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?