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For more information, please see full course syllabus of AP Physics 1 & 2
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Electromagnetic Spectrum

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Question 1 0:24
  • Question 2 0:39
  • Question 3 1:05
  • Question 4 1:51
  • Question 5 2:03
  • Question 6 2:58
  • Question 7 3:14
  • Question 8 3:52
  • Question 9 4:30
  • Question 10 5:04
  • Question 11 6:01
  • Question 12 6:16

Transcription: Electromagnetic Spectrum

Hi everyone and welcome back to Educator.com.0000

In this mini-lesson, we are going to do page 1 of the APlusPhysics worksheet on the electromagnetic spectrum.0003

You can find a link to that worksheet down below.0008

Now to do this, I would highly recommend you have a copy of the electromagnetic spectrum in front of you or somewhere you can refer to that in order to answer these problems, because they can be pretty tricky if you do not have that in front of you to answer the questions with.0010

Let us dive in to Number 1 -- Compared to the speed of a sound wave in air, the speed of a radio wave in air is -- well that is going to be much, much, much greater.0024

Radio wave is an EM wave; it is going to travel at 3 × 108 m/s.0033

Number 2 -- An electromagnetic AM band radio wave could have a wavelength of...?0039

Well to answer this, I have to go look at my electromagnetic spectrum.0044

AM band radio waves, I find in the range of somewhere about 100 - 1,000 m for their wavelength, so the correct answer here would be Number 3.0049

Number 3 -- What color of light has a wavelength of 5 × 10-7 m in air?0065

Well, the first thing I am going to do is I am going to find the frequency.0071

That helps me to determine what type of wave it is.0074

So, if V = F(λ), then frequency is V/λ, which is going to be 3 × 108 m/s/5 × 10-7 m, which is going to come out to be right around 6 × 1014 Hz.0076

Now, I go look for that -- that frequency on my electromagnetic spectrum and I find that that is just at the border of blue and green, but slightly into green on my version of the spectrum, so we are going to call that green.0096

Number 4 -- In a vacuum, all electromagnetic waves have the same speed.0111

That is that constancy, the speed of light or electromagnetic waves in a vacuum.0117

Number 5 -- An electromagnetic wave traveling through a vacuum has a wavelength of 1.5 × 10-1 m.0124

What is the period of this electromagnetic wave?0130

To find the period, I think the first thing I am going to do is realize that period is 1/frequency.0133

So, if I use V = F(λ) -- Well, frequency is 1/T, so V = λ/the period, therefore, the period is going to be the wavelength over the velocity...0139

...which implies then that the period is our wavelength, 1.5 × 10-1 m over the velocity of our wave, 3 × 108 m/s, because it is traveling in a vacuum, then I am going to come up with a period of about 5 × 10-10 seconds, so the answer is Number 1.0154

Number 6 -- Which characteristic is the same for every color of light in a vacuum?0179

Well, obviously the answer to that must be (B), speed.0183

All electromagnetic wave haves the same speed in a vacuum, 3 × 108 m/s.0187

Number 7 -- Exposure to ultraviolet radiation can damage skin. Exposure to visible light does not damage skin.0194

State one possible reason for this.0201

Well, one reason that I can think of right away is that UV radiation has a higher amount of energy; it has a higher frequency, therefore more energy and more ability to do damage.0203

I would say that UV has a higher frequency, therefore a higher energy.0213

Moving on to Number 8 -- An FM radio station broadcasts its signal at a frequency of 9.15 × 107 Hz.0230

Determine the wavelength of the signal in air.0239

Well, if V = F(λ), then our wavelength λ equals velocity over frequency, which is 3 × 108 m/s because it is in air, divided by our frequency, 9.15 × 107 Hz is going to give me a wavelength of right around 3.28 m.0242

Number 9 -- Sunlight is composed of various intensities of all frequencies of visible light.0269

The graph represents the relationship between light intensity on the y-axis and frequency on the x.0274

Based on the graph, which color of visible light has the lowest intensity.0280

Well, that is going to be over here where we have the highest frequency and the highest frequency of light is violet.0284

Check that out on your electromagnetic spectrum.0293

Which color of visible light has the lowest intensity? Violet.0295

Number 10 -- Sunlight is composed of various intensities -- same graph we have had before.0302

It has been suggested that fire trucks be painted yellow-green instead of red.0309

Use information from the graph and explain the advantage of using yellow-green paint.0313

Well, yellow-green paint is here towards the middle of the spectrum, where we have the higher intensity.0318

So in that case, you are going to have a greater intensity of incoming light to reflect off the fire truck, so hopefully the people pick up more intensity from their eyes.0324

You have more intensity of light getting to people's eyes, so I would write something like, 'You have a greater intensity for reflection in the green.'0334

Two more -- Which wave characteristic is the same for all types of electromagnetic radiation traveling in a vacuum?0359

We have seen this before and hopefully you are picking up a trend that the answer there must be 1, speed.0368

Last question -- Calculate the wavelength in a vacuum of a radio wave having a frequency of 2.2 × 106 Hz.0375

Well, we will use our wave equation, V = F(λ) and we are looking for wavelength, so that will be velocity divided by frequency or 3 × 108 m/s divided by our frequency of 2.2 × 106 Hz, should give us a wavelength of right around 136 m.0385

That concludes page 1 of the worksheet on electromagnetic spectrum. If that went well -- Terrific -- Go ahead and move on and if that did not go so well, now would be a great time to go back and review that portion of the lecture course.0410

Thanks so much for your time everyone and make it a great day!0423