Conceptual Physics Fundamentals Chapter 13: LIGHT WAVES

This lecture will help you understand: Electromagnetic Spectrum Transparent and Opaque Materials Color Why the Sky is Blue, Sunsets are Red, and Clouds are White Diffraction Interference of Light

Light Waves “The light of stars that were extinguished ages ago still reaches us. So it is with great men who died centuries ago, but still reach us with the radiations of their personalities.” —Kahlil Gibran

Light Waves Light is the only thing we can see originates from the accelerated motion of electrons electromagnetic phenomenon

Electromagnetic Spectrum Electromagnetic wave made up of vibrating electric and magnetic fields

Electromagnetic Spectrum CHECK YOUR NEIGHBOR If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a A. period of 1000 seconds. speed of 1000 m/s. wavelength of 1000 m. none of the above D. none of the above.

Electromagnetic Spectrum CHECK YOUR ANSWER If an electron vibrates up and down 1000 times each second, it generates an electromagnetic wave with a A. period of 1000 seconds. speed of 1000 m/s. wavelength of 1000 m. none of the above Explanation: The vibrating electron would emit a wave with a frequency of 1000 Hz, which is not in the list above. D. none of the above.

Electromagnetic Spectrum classification of electromagnetic waves according to frequency lowest frequency of light we can see appears red highest frequency of light we can see appears violet higher frequency of light is ultraviolet—more energetic and causes sunburns beyond are X-ray and gamma ray no sharp boundary between regions

Electromagnetic Spectrum CHECK YOUR NEIGHBOR The electromagnetic spectrum spans waves ranging from lowest to highest frequencies. The smallest portion of the electromagnetic spectrum is that of A. radio waves. microwaves. visible light. gamma rays. C. visible light.

Electromagnetic Spectrum CHECK YOUR ANSWER The electromagnetic spectrum spans waves ranging from lowest to highest frequencies. The smallest portion of the electromagnetic spectrum is that of A. radio waves. microwaves. visible light. gamma rays. C. visible light.

Which of these is fundamentally different from the others? Electromagnetic Spectrum CHECK YOUR NEIGHBOR Which of these is fundamentally different from the others? A. sound waves light waves radio waves X-rays A. sound waves.

Which of these is fundamentally different from the others? Electromagnetic Spectrum CHECK YOUR ANSWER Which of these is fundamentally different from the others? A. sound waves light waves radio waves X-rays Explanation: All are electromagnetic waves except sound, which is a mechanical wave. A. sound waves.

Transparent and Opaque Materials Light is transmitted similar to sound light incident on matter forces some electrons in matter to vibrate

Transparent and Opaque Materials How light penetrates transparent material such as glass

Transparent and Opaque Materials How light penetrates transparent material such as glass (continued) electrons or molecules in the glass are forced into vibration energy is momentarily absorbed and vibrates the electrons in the glass this vibrating electron either emits a photon or transfers the energy as heat Time delay between absorption and reemission of energy of vibrating electrons results in a lower average speed of light through a transparent material

Transparent and Opaque Materials Average speed of light through different materials vacuum—c (300,000,000 m/s) atmosphere—slightly less than c (but rounded off to c) water—0.75 c glass—0.67 c, depending on material diamond—0.41 c

Strictly speaking, the photons of light incident on glass are Transparent and Opaque Materials CHECK YOUR NEIGHBOR Strictly speaking, the photons of light incident on glass are A. also the ones that travel through and exit the other side. not the ones that travel through and exit the other side. absorbed and transformed to thermal energy. diffracted. B. not the ones that travel through and exit the other side.

Strictly speaking, the photons of light incident on glass are Transparent and Opaque Materials CHECK YOUR ANSWER Strictly speaking, the photons of light incident on glass are A. also the ones that travel through and exit the other side. not the ones that travel through and exit the other side. absorbed and transformed to thermal energy. diffracted. Explanation: Figure 13.6 illustrates this nicely. The light that exits the glass is not the same light that begins the process of absorption and re-emission. B. not the ones that travel through and exit the other side.

Transparent and Opaque Materials CHECK YOUR NEIGHBOR Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted A. is slightly less. is the same. is slightly higher. depends on the type of plastic. B. the same.

Transparent and Opaque Materials CHECK YOUR ANSWER Compared with the frequency of illuminating light on a sheet of transparent plastic, the frequency of light that is transmitted A. is slightly less. is the same. is slightly higher. depends on the type of plastic Explanation: Speed of light in plastic may vary, but the frequency transmitted doesn’t. B. higher.

The average speed of light is less in Transparent and Opaque Materials CHECK YOUR NEIGHBOR The average speed of light is less in A. air before entering glass. glass. air after emerging from glass. none of the above B. glass.

The average speed of light is less in Transparent and Opaque Materials CHECK YOUR ANSWER The average speed of light is less in A. air before entering glass. glass. air after emerging from glass. none of the above B. glass.

Transparent and Opaque Materials Reflection Light shining on metal forces free electrons in the metal into vibrations that emit their own light as reflection.

Transparent and Opaque Materials Light incident on: dry surfaces bounces directly to your eye wet surfaces bounces inside the transparent wet region, absorbing energy with each bounce, and reaches your eye darker than from a dry surface

Color Color physiological experience in the eye of the beholder

Color Color we see depends on frequency of light lowest frequency—perceived as red in between lowest and highest frequency—perceived as colors of the rainbow (red, orange, yellow, green, blue, indigo, violet) highest frequency—perceived as violet beyond violet, invisible ultraviolet (UV)

Color Selective reflection We see the color of a rose by the light it reflects.

Color Objects reflect light of some frequencies and absorb the rest. rose petals absorb most of the light and reflect red objects that absorb light and reflect none appear black object can reflect only those frequencies present in the illuminating light

Color Color of transparent object depends on color of light it transmits. colored glass is warmed due to the energy of absorbed light illuminating the glass

Color Mixed colored lights Distribution of solar frequencies is uneven most intense in yellow-green portion (where our eyes are most sensitive)

Color Radiation curve divides into three regions that match the color receptors in our eyes.

Color Additive primary colors red, green, and blue produce any color in the spectrum

Red, green, and blue light overlap to form Color CHECK YOUR NEIGHBOR Red, green, and blue light overlap to form A. red light. green light. blue light. white light. D. white light.

Red, green, and blue light overlap to form Color CHECK YOUR ANSWER Red, green, and blue light overlap to form A. red light. green light. blue light. white light. D. white light.

Color CHECK YOUR NEIGHBOR When the color yellow is seen on your TV screen, the phosphors being activated on the screen are A. mainly yellow. blue and red. green and yellow. red and green. D. red and green.

Color CHECK YOUR ANSWER When the color yellow is seen on your TV screen, the phosphors being activated on the screen are A. mainly yellow. blue and red. green and yellow. red and green. D. red and green.

A blue object will appear black when illuminated with Color CHECK YOUR NEIGHBOR A blue object will appear black when illuminated with A. blue light. cyan light. yellow light. magenta light. C. yellow light.

A blue object will appear black when illuminated with Color CHECK YOUR ANSWER A blue object will appear black when illuminated with A. blue light. cyan light. yellow light. magenta light. C. yellow.

Color Subtractive primary colors combination of two of the three additive primary colors red + blue = magenta red + green = yellow blue + green = cyan

Color The shadows of the golf ball are subtractive Magenta (opposite of green) Cyan (opposite of red) Yellow (opposite of blue)

Color Subtractive primaries are complementary to additive primaries. magenta + green = white = red + blue + green yellow + blue = white + red + green + blue example: color printing

A red rose will not appear red when illuminated only with Color CHECK YOUR NEIGHBOR A red rose will not appear red when illuminated only with A. red light. orange light. white light. cyan light. D. cyan light.

A red rose will not appear red when illuminated only with Color CHECK YOUR ANSWER A red rose will not appear red when illuminated only with A. red light. orange light. white light. cyan light. D. cyan light.

Color Only three colors of ink (plus black) are used to print color photographs—(a) magenta, (b) yellow, (c) cyan, which when combined produce the colors shown in (d). The addition of black (e) produces the finished result (f).

Why the Sky is Blue Why the sky is blue results of selective scattering of smaller particles than the wavelength of incident light and resonances at frequencies higher than scattered light the tinier the particle, the higher the frequency of light it will reemit

Why the Sky is Blue Why the sky is blue (continued) due to selective scattering blue scattered light predominates in our vision varies in different locations under various conditions clear dry day—much deeper blue sky clear, humid day—beautiful blue sky lots of dust particles and larger molecules than nitrogen and oxygen in the atmosphere—less blue sky with whitish appearance after heavy rainstorm (washing away of airborne particles)—deeper blue sky

A white sky is evidence that the atmosphere contains Why the Sky is Blue CHECK YOUR NEIGHBOR A white sky is evidence that the atmosphere contains A. predominantly small particles. predominantly large particles. a mixture of particle sizes. pollutants. C. a mixture of particle sizes.

A white sky is evidence that the atmosphere contains Why the Sky is Blue CHECK YOUR ANSWER A white sky is evidence that the atmosphere contains A. predominantly small particles. predominantly large particles. a mixture of particle sizes. pollutants. C. a mixture of particle sizes.

Why Sunsets are Red Light that is least scattered is light of low frequencies, which best travel through air. red orange yellow

A variety of sunset colors is evidence for a variety of Why Sunsets are Red CHECK YOUR NEIGHBOR A variety of sunset colors is evidence for a variety of A. elements in the Sun. apparent atmosphere thickness. atmospheric particles. primary colors. C. atmospheric particles.

A variety of sunset colors is evidence for a variety of Why Sunsets are Red CHECK YOUR ANSWER A variety of sunset colors is evidence for a variety of A. elements in the Sun. apparent atmosphere thickness. atmospheric particles. primary colors. C. atmospheric particles.

Why Sunsets are Red CHECK YOUR NEIGHBOR If molecules in the sky scattered orange light instead of blue light, sunsets would be A. orange. yellow. green. blue. D. blue.

Why Sunsets are Red CHECK YOUR ANSWER If molecules in the sky scattered orange light instead of blue light, sunsets would be A. orange. yellow. green. blue. Explanation: Of the colors listed, blue is closest to being the complementary color of orange. D. blue.

Why Clouds are White Clouds clusters of various sizes of water droplets

Why Clouds are White Size of clusters determines scattered cloud color tiny clusters produce bluish clouds slightly large clusters produce greenish clouds larger clusters produce reddish clouds overall result is white clouds slightly larger clusters produce a deep grey still larger clusters produce raindrops

Diffraction Diffraction bending of waves by means other than reflection and refraction property of all kinds of waves seen around edges of many shadows

Diffraction Waves diffract after passing through a narrow opening. Plane waves passing through openings of various sizes. The smaller the opening, the greater the bending of the waves at the edges.

Diffraction Amount of diffraction depends on wavelength of the wave compared to the size of the obstruction that casts the shadow.

Diffraction Features of diffraction limitations with focusing images in optical instruments object about the same size as wavelength of light, diffraction blurs object smaller than wavelength of light, no image limitations avoided with an electron beam having extremely short wavelengths

Diffraction Features of diffraction (continued) electron microscopes use electric and magnetic fields to focus and magnify images better radio reception with long radio waves for dolphins, use of shorter wavelengths see finer detail—ultrasound

Interference of Light Superposition of waves

Interference of Light Interference pattern caused by interference between a pair of waves

Interference of Light Interference pattern (continued) constructive interference produces bright region where waves reinforce each other (waves arriving in phase) destructive interference produces dark region where waves cancel each other (waves arriving a half wavelength out of phase)

Interference of Light Detail of interference pattern

The phenomenon of interference occurs for Interference of Light CHECK YOUR NEIGHBOR The phenomenon of interference occurs for A. sound waves. light waves. both A and B neither A nor B C. both of these.

The phenomenon of interference occurs for Interference of Light CHECK YOUR ANSWER The phenomenon of interference occurs for A. sound waves. light waves. both A and B neither A nor B Explanation: Interference is the property that characterizes waves in general. C. both of these.

Diffraction Diffraction grating composed of a large number of close, equally spaced slits for analyzing light source produced by spectrometers that disperse white light into colors

Interference Interference colors by reflection from thin films The thin film of gasoline is just the right thickness to result in the destructive interference of blue light.

Interference of Light CHECK YOUR NEIGHBOR If the thin film of gasoline was a bit thinner, the wavelength to be cancelled would be A. shorter than that of blue. longer than that of blue. white. none of the above A. shorter than that of blue.

Interference of Light CHECK YOUR ANSWER If the thin film of gasoline was a bit thinner, the wavelength to be cancelled would be A. shorter than that of blue. longer than that of blue. white. none of the above A. shorter than that of blue.

Interference of Light CHECK YOUR NEIGHBOR If violet light were cancelled by the double reflection of sunlight from gasoline on a wet surface, the resulting color would likely be A. red. orange. green. violet. B. orange.

Interference of Light CHECK YOUR ANSWER If violet light were cancelled by the double reflection of sunlight from gasoline on a wet surface, the resulting color would likely be A. red. orange. green. violet. Explanation: Orange is the complementary color of violet. B. orange.

Interference of Light CHECK YOUR NEIGHBOR If you see the color blue reflected in the interference from gasoline on water, and you lower your head so a greater angle from the normal results, you’ll likely see a color having a wavelength A. shorter than that of blue. longer than that of blue. with a white appearance. none of the above A. shorter than that of blue.

Interference of Light CHECK YOUR ANSWER If you see the color blue reflected in the interference from gasoline on water, and you lower your head so a greater angle from the normal results, you’ll likely see a color having a wavelength A. shorter than that of blue. longer than that of blue. with a white appearance. none of the above Explanation: The path through the gasoline would be longer, and a longer wavelength would be cancelled. The result of a long wave being cancelled is a shorter wave. B. Yes for 1 and 2.

Interference of Light Interference colors Note the colors in the bubble are subtractive primaries—magentas, yellows, and cyans.

What can the human eye not see? Color CHECK YOUR NEIGHBOR What can the human eye not see? A. infrared radiation ultraviolet radiation both A and B neither A nor B C. Both of the above.

What can the human eye not see? Color CHECK YOUR ANSWER What can the human eye not see? A. infrared radiation ultraviolet radiation both A and B neither A nor B C. Both of the above.