Light: Waves and Particles

1. Light: Waves and Particles • Like a wave … light shows interference (reflect, refract, diffract, standing waves) does not require a medium • Like a particle, light can move electrons • So light is described as having a dual nature • Comes in “packets” called photons(more like bundles of energy) • Amount of energy directly proportional to frequency. Higher freq. = more energy. • Indirectly proportional to wavelength. Bigger wave = less energy • Speed of light 3 x 108m/s in vacuum • Speed changes for medium, but not significantly, slows down for denser media

2. Electromagnetic spectrum • all possible kinds of light, classified according to frequency and wavelength • Radio/TV Waves…lowest energy, used in communication • Microwaves…used in cooking and communication • Infrared Waves… felt as warmth • Visible light…we can see ROYGBIV • Ultraviolet waves…cause sunburn • X Rays… used in medicine • Gamma Rays… used in medicine • Stars, like the sun, give off all of these forms of energy

3. Light Rays • use lines called raysto see where light goes • rough surfaces reflect the rays in all directions • smooth surfaces reflect the light in one direction • the angle light rays come in at equals the angle it’s reflected at. • Angle of Incidence equal the Angle of Reflection (Law of Reflection) • Angles are measured between the ray and an imaginary line perpendicular to the surface called the normal. • movie: Law of Reflection

4. Mirrors • Plane (flat) mirrors form virtual images inside of the mirror. Virtual imagesare from where light appears to come from, not the actual path. • Curved mirrors distort the image, but make realimages. Real imagesform when light rays come together at a certain point • movie: comparing real and virtual images

5. Color • Light hits objects. • Many wavelengths are absorbed by the object and not seen by you. • Wavelengths that are reflected are seen by you as color. • Technically, you don’t see something, you see light bouncing off of something. • Colors can add together to make new colors (e.g. light. White light = all colors of light, black = absence of light) • Colors can subtract to make new colors (e.g. pigments, black paint = all colors of paint, white paint = absence of color) • movie: additive color mixing

6. Refraction • Light bends as it goes from one substance (medium) to another because it changes speed • Compare the bend to the normal, just like reflection • Light bends towards the normal when it enters a denser medium and slows down • Light bends away from the normal when it enters a less dense medium and speeds up

7. Lenses • Mirrors and lenses can be curved out (convex) or can curve in (concave … like a cave). • movie: converging anddiverging lenses

8. Light Rays • There’s an angle called the critical angle, where light will not go back out of something and just keeps bouncing back around inside. • Diamonds are cut like this to keep the light from leaving and fiber optic cables are made so that light does not escape. • movie: Total Internal Reflection