Light Electromagnetic waves

E+M Waves Electromagnetic waves are transverse waves that can travel through a vacuum or uniform mediums Light is created by vibrations of electrons which creates both electric and magnetic fields Light moves in a straight line- called the Ray Model of light, treats light as a particle… Light also has Wave properties… Duality of Light: Light has the properties of waves and particles – Examples- shadows around corners, photoelectric effect

Light Speed o Before the 17 th century people thought light was instant o Early experiments showed light had a finite speed: Michelson “mountain” test o Speed = 299,792,458 m/s o The generally accepted value is 3.0 x 10 8 m/s o Symbol = c

Source of light Luminous bodies : – Objects that emits light waves – Example: Sun, light bulb Illuminated bodies : – Objects that reflect light produced from another source – Examples: moon, pictures

Light and Matter Interactions 3 things can happen when light reaches matter – When light passes through an object without distortion, the object is transparent. – When light passes through an object but is distorted, the object is translucent. – When light is reflected or absorbed by a material, the object is opaque. When objects reflect light… we see colors…

Visible Light Very small portion of the E&M spectrum The difference between red and violet is only about 4 nanometers (4 millionths of a meter)

Color Newton experimented with a beam of light passing through a prism. Each color has a specific wavelength in the E+M spectrum

Color by Addition Light has 3 primary colors – Red, Blue, green – These are the colors (wavelengths) that the human eye can perceive (look close at a TV screen) Cones- part of eye that detects color Adding pairs make the secondary colors Red + Blue = Magenta Red + Green = Yellow Blue + Green = Cyan

So why is grass green? What colors can your eyes detect? – Red, green, blue If you see green, what is being reflected? – Green- that is what you perceive in your eyes Then what color(s) is being absorbed? – Red and blue If a shirt appears to be yellow to you, what colors are being absorbed and reflected?

Subtractive Colors (paint pigments) Pigments are materials that absorb some light waves and reflect others. The three Primary colors are yellow, magenta and cyan (not red, yellow and blue) – Example: an object that appears red must absorb cyan but reflect yellow and magenta Color is absorbed and “subtracted” from what is seen. Therefore, you see what is left behind.

Pigments

A student walks in with a bright yellow shirt. – What color light is hitting the shirt? – What colors does the shirt absorb? – What colors does the shirt reflect?.

A lab table appears to be black. – What color light strikes the table? – What color light is reflected? – What colors of light are absorbed? The starts of the flag appear white. – What color light is striking the flag? – What color light is reflected? Absorbed?

Polarization Polarized light waves are light waves in which the vibrations occur in a single plane The process of transforming un-polarized light into polarized light is known as polarization there are 4 ways to polarize light – Transmission – Reflection – Refraction – scattering

Transmission “filtering” The most common way is to “filter” out waves of a an alignment you do not want. Example: Polarized lenses on cameras Class example: think about the glasses!

Polarization by reflection Example: night time driving in the rain, the headlights give a glare from the reflection on the road. Light hits the road un-polarized but reflects off as polarized- causing glare

Snell’s Law Refraction occurs when waves move from one medium to another. – Each medium has an “index of refraction” (n)- a ratio of the speed of light in a vacuum versus the speed of light in the material. The equation is: n i *(sinθ i ) = n R *(sinθ R ) Θ is ALWAYS measured from the normal line! – Normal means “perpendicular”

Practice problem A drinking glass full of water has light reflecting off a pencil inside of it. As the light leaves the pencil it strikes the container edge at a 25° to the normal line. If it enters air on the other side, what angle will it leave the surface at? Air Ice1.31 Water1.333 Ethyl Alcohol1.36 Plexiglas1.51 Crown Glass1.52 Light Flint Glass1.58 Dense Flint Glass1.66 Zircon1.923 Diamond2.417 Rutile2.907