Electromagnetic Waves

Transcription

1 Electromagnetic Waves

2 What is an Electromagnetic Wave? An EM Wave is a disturbance that transfers energy through a field. A field is a area around an object where the object can apply a force on another object without touching it.

3 What is an Electromagnetic Wave? Electricity can be static, like what holds a balloon on the wall or makes your hair stand on end. Magnetism can be static, like a refrigerator magnet. When they move or change together, they make EM waves!

4 How does an EM wave form? Electrons (negatively charged particles) move at different distances from the nucleus depending on the amount of energy the electron has. When electrons absorb energy, they jump to a higher position (away from the nucleus). If an electron returns to its original position, it releases energy as a Photon. A stream of photons starts a disturbance of vibrating magnetic and electric fields which carry the energy released by the electron!

5 How does an EM wave form?

6 How does an EM Wave Move? A vibrating electric field generates a magnetic field. A vibrating magnetic field generates an electric field. The fields move perpendicular to each other and in the direction of travel of the wave. Together they form a single EM wave traveling as a transverse wave.

7 How does an EM Wave Move? Video Animation

8 What is the Source of most of the Earth s EM Waves? The Sun s high energy allows it to give off countless EM waves. Other stars produce EM waves, but because they are so far away, few of their waves reach Earth

9 Why can EM Waves Travel Through Space? EM waves are made up of fields and not matter (photons have no mass), they can transfer energy through space without a medium. The transfer of energy in the form of EM waves is called radiation. Because EM waves do not vibrate matter to move, they do not lose energy as they travel. EM waves from the Sun travel about 93 million miles to reach the Earth (~150 million km).

10 How do EM Waves Travel Through Space? In a vacuum, EM waves spread outward in all directions from the source of the disturbance. The wave travels in a straight line until something interferes with it. The farther the waves move from their source, the more spread out they are. Only a small part of the energy radiated from the Sun is transferred to the Earth.

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12 How fast do EM waves travel? In a vacuum, EM waves travel at a constant speed. Known as the speed of light, EM waves travel at ~300,000,000 m/s Even traveling Speed of Light, the Sun s rays take about 8 minutes to reach the Earth.

13 Are all EM waves the same? Even though all EM waves travel at the same speed, they can have different frequencies and wavelengths. The entire range of possible EM waves is called the Electromagnetic Spectrum

14 Are all EM waves the same? The spectrum is a smooth, gradual progression from the lowest frequencies (longest wavelengths) to the highest frequencies (shortest wavelengths). The divisions between the EM Spectrum areas are not exact.

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16 Radio Waves Radio waves have the longest wavelengths, lowest frequencies, and carry the least energy. Different radio stations broadcast at different frequencies. The numbers (101.1 MHz, 99.5 MHz) are the frequencies they transmit at!

17 Radio Waves Just transmitting EM waves at different frequencies is not enough to send music or words. The radio transmitter attaches the information by MODULATING (changing) the waves. AM stations use Amplitude Modulation. FM stations use Frequency Modulation.

18 Radio Waves AM Radio waves can reflect off the ionosphere Helps AM waves travel long distances FM radio waves can pass through the ionosphere. Cannot travel as far as AM waves There are only 100 FM radio stations available to avoid frequency interference TV also uses radio waves Picture part is transmitted with AM waves Sound part is transmitted with FM waves

19 Microwaves Waves Microwaves are EM waves with shorter wavelengths, higher frequencies, and higher energy than radio waves Radar, cell phones, and Microwave ovens use microwaves!

20 Microwaves Radar stands for radio detection and ranging. First used in WWII to detect aircraft and ships from a distance. Radar works by transmitting microwaves and receiving reflections of those waves when they hit objects. Radar led to the invention of the microwave oven (accidentally)

21 Infrared Waves Infrared light waves are between microwaves and visible light on the spectrum. Infrared is commonly associated with heat, sometimes called Heat Rays.

22 Infrared Waves Humans are unable to see infrared waves. We can sense their presence by feeling the warmth Pit vipers and other snakes can see infrared light.

23 Infrared Waves Humans can only see these frequencies using a device like an infrared camera. Warmer areas appear red, orange, or white. Cooler areas appear blue, green, or purple

24 Visible Waves Visible waves are a very narrow part of the EM spectrum. Humans see different wavelengths as different colors.

25 Visible Waves Longest wavelengths have lowest energy Seen as RED Shortest wavelengths have the most energy Seen as Violet or Purple In order of increasing energy R O Y G B I V

26 Visible Waves When all the colors are combined White light is seen (different from paint!) Most visible light on Earth comes from Sun, but some materials can give off visible light

27 Visible Waves When some materials get hot enough, light is produced Called Incandescence In incandescent light bulb, a tungsten wire has high electric resistance, causing the wire to get hot and then emit light. Actually produces more infrared than visible, this is why light bulbs hot

28 Ultraviolet Waves UV waves have higher frequencies, shorter wavelengths, and more energy than visible waves. Can damage tissue by burning your skin. Use sunblock and UV-sunglasses to filter these out

29 UV Waves UV waves are used to sterilize eye googles in the classroom! UV waves help create Vitamin D, essential to good health. Bees are able to see in the UV range.

30 X-Ray Waves X-rays have very short wavelengths, high frequencies, and very high energy. X-rays easily pass through human skin, but are blocked by bone.

31 X-Ray Waves Placing photographic film (or an electronic detector) will expose the film and allow doctors to diagnose bone fractures and dense tumors. Too much exposure to x-rays can damage tissue and cause cancer. Lead blocks x-rays

32 Gamma Ray Waves Highest energy EM waves, shortest wavelength, highest frequency Produced by radioactive substances and stars

33 Gamma Ray Waves Gamma rays can penetrate skin and hard tissue of the body. Used in cancer treatment to kill cancer cells Unfortunately all cells in beam path can also be killed

34 Wave Interaction

35 How Do EM Waves Interact with a Medium? Transmission is the passage of an EM wave through a medium If the light reflected from objects did not pass through the air, windows, or most of the eye, we would not be able to see the objects!

36 Material Classification Transparent Most light passes through. Air, water, clear glass Translucent Some light is transmitted, but some is reflected into new directions Lamp shade, frosted windows, notepaper Opaque No light is transmitted. All is either reflected or absorbed. Heavy fabrics, ceramics, wood, rock

37 Absorption Absorption is the disappearance of an EM wave into a medium. i.e. when a dark colored object has light shining on it, the object heats up as the light energy is absorbed.

38 Scattering The spreading out of light waves in all directions because particles reflect the light in different directions. Scattering makes the sky blue Air molecules are perfect size to scatter blue light more than any other color!

39 Refraction Light bending as it passes at an angle from one medium to another. Light refracts based on its wavelength Prisms refract light waves twice entering and exiting Violet is refracted more than red Rainbows are caused by light refracting through water

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41 Law of Reflection Occurs when light bounces off an object Reflection angle = incident angle!

42 Reflection Regular reflection, like from a mirror, occurs when the surface is smooth. Diffuse reflection occurs when the surface is rough.

43 Why do we see colored objects Color seen is determined by the wavelength an object absorbs and reflects.

44 What are Primary Colors? The human eye is only capable of detecting three colors red, green, and blue!

45 What are pigments? Pigments give substances color by absorbing some of the colors and reflecting others. Each pigment absorbs at least one color of light Mixing pigments together is called color subtraction. When you mix pigments together, more colors are absorbed or subtracted from what you see!