The path of light is bent. Refraction and Lenses Unit 11 These are not photographs, but rather computer generated graphics based on the artist s understanding of the index of refraction. The angle of incidence equals the angle of reflection. Not so for refraction. Refraction when waves enter a new material, they change speed. As a result, they change direction. Refraction the bending of light due to a medium change Light does not travel at the speed of light when it is moving through a medium. It travels much slower because it must be absorbed and re-emitted by all of the molecules and atoms. The more tightly packed the molecules and atoms, the slower the light will move. 1
5/3/2018 Solider/band analogy The solider cannot walk as fast in the yellow sand as they can on concrete, so he slows down What would happen to soldiers if they all hit the yellow sand at the same time? Applications and Optical Illusions Index of Refraction of a material is the ratio of the speed of light in vacuum to the speed of light in that material. c n= v Where: n is the index of refraction c is the speed light 3E8 m/s v is the average speed of light in the material Snell s Law Because light slows down, it will also bend or change directions this bending is known as refraction. How much bending depends on the angle of incidence and the index of refraction. But this is pretty hard to measure in the lab! 2
How much the light bends depends on how much the light slows down. If the index of refraction of the new medium is less than the index of refraction of the old medium nv = c, velocity will increase. And the light will bend away from the normal Light slows down more in glass (2/3c) than it does in water (3/4c) and therefore bends more in glass than in water. Materials that slow light down a lot are said to be optically dense. Note: The more optically dense the material, the slower the speed of light in that material. Thus n > 1 for all materials, and increases with increasing optical density. n = 1 in vacuum (and pretty close to 1 for air). The frequency of light does not change when it passes from one medium to another. Glass is more optically dense that water is. Both have a greater optical density that air. Snell s Law Snells Law Lab Part 1 Index of refraction of glass 3
Instructions Each student must do their own drawings Entry angles are 30 and 45 degrees Make a separate drawing for each angle Be very careful with the expensive glass blocks! Measuring the index of refraction (n) in glass. Make the entry line dark Sight the exit line with a ruler Connect the path through the glass block (dashed line) Use the top i and r only for calculation Yesterday you measured the index of refraction for glass. Light entered at B, slowed down and bent, then it left the block at C and sped up going into air and bent Direction of bending FST, Fast to Slow, Towards normal SFA, Slow to Fast, Away from normal Could you use the same process to measure the index of refraction for water? Recap: Refraction the bending of light due to a medium change Light does not travel at the speed of light when it is propagating through a medium. It travels much slower because it must be absorbed and re-emitted by all of the molecules and atoms. The more tightly packed the molecules and atoms, the slower the light will propagate. Does this mean that if light doesn t change speed, it doesn t change direction? If glass if clear, how can we see it? What would happen if glass were surrounded by a liquid that carried light at the same speed? Would it disappear? Demo- oil and glass 4
Index of Refraction Water -No Pin is Needed at Point B There is already a scratch mark there instead. -Fill the dish 3/4 full with water. Why is there no refraction as the light leaves the water dish? You should end up with something like this. You need: Two (maybe 3) Pins A Pink Foam Sheet Scratch Paper Dish is already at your station Homework Turn in Lab by the end of class Direction of Bending worksheet from Physics Classroom due tomorrow Use https://tinyurl.com/hoffspanglens6 for help Lesson 1 from Refraction unit at PC Snell s Law Calculations Example Problems 5
Find the angle of refraction. Find the index of refraction. Find the angle of refraction. 6
When light travels from a fast material like air into a slow material like glass, Snell s Law always works. When light travels out of glass or water and back into air, something unusual happens as the angle of incidence gets larger. What is the critical angle for water? If the angle of incidence exceeds the critical angle, refraction no longer occurs. Instead the light is internally reflected! 7
Fish Tank TIR Fish View of the World Sparkling Diamond Homework: Read here: https://tinyurl.com/hoffspangtir two lessons on TIR and Critical Angle Lenses Some books call them concave and convex but that is not really correct! Snell s Law WS for tomorrow Converging Diverging http://tinyurl.com/pvlens 8
Lens Diagrams for Thin Lenses Converging Fat Middle A ray traveling in parallel will go through the lens and converge through the far focal point A ray traveling in through the near focal point will pass through the lens and travel parallel A ray passing through the center of the lens will pass through unchanged Lens Diagrams for Thin Lenses Diverging Thin Middle A ray traveling in parallel will go through the lens and diverge on a path that includes the near focal point A ray traveling in through the far focal point will pass through the lens and travel parallel A ray passing through the center of the lens will pass through unchanged Formulas apply to both converging and diverging lenses. 1 f h h i o 1 1 = + d o d i di = d o = M f is the focal length of the lens M is the magnification d i is the image distance d o is the object distance h o is the height of the object h i is the height of the image. Converging f is positive +di means real image -di means virtual image +hi means upright image -hi means inverted image Conventions (Different than mirrors!!!) Diverging f is negative di is always negative virtual image hi is always positive upright image https://goo.gl/7vqhsy 9
Assignment 18L This interactive helps: https://tinyurl.com/hoffspanglens2 And all tutorial lessons 10