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Chapter 27
Light
All things either reflect or emit light.
Emit -- to give off
What is light? Particle or wave
• Newton and most philosophers before him thought that light consisted of particles.
• Light seemed to move in straight lines instead of spreading out as waves do.
• Huygens provided evidence that under some circumstances light does spread out.
• Other scientists later found more evidence to support the wave theory.
Then came Einstein
• In 1905 Einstein published a theory explaining the photoelectric effect. According to this theory, light consists of particles called photons.
• Photons are massless bundles of concentrated electromagnetic energy.
So what is light?Particle or Wave?
• Light is both a wave and a particle.
• We call this the dual nature of light.
• Sometimes light acts as a particle
• Sometimes light acts as a wave.
• It was not known whether light travels instantaneously or with finite speed until the late 1600s.
The Speed of Light
Now we just ask google
Measuring the Speed of Light- Michelson in 1880
The Results• Michelson’s experimental value for the speed of light was 299,920,000 m/sec.
We shall use 300,000,000 m/sec.In scientific notation, it is 3x108 m/sec.
•Michelson receivedthe 1907 Nobel Prizein physics for this experiment.
•He was the first American scientist to receive this prize.
Electromagnetic Waves
• Electromagnetic– Part electric part magnetic.
• All light is made up of an alternating electric and magnetic field.
• Light is a small part of what we call the electromagnetic spectrum.
The Electromagnetic Spectrum
• The electromagnetic spectrum consists of:• Radio waves• Microwaves• Infrared• Visible light• Ultraviolet• X-rays• Gamma Rays
Increasing Frequency
Increasing energy
Decreasing Wavelength
More about light.
• The light that comes from the sun is made up of many different wavelengths.
• All light can be broken down into all of the wavelengths that make up the light.
• All elements have their own signature of light when they burn.
• Astronomers use this to see what distant stars and galaxies are made of.
Continuum, Absorption, Emission
Emission Lines• Every element has a DIFFERENT finger print.
Polarized Light• Light being emitted from a source is
said to be polarized if all the amplitude of the waves is aligned in the same direction.
Try these• I like to listen to KSJS on the radio. I tunes my
radio to 90.5MHz. What is the wavelength of the station?
• Visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. Let’s calculate their frequencies.
• The X-Ray range starts around 3 x 1016 Hz. What is the wavelength of these waves?