Chapter 33: Electromagnetic Waves

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1 Physics 4C Chapter 33: Electromagnetic Waves An Electromagnetic Wave The EM Spectrum Refraction Apparent Depth Chromatic Dispersion Total Internal Reflection An electromagnetic wave is a transverse wave consisting of oscillating electric and magnetic fields. The Visible Spectrum The Electromagnetic Spectrum Imagine if you can the world view of little creatures who could see only a very tiny portion of the visible spectrum, creatures who are color blind to all other parts. Their world view would be very limited. Guess what? We are like those little creatures, in that the spectrum of colors we can see are a tiny portion of the electromagnetic spectrum! The electromagnetic spectrum consists of electromagnetic waves of all frequencies. 1

2 The Electromagnetic Spectrum For an EM wave, the plane containing the electric field is called the plane of oscillation. Visible light makes up a very small portion (less than one millionth of 1%) of the measured electromagnetic spectrum. of EM wave is represented by showing a head-on view of plane of oscillation. Transverse wave polarized in vertical plane Transverse wave polarized in horizontal plane 2

3 Refraction of Light Refraction of Light To understand why light bends when passing from one transparent material to another, imagine a cartwheel rolling from a sidewalk onto grass. Refraction: The bending of light when it passes from one transparent material to another. Because the wheels roll slower in the grass, the left wheel will slow down first causing the cartwheel to change directions. Refraction of Light Apparent Depth Similarly, when light hits a transparent material like water where the speed of light is slower, one side of the wavefront slows down before the other side. Because light travels slower in the water, the left side of the wavefront will slow down first, causing the light to bend. Because of refraction, the apparent depth of objects in water is less than their actual depth. (Objects are deeper than they appear.) 3

4 Place a glass test tube in water and you can see the tube. Place it in clear soybean oil, and you may not be able to see it. What does this tell you about the speed of light in the oil and in the glass? Chromatic Dispersion The index of refraction (n) in a transparent material depends upon the frequency of the light. In general, n is greater for higher frequencies (violet) than for lower frequencies (red). Therefore violet light is bent more than red light. The speeds in both glass and soybean oil are the same, so there is no reflection or refraction between the glass and oil. Chromatic Dispersion Dispersion occurs when sunlight hits a water droplet. This is what causes rainbows. The incoming sunlight enters the water droplet and different frequencies are bent (refracted) by different amounts. Chromatic Dispersion: The separation of light into different colors arranged according to their frequency. When this refracted light hits the back of the water droplet, some of it is transmitted back into the air (not shown) and some of it is reflected back into the water droplet. The light reflected from the back of the water droplet then gets bent again (refracted) as it leaves the water droplet. 4

5 The different colors of a rainbow are the result of the dispersion of sunlight by millions of water droplets that act like tiny prisms. Although each water drop disperses a full spectrum of colors, a person can only see one color of light from each drop. Because of this, two people in different positions do not see the same rainbow. are actually complete circles, however the ground cuts off the bottom half. Sunlight can sometimes be reflected twice within a single water droplet. Single reflection single rainbow. Double reflection double rainbow. 5

6 Sometimes a momentary flash (lasting 1 or 2 seconds) of green light can be seen when the sun sets. A simplified explanation of green flashes is that our atmosphere acts like an upside down prism. White light is dispersed with blue on top, green near the top, and red on the bottom. At the moment of setting, the red is cut off by the Earth, the blue is removed because of atmospheric scattering, and the green survives to give the green flash. 6

7 Blue Flash Total Internal Reflection Under ideal atmospheric conditions, it is possible to see a blue flash. Total Internal Reflection When light tries to go from one transparent material into another transparent material where the speed of light is greater, the light will get totally internally reflected (no light gets refracted out) if the angle the light hits at is greater than a certain angle called the critical angle. 7