Chapter 18 The Electromagnetic Spectrum and Waves Section 18.1 Electromagnetic waves

Section 18.1 Electromagnetic waves Electromagnetic waves Are transverse waves consisting of changing electric fields and changing magnetic fields. They are similar to mechanical waves in that they carry energy from place to place.

Section 18.1 electromagnetic waves They differ in how they are produce and how they travel They are produced by constantly changing fields. 1. Electric field- is a region of space that exerts electric forces on charged particles. They are produced by electrically charged particles and by changing magnetic fields.

Section 18.1 electromagnetic waves 2. Magnetic field- a region in space that produces magnetic forces. They are produced by magnets, by changing electric fields and by vibrating forces. Electromagnetic waves are produced when an electric charge vibrates or accelerates. Figure 2 page 533

Section 18.1 electromagnetic waves Electromagnetic waves do not need a medium It can travel through a vacuum, empty space as well as matter. When these waves travel through matter or across space it is called electromagnetic radiation.

Section 18.1 electromagnetic waves Light travels faster than sound. Ex: You see lightning before you hear thunder. In 1926 Albert Michelson measured the speed of light more accurately than the experiments performed in the 1800’s. Read page 534 Michelson’s Experiment

Section 18.1 electromagnetic waves All light and electromagnetic waves travel at the same speed when in a vacuum. The speed of light is 3.00 × 108 m/s. The symbol is c (it is a constant) Electromagnetic waves may travel at the same speed but they are not all the same. They vary in wavelength and frequency. P. 535 calculating wave speed

Section 18.1 electromagnetic waves Electromagnetic radiation behaves sometimes like a wave and sometimes like a stream of particles. Isaac Newton was the first to propose the particle explanation. Based on 2 pieces of evidence: light travels in straight lines and it casts a shadow.

Section 18.1 electromagnetic waves Evidence of the wave model In 1801 Thomas Young showed that light behaves like a wave. He observed alternating bright and dark bands. These bands were evidence that light produced an interference pattern. Bright bands- constructive interference Dark bands- destructive interference. Interference only occurs when 2 or more waves overlap.

Section 18.1 electromagnetic waves Evidence for the Particle Model Photoelectric effect is the emission of electrons from a metal caused by light striking the metal. Blue light causes electrons to be emitted (no matter how dim) but not red light (no matter how bright)

Section 18.1 electromagnetic waves In 1905 Albert Einstein proposed that light, and all electromagnetic radiation consists of packets of energy called photons. Each photons energy is proportional to the frequency of light. Meaning that the greater the frequency of an electromagnetic wave the more energy each of its photons has.

Section 18.1 electromagnetic waves Red light- low frequency- so less energy of photons compared to blue light. Blue light- high frequency- more energy in photons than red light and able to emit electrons.

Section 18.1 electromagnetic waves Intensity The closer you are to a source of light the brighter it will appear. 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 more intense. Think of it as brightness The intensity of light decreases as photons travel farther from the light source (more spread out).

18.2 Electromagnetic Spectrum and Light Electromagnetic Spectrum is the full range of frequencies of electromagnetic radiation. The spectrum consists of: Long Wave Length Radio wave Low Frequency Infrared rays Visible light Ultraviolet rays X-rays Short wavelength Gamma Rays High Frequency