YOU DO NOT NEED TO COPY THIS PARAGRAPH FOLLOW THE SAME PROCEDURE AS WE DID WITH THE MIRRORS AND DESCRIBE WHAT YOU SEE. THIS TIME, PLACE THE LENS CLOSE TO YOUR JOURNAL AND SLOWLY BACK IT AWAY TOWARD YOUR EYE. Journal #88 April 28, 2010

CHAPTER 18 Chapter QUIZ: FRIDAY, May 7 th TEST: Tuesday, May 11 th or schedule to take early if you know you will miss that day. Class grades will be locked in as of May 12 th and your only remaining chance to change your grade will be the final exam Refraction and Lenses

Refraction of light Light changes direction (bends) as it crosses a boundary between 2 media in which the light moves at different speeds Amount of refraction of light depends on properties of media (material type, temperature or density) and angle at which it hits the boundary

Examples of Light Refraction Pond or pool looks shallower than it actually is Straw or spoon in a glass appears bent White light comes out of prism as rainbow Air above hot stove seems to shimmer Stars twinkle

Index of Refraction The index of refraction (n) of a medium is equal to the speed of light in a vacuum divided by the speed of light in the medium. In a vacuum, n is equal to 1. The larger n is, the slower light will travel through a substance meaning it will bend more sharply as it enters and leaves that substance. Common Indices of Refraction Mediumn Vacuum1.00 Air Water1.33 Diamond2.42

Snell’s Law of Refraction Though you will not be required to work problems using this formula, it is important to know that you can calculate the incident or refracted angle of light mathematically. To solve for any variable, you must know all the remaining variables. To solve for an angle, you would use sin -1 as your last step (and you must be in degree mode).

Critical Angle and Total Internal Reflection When light passes from a substance with a higher index of refraction to a lower index of refraction (such as from water to air), an interesting phenomenon can occur. If the angle of incidence is increased too much, the incident light will bend so greatly that it cannot escape the substance. This phenomenon is called total internal reflection. The angle at which this happens is referred to as the critical angle.

Critical Angle and Total Internal Reflection Figure A: Ray A is partially refracted and partically reflected. (Figures from P. 498) Figure B: Ray B is refracted along the boundary of the medium and forms the critical angle.

Critical Angle and Total Internal Reflection Figure C: An angle of incidence greater than the critical angle results in the total internal reflection of Ray C, which follows the law of reflection.

Fiber Optics Use total internal reflection Occurs within light pipes or optical fibers Light is reflected over and over many times

Advantages of Fiber Optic Technology Used to get light to inaccessible places such as car engines, inside a patient’s body, and in communications transmitting telephone messages – replacing electrical circuits and microwave links in communication technology Can carry more info in high frequencies of visible light than in lower frequency electrical current Thin glass fibers replace bulky expensive copper cables – more practical in weight, size, cost

Lenses A lens is made of transparent material, such as glass or plastic, with a refractive index larger than that of air, causing light to bend (refract) as it passes through it. A lens has a curved surface on one or both sides.

Plano-convex Double-convex Plano-concave Double-concave Some Examples of Types of Lenses

Convex vs. Concave lenses A convex lens causes parallel light rays to eventually converge and a concave lens causes parallel light rays to eventually diverge.

Convex Lens: Beyond 2F Image is Real, Inverted, and Smaller Check Line

Convex 2F Image is Real, Inverted, and same size Check Line

Convex Lens: Between FP and 2F Image is Real, Inverted, and Enlarged Check Line

Convex No image is formed. Check Line

Convex Lens: Between FP and Lens The refracted light rays diverge. The image forms on the same side of the lens at the object. The image is Virtual, Upright, and Enlarged Check Line

The Concave Lens Rays diverge after they hit the lens. Image will always be virtual, upright, and reduced in size. The Concave Lens Check Line

Lens Chart Lens Type/ Placement of Object Size of image compared to Object Real or Virtual Upright or Inverted Double Convex beyond 2F Double Convex at 2F Double Convex between 2F & F Double Convex at F Double Convex in front of F Double Concave (any placement)

Applications that Use Lenses Hand lenses/Magnifying glasses Projectors Refracting telescopes Binoculars Cameras Microscopes Corrective Eyeglasses and Contact lenses

Practice Problems Homework: Due on Monday, March 30  P #15-19  P #20-23 Use same formulas from last chapter!!! Tutorials Thursday after school with me or with other teachers at 7:30 AM Wed or Thurs!

Lens Chart Answers - Lens Type/ Placement of Object Size of image compared to Object Real or Virtual Erect or Inverted Double Convex beyond 2F SmallerRealInverted Double Same SizeRealInverted Double Convex between 2F & F EnlargedRealInverted Double F No Image Double Convex in front of F EnlargedVirtualUpright Double Concave (any placement) ReducedVirtualUpright

Nearsightedness Nearsightedness (Myopia) occurs when the eyeball is too long, so focal length is too short Image forms in front of the retina; causes distant objects to be blurry. Corrected by a concave lens that forces light to diverge to a farther point on back of retina

Farsightedness Farsightedness (Hyperopia) occurs when the eyeball is too short so focal length is too long, also happens with aging as muscles holding the shape of lens relax and allow it to flatten Image forms behind wall of the retina; causes objects located close to the eye to become blurry Corrected by convex lens that forces light to converge at a closer point on the back of retina

Various Vision Impairments

Dispersion When white light passes through a prism and is refracted into the entire spectrum of colors it is called dispersion.