the light is in the more dense medium and approaching the less dense medium. the angle of incidence is greater than the socalled critical angle. Diamonds Why does the cut of a diamond matter? Diamonds Penny activity (per table) 1. Grab a beaker from the sink and fill almost to the top with water. Grab a penny and a paper plate. Refraction Mirages Inferior Mirages When the air is warmer on the ground light will travel faster through it and bend upward Image appears lower than it is and upside down Example: “puddle” on the road Refraction Mirages Superior Mirages When the air near surface is cooler than air above (temperature inversion) light is bent downward Image appears higher than it is and right side up Example: Flying Dutchman and ghost ships Rainbows Rainbows Blue wavelengths of light bend more than red and get dispersed in a raindrop Double Rainbow The Flying Dutchman Warm-up 1. Where is an image projected in a superior mirage? 2. What color is on the inside of a primary rainbow? 3. What causes the colors to be reversed in a secondary rainbow? 4. Why can’t you always see the secondary rainbow? Lenses Ray Diagrams A graphical method for determining how images will turn out. 3 rays can be drawn according to known patterns Where the 3 rays intersect or appear to intersect there will be an image. Concave mirror Warm-up 1. 2. th (6 period) What is the speed of light in ethanol if the refractive index is 1.36? Does light go faster or slower in water? See board Ray #1: Center Ray A ray through the center will not change direction Mirrors: The ray will reflect at equal angle at the center Lenses: The ray will continue straight through the lens without changing direction. Ray #2: First Focal Point Ray A ray lined up with the first focal point will come out parallel to the axis Mirrors: The ray lined up with the focal point will reflect parallel to the axis Lenses: The ray lined up with the first focal point (second focal point if concave) will come out parallel. Ray #3: Second Focal Point Ray A ray parallel to the axis will come out lined up with the focal point Mirrors: The ray parallel to the axis will reflect so that it is lined up with the focal point Lenses: The ray parallel to the axis will reflect so that it is lined up with the second focal point (first focal point if concave). Concave Putting it all together … A Person 2 meters tall stands 6 meters in front of a Concave Mirror with a focal length of 2 meters. Lenses Convex Objects close (within a focal length) to a convex lens will produce a magnified, upright, virtual image Objects further away will produce minified, inverted, real images Concave Images produced will always be minified, upright and virtual. Examples Warm-up 1. 2. th (5 period) You are underwater. You want to spear a buffalo that is drinking from the watering hole. Draw two rays from the buffalo to you and figure out where you should aim. See board Warm-up Light travels through gallium phosphide at 8.6x107 m/s. What is the index of refraction (n) for this compound? 2. Label the following from the picture on the board a) Incident ray b) refracted ray c) perpendicular d) angle of incidence e) angle of refraction 1.