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Light
Light Terminology
Which is not a measure we use to identify a type of light?
A. Wavelength
B. Speed
C. Frequency
D. Energy
Speed of Light
• Light is made of particles called photons.
• All photons move at a constant speed c in the vacuum of space.
c = 3 105 km/s = 186,000 miles/sec
10-9 s in 1 foot = 1 nanosecond/foot
0.02 sec to cross the US
8 min to get from the Sun to the Earth
Measuring Waves
• Photons can act as waves.
• Wavelength () is the distance between two waves.
• The period is the time between waves.
• Frequency () is the inverse of the period.– Frequency is measured in
hertz (Hz).
• Speed, wavelength and frequency are related.
1 wavelength
c
Measuring Particles
• Photons can act as particles.
• Particles are described by their energy (E).– Measured in joules (J)
– Golf ball dropped from 7 ft
• Waves are related to particles.
• Planck’s Law describes the amount of energy in a photon.– h is Planck’s constant
– h = 6.6 10-34 J s
hc
hE
Visible Light
• Visible light is radiation in a narrow band of wavelength or energy.– Narrower ranges are colors
• Visible light is part of a larger electromagnetic spectrum.
wikipedia
Wavelength in nm
Radio Waves
• Radio waves come from electrical oscillations.– Microwaves with shorter
wavelengths
• The long wavelengths travel easily and are not interfered by atoms.
Radio waves > 1 m < 3 x 108 HzMicrowaves 1 mm to 1 m 3 x 1011 Hz to 3 x 108 Hz
Infrared and Ultraviolet
• Visible light is often produced by vibrating atoms and molecules.– Infrared lower energy than
visible light
– Ultraviolet higher energy than visible light
Infrared 700 nm to 1 mm 4 x 1014 Hz to 3 x 1011 HzVisible light 400 nm to 700 nm 8 x 1014 Hz to 4 x 1014 HzUltraviolet 10 nm to 400 nm 3 x 1016 Hz to 8 x 1014 Hz
High Energy Photons
• X-rays come from energetic transitions in atoms.
• Gamma rays come from nuclear and subatomic reactions.
X-rays 0.01 to 10 nm 3 x 1019 Hz to 3 x 1016 HzGamma rays < 0.01 nm > 3 x 1019 Hz
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