1 2 4 • How many l between 1 & 2 ? • How many l between 1 & 4? • How many nodes are there below?
How is a standing wave formed? • 2 waves or pulses same speed, l, f, undergo superposition interfering at fixed points of constructive & destructive interference forming pattern.
Light is a small part of the EM spectrum.All EM waves have the same behavior.
What is Light/EM waves 2 Models 1) Particles/Photons Packets of energy 2) Electromagnetic Waves energy-carrying waves emitted by vibrating charges. Light displays both types of behavior.
Light & all EM waves are transverse, non-mechanical waves. EM waves begin as motion of charged particles.
Electromagnetic Wave Velocity • The speed c is the same for all forms of EM waves in a vacuum of space. • It is ~ 3.0 x 108 m/s. • No mass can go the speed of light. • Nothing can go faster.
The velocity of EM radiation is almost the same in air as it is in space. • In other substances it travels more slowly.
Ex: The wavelength of a certain color of visible light is 429 nm. Using your table, determine the color of the light. • v = lf. • f = v/l • For light in air or vacuum, v = 3x108 m/s • Find f = 3x108 m/s = ________________ 429 x 109m. • f = 7 x 1014 Hz = violet.
Amplitude is related to the wave energy. So is frequency.High Amplitude means bright.Higher f = higher energy change in color.
The Electromagnetic SpectrumWaves become more energetic as the wavelength gets shorter (freq. gets higher) • Radio Waves - communication • Microwaves - used to cook • Infrared - “heat waves” • Visible Light - detected by your eyes • Ultraviolet - causes sunburns • X-rays - penetrates tissue • Gamma Rays - most energetic
The Visible Spectrum • A range of light waves extending in wavelength from about 400 to 700 namometers.
Questions • Is it correct to say that radio wave is a low-frequency light wave? • Is a radio wave also a sound wave? *
We see things because they reflect or transmit light into our eyes: Physics Rules
Luminous objects – generate their own light (the sun) • Illuminated objects – reflect light (the moon) • Line of Sight – The way you locate objects. You see a line from an object or image to your eyes (light from the object travels along this line to your eyes). You sight along the line.
Reflection of Light Rayincident ray hits new material, reflected ray bounces back. Normal – line perpendicular to the mirror surface • Incident Ray Reflected Ray
Law of Reflectionincident angle Qi = Qr reflection angle • Qi – angle between incident ray and normal • Qr – angle between reflected ray and normal.
Driving at night on a wet roadway results in an annoying glare from oncoming headlights.
Problem • A ray of light is incident on a mirror with Qi = 20o. Using a protractor and ruler, sketch the incident ray, the normal to the surface, the reflected ray. • Label the angles. • Sketch 5 wavefronts on the rays that show a wavelength of 1 cm.
A ray of light hits a boundary between 2 different materials, state what behavior(s) it might exhibit at the boundary.
A wave entering a new material will change its velocity.This will result in a bending of the wave either toward or away from a perpendicular to the interface.
Law of Refraction - Snell’s Law A ray passing from a faster medium to a slower medium bends toward normal. Qi greater than Qr. What happens to l?
For light traveling from a slow medium to a faster one, the refracted ray bends away from the normal. The light coming off fish bends at water/air interface.
Refraction of sunlight allows us to see sun a few minutes after it has set below horizon.
Amount of “Bending” The amount of bending of light is dependent on the ratio of the speed of light in the two mediums. The greater the Dv between the two materials, the greater the bend.
“Absoluteindex of refraction, n,” is the ratio of the speed of light in a vacuum to its speed in a medium. n = cn= index of refractionv v = velocity in medium c = speed of light in vacuum - 3 x 108 m/s
Light in a vacuum has an index of 1 since: n = c/v = c/c n = 1 All other materials have velocity less than c, so n is greater than 1. Air is so close that n ~1.
1. The speed of yellow light in calcite is 1.97 x 108 m/s. What is the absolute index of refraction for calcite?
A substance has an index of refraction n, of 0.67. What is wrong with this statement? All indices of refraction must be ≥ 1 since no substance can allow light to pass at a faster speed than c.
1. Light travels at 1.1 x 108 m/s in material X. What is the absolute index of refraction for material X. 2. What is the speed of light in Lucite? 3. Light travels at 2.66 x 108 m/s in Popalite. What is the index of refraction for Popalite?
Snell’s Law – Law of Refraction • Relates angle of incidence, angle of refraction and the 2 indices of refraction as well as velocity. n1sin q1 = n2sin q2 . n1 = indx refrc med 1. n2 = indx refrc med 2. q1 = angle of incidence q2 = angle of refraction
Variations of Snell’s Law sin q1 = v1sin q2 v2also n2 = v1 = l1n1 v2l2 .All in ref table.
1. A ray of light strikes crown glass from air at an angle of 30o from the normal to glass. What is the angle of refraction?Hint: Use Snell’s Law
n1 sin q1 = n2 sin q2(1.00)(sin 30) = (1.52)(sin q2) sin q2 = 0.5 = 0.33 1.52 q2 = 19o
2. A ray of light enters diamond from glycerol at an incident angle of 40o. Make a scaled sketch showing the: Incident ray Reflected Ray Refracted Ray (use Snell’s Law to calculate).
3. A ray of light travels from water into air. The incident angle is 40o. Make a scaled sketch showing the:Incident rayReflected RayRefracted Ray (use Snell’s Law to calculate).