Sec. 18.1- Electromagnetic Waves

Waves- all around us X-rays, microwaves, ovens, cell phones, wireless internet, heat lamps all use waves in order to travel

Without waves, we would not be able to receive energy from the Sun, watch TV, listen to the radio, use wireless internet, use cell phones to call or text… we wouldn’t even be able to see anything! Many of these waves are actually invisible to us!

What are electromagnetic waves? Electromagnetic waves display some behaviors similar to those of mechanical waves Even though there are similarities, there are many behaviors that are unique to electromagnetic waves

Electromagnetic waves- transverse waves consisting of changing electric fields and changing magnetic fields Electromagnetic waves carry energy from one place to another (like mechanical waves)- they just carry it differently

Electromagnetic waves are produced by constantly changing fields Electric field- region of space that exerts an electric force on charged particles (ions) Produced by charged particles and changing magnetic fields

Magnetic field- region of space that produces magnetic forces Produced by magnets, changing electric fields, and vibrating charges Electromagnetic waves are produced when an electric charge vibrates or accelerates

How electromagnetic Waves Travel Changing electric fields cause magnetic fields to change, and changing magnetic fields cause electric fields to change Due to this, electric and magnetic fields regenerate each other- cyclic process that repeats itself in a pattern

Picture of Electromagnetic Wave:

As the electric and magnetic fields regenerate each other, their energy travels in the form of a wave Electromagnetic waves do not need a medium- they can travel through vacuums Travel because of the pulsating/changing of the electric and magnetic fields- no matter is required for this to occur

Even though they don’t need mediums, electromagnetic waves are still able to travel through any form of matter as well The transfer of energy by electromagnetic waves is called electromagnetic radiation

Electromagnetic Waves- Speed All electromagnetic waves travel at the same speed through a vacuum- these waves may be slowed down if they travel through matter Through a vacuum, electromagnetic waves all travel at the speed of light Speed of light = 3.00 x 108 m/s

Electromagnetic Waves- Wavelength and Frequency Even though electromagnetic waves travel at the same speed, they are not the same- in fact there are a large variety of different types of electromagnetic waves Electromagnetic waves can have different wavelengths and frequencies

Since the speed of an electromagnetic wave is constant (all will be assumed to have the speed of light), wavelength and frequency of the waves are inversely proportional to each other (just like for mechanical waves)

We can use the wave speed equation to calculate wavelengths and frequencies of electromagnetic waves This can further give us information on the type of electromagnetic wave we are dealing with, and how much energy it is carrying

Wave speed equation: A radio station broadcasts a radio wave with a wavelength of 3.0 meters. What is the frequency of the wave?

Light acts as both a wave and a particle Photons Light acts as both a wave and a particle In certain situations, we can see the wave behavior of light In other situations, we can observe how light acts as a particle

When light, as well as all electromagnetic radiation, travels, it travels in packets of energy These packets of energy are called “photons” Photons will carry different amounts of energy depending on the frequency of the electromagnetic wave

Photons containing high frequency electromagnetic waves will carry more energy So if frequency of an electromagnetic wave increases, its energy also increases and vice versa Blue light has a higher frequency than red light. If you have a photon of blue light, and a photon of red light, which one will have more energy?

Intensity: rate at which a wave’s energy flows through a given area Intensity is related to the brightness of a light High intensity light= very bright light

What happens at night when you walk further and further away from a street light? What does this mean about the intensity of light as you increase the distance from the light source? If you get farther away from a light source, what happens to the distance photons have to travel to get to you?

Intensity of light decreases as photons travel farther from the source The farther the photons have to travel, the more spread out they get- creating light that is not as concentrated/intense This phenomena can be modeled by a can of spray paint

When the can is close to a piece of paper, the paint forms a small, dark spot When the can is farther away from a piece of paper, the paint forms a large, fainter spot Same amount of paint spread out over a larger area