The path of light is bent. Refraction and Lenses 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 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. 1
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? 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? Index of Refraction of a material is the ratio of the speed of light in vacuum to the speed of light in that material. n = c v Where: n is the index of refraction c is the speed light v is the average speed of light in the material But this is pretty hard to measure in the lab! 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. 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 2
How much the light bends depends on how much the light slows down. Materials that slow light down a lot are said to be optically dense. Light slows down more in glass (2/3c) than it does in water (3/4c) and therefore bends more in glass than in water. Glass is more optically dense that water is. Both have a greater optical density that air. Note: Snell s Law 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. Applications and Optical Illusions Refraction of the Sun 3
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 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! Yesterday you measured the index of refraction for glass. Snells Law Lab Part 1 Could you use the same process to measure the index of refraction for water? Index of refraction of glass 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? 4
You should end up with something like this. Two Pins A Pink Foam Sheet Scratch Paper Dish You need: Lenses Your book calls them concave and convex but that is not really correct! Lens Diagrams for Thin Lenses Converging Fat Middle (p495) 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 Converging Diverging Lens Diagrams for Thin Lenses Diverging Thin Middle (p498) 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 d = d o i = 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. 5
Conventions Converging f is positive +di means real image -di means virtual image +hi means upright image -hi means inverted image Diverging f is negative di is always negative virtual image hi is always positive upright image Example Problems page 496 Let s Practice Pass out sample diagrams. We will practice 1 and 4. 6
Assignment 18L Snell s Law Calculations Example Problems Find the angle of refraction. 7
Find the index of refraction. Find the angle of refraction. 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. 8
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! Fish Tank TIR 9
Fish View of the World Sparkling Diamond TIR and Fiber Optics Problem Set 18A Fiber Optics 10
TIR and Prisms Dispersion Each color of light has a slightly different index of refraction, so each color bends a different amount. Chromatic Aberration A Lens Defect Fixing Chromatic Aberration for Lenses Good Lens and Cheap Lens Structures of the Human Eye 11
Human Eye Diagram (p500) Refraction Structures Cornea (3/4) - fixed Lens (1/4) - adjustable Common Disorders Cataract cloudy cornea, fixed with surgery. Glaucoma high pressure in eye fluid, fixed with medicine or surgery. Macular Degeneration leading cause of blindness over 50. Thinning and degeneration of the central part of the retina. Corrective Lenses Nearsightedness Farsightedness Astigmatism Rainbows Double Rainbow 12
Double Rainbow Mirage Superior Mirage Superior Mirage "Crocker Land", a mountain range that appeared on arctic maps from 1909 to 1916, was nothing more than a superior mirage. 13