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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