Electromagnetic Waves

Evidence for the Wave Model

In 1801, the English physicist Thomas Young showed that light behaves like a wave. Young passed a beam ofl ight first through a single slit and then through a double slit. Where light from the two slits reached a darkened screen, Young observed alternating bright and dark bands. The bands were evidence that the light had produced an interference pattern. Bright bands indicated constructive interference, and dark bands indicated destructive interference. Interference occurs only when two or more waves overlap. Therefore, Young's experiment showed that light behaves like a wave.

Photons

In 1905, lbert Einstein proposed that light, and all electromagnetic radiation, consists of packets of energy. These packets of electromagnetic energy are now called photons. Each photon's energy is proportional to the frequency of the light. The greater the frequency of an electromagnetic wave, the more enrgy each of its photons has. Blue light has a higher frequency than red light, so photons of blue light have more energy than photons of red light. Blue light consists of photons that have enough energy to cause electrons to be emitted from a metal surface. So blue light can cause emission of electrons. Red light has a lower frequency than blue light, so photons of red light have less energy than photons of blue light. Red light consists of photons that have too little energy to cause any electrons to be emitted from a metal surface. So red light does not cause emission of electrons.

Michelson's Experiment

In 1926, the American physicist Albert Michelson measured the speed of light more accurately than ever before. On top of Mount Wilson in California, Michelson placed an eight-sided rotating mirror. He placed another mirror, this one stationary, on Mount San Antonio, 35.4 kilometers away. Michelson shined a bright light at one face of the rotating mirror. The light reflected to the stationary mirro on the other mountain and then back to Mount Wilson, where it struck another face of the rotating mirror. Michelson knew how fast the eight-sided mirror was rotating and how far the light traveled from mountain to mountain and back again. With those values he was able to calculate the speed of light quite accurately. His findings were similar to modern measurements.

Wavelength and Frequency

In a vacuum, all electromagnetic waves travel at the same speed. But not all electromagnetic waves are the same. Electromagnetic waves vary in wavelength and frequency. The speed of an electromagnetic wave is the product of its wavelength and frequency. Because the speed of electromagnetic waves in a vacuum is constant, the wavelength is inversely proportional to the frequency. As the wavelength increases, the frequency decreases. If you know the wavelength of an electromagnetic wave, you can calculate its frequency.

Wave or Particle?

In the late 1600s, the English physicist Isaac Newton was the first to propose a particle explanation. He based this hypothesis on two pieces of evidence: light travels in a straight line and it casts a shadow. Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles.

Intensity

Photons travel outward from a light source in all directions. Near the light source, the photons spread through a small area, so the light is intense. Intensity is the rate at which a wave's energy flows through a given unit of area. You can think of intensity as brightness. Farther from the source, the photons spread over a larger area. The intensity of light decreases as photons travel farther from the source.

The Speed of Light

Since Michelson, many other scientists have measured the speed of light. Their experiments have confirmed that light and all electromagnetic waves travel at the same speed when in a vacuum, regardless of the observer's motion. The speed of light in a vacuum, c, is 3.00*10^8 meters per second.

Wave Speed of Light

Speed (m/s)=wavelength (m)*frequency (Hz)

Photoelectric Effect

The emission of electrons from a metal caused by light striking the metal is called the photoelectric effect. Discovered in 1887, the photoelectric effect was puzzling. Scientists did not understand why dim blue light caused electrons to be emitted from metal but even bright red light did not.

What are electromagnetic waves?

These are transverse waves made up of alternating electric waves and magnetic waves. Electromagnetic waves are made up of energy and particles known as photons.

Evidence for the Particle Model

When dim blue light hits the surface of a metal such as cesium, an electron is emitted. A brighter blue light causes even more electrons to be emitted. But no red light, no matter how bright it is, does not cause the emission of any electrons in this particular metal.

Objectives

-Describe the characteristics of electromagnetic waves in a vacuum and how Michelson measures the speed of light -Calculate the wavelength and frequency of an electromagnetic wave given its speed -Describe the evidence for the dual nature of electromagnetic radiation -Describe how the intensity of light changes with distance from a light source

Key Words

-Electric Field -Electromagnetic Radiation -Electromagnetic Waves -Intensity -Magnetic Field -Photoelectric Effect -Photons

If you know the speed of light is 3.0*10^8 m/s, and the wavelength is 10 meters, what is the frequency?

3.0*10^8 m/s=10m*frequency (Hz); Frequency=30,000,000 or 3*10^7 Hz

Magnetic Field

A magnetic field in a region of space produced magnetic forces. Magnetic fields are produced by magnets, by changing electric fields, and by vibrating charges.

Electric Field

An electric field in a region of space exerts electric forces on charged particles. Electric fields are produced by electrically charged particles and by changing magnetic fields. Electromagnetic waves are produced when an electric charge vibrates or accelerates.

How They Travel

Because changing electric fields produce changing magnetic fields, and changing magnetic fields produce changing electric fields, the fields regenerate each other. As the field regenerate, their energy travels in the form of a wave. Unlike mechanical waves, electromagnetic waves do not need a medium. Electromagnetic waves can travel through a vacuum, or empty space, as well as through matter. The transfer of energy by electromagnetic waves traveling through matter or across space is called electromagnetic radiation.

Where do you find electromagnetic waves?

Electromagnetic waves are everywhere, including the sun, television, radio, lights, fire, and atoms that make up matter.

Electromagnetic Waves

Electromagnetic waves are transverse waves consisting of changing electric fields and changing magnetic fields. Like mechanical waves, electromagnetic waves carry energy from place to place. Electromagnetic waves differ from mechanical waves in how they are produced and how they travel.

Photons

Electromagnetic waves are unique in that they not only travel as waves, but they also act like particles known as photons. Photons are units of energy that are proportional to the frequency of the light. This speical property of electromagnetic waves is known as the wave-particle duality. The casting of a shadow next to an object is believed to be evidence that electromagnetic waves travel as waves and as particles because they experience interference like a wave, and electrons are emitted like a particle.

What can electromagnetic waves travel through?

Electromagnetic waves can travel through air, water, and even vacuums. Electromagnetic waves do not need a medium to travel through.