What is Light? Light is an electromagnetic wave. These consist of electric and magnetic fields that are at right angles to each other. Electromagnetic waves are transverse waves.

Light consists of a varying electric and magnetic field

Does Light Travel Through a Medium? Unlike mechanical waves, light does not require a medium. The vibrating fields emanate from the source, they’re mutually inductive, allowing the wave to transmit in a vacuum.

Different Wavelengths Lead To:

It moves at about 300,000 km/sec!

Example 1 How long does it take for light from the sun to reach the earth if the sun is 1.50 x 1011 m away? Given: d = 1.50 x 1011 m c = 3.00 x 108 m/s c = d/t t = d/c = 1.50 x 1011 m 3.00 x 108 m/s = 500 s  8 min

Example 2 Microwaves emit waves of about 2450 MHz. What is the wavelength of this light? Given: c = 3.00 x 10 8 m/s f = 2450 x 10 6 Hz c =  f = c/f = 3.00 x 10 8 m/s 2450 x 10 6 Hz = 0.122 m

The wave equation becomes: c = l f Wave Nature of light The characteristics of waves we have described so far-– all apply to light. The wave equation becomes: c = l f

Wave nature of light Light exhibits interference effects. Young’s double slit experiment.

But there’s evidence of particle behavior! Newton’s Corpuscular Theory of Light - light consists of small particles, because it: • travels in straight lines at great speeds • is reflected from mirrors in a predictable way Einstein: Photoelectric Effect Light of high enough frequency can knock electrons loose from surface atoms of some metals!

Light is Packets of Energy Called Photons

Wave- Particle duality Lower energy (longer wavelength) light acts predominately like a wave High energy (shorter wavelength) light acts predominately like a particle Like a wave when it travels Like a particle when it interacts with matter

Cool Things Light Can Tell Us It can tell us what objects are made out of It can tell us if objects are moving toward or away from us It can tell us how far away an object is or (if we already know that) how energetic it is It can tell us the object’s temperature

Intensity of light Light emits equally in all directions. This affects the amount of light you will measure in any given square cm as you move further away from the light source.

The luminous flux, symbol P, is the rate at which visible light is emitted – measured in lumens (lm). Illuminance, symbol E, is the luminous flux per unit area – measured in lm/m2

Light intensity of light follows an inverse – square relationship. E = illuminance P = luminous flux

Example 3 What is the illumination on your desktop if it is lighted by a 1750 lm lamp that is 2.50m above your desk?

P = 1750 lm (luminous flux) d = 2.50 m E = ? (illuminance)