An Introduction to Light

What is light?

• Sun is our closest star• Source of all energy

on Earth• Energy produced by

nuclear reactions in core reach Earth in form of light (and heat)

• Source of all life on Earth

What is light really?Electromagnetic radiation waves

• Light waves are three dimensional.

• Light waves vibrate in all planes around a center line.

• Waves have high points called “crests.”

• Waves also have low points called “troughs.”

• Distance from one crest to the next crest is called a “wavelength.”

• Number of waves passing a given point in one second is called the “frequency.”

wavelength

James Clerk Maxwell• English Physicist• In 1864, predicted

that electricity forms a change travelling through space (electromagnetic waves)

• Also knew that it would travel at the speed of light

• Great beard!

Heinrich Hertz

• German Physicist• In 1887, he proved

what Maxwell had predicted 23 years earlier

• Guess who got credit for the work?

• Good beard…at best

The Electromagnetic Spectrum

Notice the wavelength is long(Radio waves) and gets shorter (Gamma Rays)

Electromagnetic SpectrumIncreasing Energy

Radio waves

• Lowest energy Electromagnetic (EM) waves

Microwaves

• Next highest energy EM waves

• Used in telecommunications, microwave ovens and astronomy (measuring background radiation from Big Bang Theory)

Infrared Light

• Used in remote controls, lasers, heat detection, physical therapy and food warming

Visible light

• Only portion of the spectrum that can be seen

• Examples: human vision, rainbows, visible lasers

Activity

• Let’s stop there and have a look at an experiment first done by Sir Isaac Newton in the year 1666

Ultraviolet light• Invisible• Causes skin to tan

and burn• Kills bacteria in food

and water• “black” lights

X-rays

• Very high energy EM waves

• Short wavelength• Used in medical

imaging, security equipment and cancer treatment

Gamma rays

• Highest energy EM waves

• Used in cancer treatment

• Product of some nuclear decay

• Incredible Hulk???

How is light produced?

I mean besides from the Sun…

Light from Incandescence

• Producing light as a result of high temperature

• Eg. Stove burner on high, light bulb, molten glass, sparks, etc.

Light from Electric Discharge

• Producing light as a result of an electric current running through a gas

• Eg. Lightning, neon lights, etc.

Light from Phosphorescence

• “glow-in-the-dark” objects are coated with phosphors (material that gives off light)

• Eg. Toys, watch dials, etc.

Light from Fluorescence

• When an object absorbs UV light and immediately releases the energy as visible light

• Can laundry soaps really make clothes brighter?

Light from Chemiluminescence

• Production of light as a direct by-product of a chemical reaction

• Almost no heat produced, called “cold light”

• Eg. Glow sticks, Luminol (CSI), etc.

• When chemi-luminescence occurs in nature

• Quite common!• Eg. Bacteria,

fungi, fish, and of course glow-worms and fireflies

Light from Bioluminescence

Light from a Light-Emitting Diode

• Aka “LED” light• Electronic device

allows electricity to flow in one direction

• With current, LED emits light

• Cooler and more energy efficient than incandescent

Triboluminescence

• Production of light when certain crystals are scratched, crushed or rubbed together

• Has no real use at this time

Activity

• Try Triboluminescence