Light & Optics Physics I By Lynn Johnson

Speed of Light (entire spectrum): c = 3x108 m/s The Electromagnetic Spectrum NASA, (2011) Visible light: 400 nm – 750 nm 1 nm = 1x10-9 m Speed of Light (entire spectrum): c = 3x108 m/s

Definitions: Opaque – Does not transmit any light – solid color object Translucent – Transmits some light – fuzzy images Transparent – Transmits nearly all light – clear images Coherent Light: Crests of one wave line up with crests of another. Example: Lasers Light from light bulbs or the sun, however, are incoherent light. Image: (Phet, 2010)

Polarization

Glare The lines on the lenses of the eyeglasses indicate the plane of polarization through which light can pass.                                                                                           Which pair of glasses has the plane of polarization in the best orientation for reducing road glare while driving?

Diffraction of Light Diffraction is the ability of light waves to bend around obstacles placed in their path. Ocean Beach Fuzzy Shadow Light rays Water waves easily bend around obstacles, but light waves also bend, as evidenced by the lack of a sharp shadow on the wall. Slide Author: (Tippens, 2007c)

Interference – Young’s Double Slit Experiment (Particle theory prediction) If light is a wave, light passing through one of the two slits should interfere with light passing through the other slit. Wave fronts (Lyon, 2010) Giancoli Physics

Young’s Interference Pattern Constructive Bright fringe Destructive Dark fringe Constructive Bright fringe Slide Author: (Tippens, 2007c)

Single Slit Double Slit Diffraction Grating (Redbelly98, n.d)

Properties of Light Any study of the nature of light must explain the following observed properties: Light travels in straight lines. Reflection: Light striking a smooth surface turns back into the original medium. Refraction: Light bends when entering a transparent medium. Slide Author: (Tippens, 2007a)

Definitions Focal length: The straight-line distance f from the surface of a mirror to focus of the mirror. Magnification: The ratio of the size of the image to the size of the object. Real image: An image formed by real light rays that can be projected on a screen. Created by real rays. Virtual image: An image that appears to be at a location where no light rays reach. Created by virtual rays. Converging: How the reflected or refracted rays intersect at one point Diverging: How the reflected or refracted rays scatter

The Plane Mirror A mirror is a highly polished surface that forms images by uniformly reflected light. Image appears an equal distance behind the mirror. However, images are right-left reversed.

Real and Virtual Real images and objects are formed by actual rays of light. (Real images can be projected on a screen.) Virtual images and objects do not really exist, but only seem to be at a location. Virtual images are on the opposite side of the mirror from the incoming rays.

Concave Mirror                                                                                              The object is within the focal length of the mirror.

Convex Mirrors  

Molecular Expressions – Concave Mirrors When the object is placed _________ the focal point, a _______________image is formed. The real image is inverted or upright?

Molecular Expressions – Concave Mirrors When the object is placed _________ the focal point, a _______________ image is formed. The virtual image is inverted or upright?

Molecular Expressions – Concave Mirrors When the object is placed _________ the focal point, no image is created due to _____________ reflected rays.

Convex Mirrors – Molecular Expressions No matter where the object is placed, it always produces an ________, _________ image.

Concave Mirror (Converging) do = Object Distance di = Image Distance f = focal length r = radius of curvature

Signs to Remember: Positive di = Real Image (Forms in Front – same side as eye) Negative di = Virtual Image (Forms behind mirror) Converging Mirrors have Positive focal lengths An object 25.4 cm high is located 91.5 cm from a concave mirror. The focal length is 15 cm. a) Where is the image located? b) What type of image is it?

Your Turn! An object located 20 cm in front of a mirror forms an image 10 cm behind (indicating virtual image) the mirror. What is the focal length of the mirror? Ans: f = -20 cm (Did you make the di negative because it was a virtual image?)

Wavelength and Refraction The energy of light is determined by the frequency of the electromagnetic waves, which remains constant as light passes into and out of a medium. (Recall v = fl.) Glass Air n=1 n=1.5 lA lG fA= fG lG < lA Slide Author: (Tippens, 2007b)

Refraction Refraction is the bending of light as it passes from one medium into another. Water Air refraction N qr qi Note: the angle of incidence qi in air and the angle of refraction qr in water are each measured with the normal N. Slide Author: (Tippens, 2007b)

Index of Refraction, n 60 60 34 41 glass water 60 60 n (glass) = 1.52 n (water) = 1.33 n(air) = 1.00

Analogy for Refraction Sand Pavement Air Glass 3 x 108 m/s 2 x 108 m/s vs < vp 3 x 108 m/s Slide Author: (Tippens, 2007b)

Snell’s Law (Law of Refraction): n1sin 1 = n2sin 2 (J.S. Wetzel, Wisebridge Learning Systems, 2005)

 is a phenomenon which occurs when a propagating wave strikes a medium boundary at an angle larger than a particular critical angle with respect to the normal to the surface. If the refractive index is lower on the other side of the boundary and the incident angle is greater than the critical angle, the wave cannot pass through and is entirely reflected. Total Internal Reflection c = Critical Angle                   (J.S. Wetzel, Wisebridge Learning Systems, 2005)

  Optical Fiber                                             

Dispersion by a Prism Dispersion is the separation of white light into its various spectral components. The colors are refracted at different angles due to the different indexes of refraction. Note that the shorter wavelengths of light are bent more than the longer; blue more than red.

Refraction Distorts Vision Water Air Water Air Slide Author: (Tippens, 2007b)

The broken pencil

Why do stars twinkle?                                                 (Enchanted Learning.com, 2010)

Explaining the Water Mirage   Hot Pavement

Refraction Sample Problems   1.A ray of light travels from air into water at an angle with the surface of 60. The index of refraction of the water is 1.33. Find the speed of light in the water. b)      Equation: Substitute: Answer: v = 2.25x108 m/s