Light Introduction General Information. Light is a Wave Exactly as ocean waves, sound waves, slinky waves... Amplitude Wavelength Frequency Wave speed.

Light Introduction

General Information

Light is a Wave

• Exactly as ocean waves, sound waves, slinky waves. . .

• Amplitude

• Wavelength

• Frequency

• Wave speed

v = f

The speed of light is constant (in a given material)

• 3 x 108m/s (186,000 miles per second)

• Nothing travels faster – Although light does slow as it enters a

more dense medium

• Therefore,

3x108m/s = f

So if v is constant . . .

•v = f • As f goes up goes down

• As f goes down goes up

• Also E is related to amplitude –Big wave high energy (tsunami)

Light is different from most waves

• Because it does not need a medium to be transmitted– A water wave needs water– A sound wave needs air– A light wave needs nothing!

Light needs no medium because

• It is an electro-magnetic wave

• An electron moves– A changing electric field is produced– The electric field produces a magnetic field– The changing magnetic field produces an

electric field– A changing electric field…

Moving at websitehttp://www.phy.ntnu.edu.tw/java/emWave/emWave.html

Light is Electromagnetic Waves

Only a small part are visible, most are not.The spectrum is organized by frequency and wavelength

All light is a wave

• Can only see visible light• Can’t see the wave form• We can see white light which is all colors

together • Gamma has high frequency small wavelength• Radio has low frequency large wavelength

Energy is related to frequency

• As you increase frequency you increase energy

• As you decrease frequency you decrease energy

• Gamma high f, high E

• Radio low f, low E

High Energy

Low Energy

Wait!

• In the wave unit we learned energy was proportional to amplitude– In a classic wave there is no connection

between f and E

• Again, light is different from a traditional wave

Why is energy related to frequency for light not for other waves?

• We describe light as having a dual nature– We try to explain light in terms of human

experience, but we can’t really describe it– Sometimes we notice light acts like a

particle– Sometimes we notice light acting like a

wave– Actually, light is both and neither.

Energy• High frequency = high Energy• Short wavelength = high Energy

E = hf

What is more dangerous:

• Visible light or microwaves?

• How does a microwave work then?

• Why doesn’t it hurt us?

• Cell phones use microwaves

Intensity

• Brightness

• The number of light waves per area

• Nothing about the wave changes

We think of light as just brightness

• Really light is a SPECTRUM

• All wavelengths of light together creates white light– When a room is dark you see - black!

Black is the absence of light – A prism separates the light into colors– But light is more than just the colors

Remember

• Light is electromagnetic radiation

• From gamma to radio waves

• Energy is proportional to frequency

• Light can be described as a wave and particle

• Otherwise, light behaves as any other wave