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