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.

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Section 18.1 electromagnetic waves They differ in how they are produced 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

Travel of Electromagnetic Waves Fields regenerate each other. Their energy travels in the form of waves.

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

Section 18.1 electromagnetic waves The Speed of 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 he 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

Electromagnetic Spectrum Page 540

AM=Amplitude modulation Amplitude is varied, Frequency is same Section 18.2 Page 540 Notes 1. Radio Waves – lower frequency, longest wavelength 300,000 m Hz + lower Used in radio and TV technologies as well as microwave ovens and radar. A) Radio AM=Amplitude modulation Amplitude is varied, Frequency is same FM=Frequency modulation Frequency is varied, amplitude is same

AM stations can be heard for greater distances because particles in the Earth’s upper atmosphere reflect lower-frequency waves better.

B) Television Similar to radio signals Difference- carry pictures and sound Reception affected by weather Satellite allows you to receive the signal directly. Cable sends signal to your house. C) Microwaves Shortest-wavelength radio waves Cook and reheat by heating up materials to higher to thermal energy.

Also carry cell phone conversations C) Radar ( Radio Detection and ranging) Sends short burst of radio waves that reflects back to a transmitter to interpret results 2. Infrared – wavelength 1mm-750mm -used as source of heat and to discover areas of heat differences - infrared lamps -Thermograms (infrared sensors)- color- coded pictures that show variations in temperature.

3. Visible Light -ROYGBIV -Use to see, help keep safe, and communicate (read, traffic signals, headlights) 4. UV Rays-higher frequency than violet light -use in health, medicine and agriculture -helps skin produce Vitamin D -excessive cause sunburn, wrinkles, skin cancer, eye damage -kill microorganisms - disinfect air in cooling systems - help plants grow

5. X – Ray- short wavelengths 12 nm to .005 nm. -Medical , industry, and transportation to make pictures of the inside of solid objects -Photos of teeth and bones - Too much can kill or damage tissue. 6. Gamma Rays- shortest wavelengths in electromagnetic spectrum, .005 nm or less. Used in medical field to kill cancer cells, make pictures of the brain, and in industrial situations as an inspection tool. Overexposure can be deadly.