The Nature of Light Visible Light: Electromagnetic waves are created by vibrating electric charges having frequencies that fall within the range of sight “Visible light” and all electromagnetic waves are transverse waves In physics, all electromagnetic waves are considered “light waves”. We use the term “Visible light” to distinguish the types of waves we can observe with our eyes.

The Electromagnetic Spectrum “Visible light” is really just one small portion of the electromagnetic spectrum. All waves in the electromagnetic spectrum do not need a medium. Visible light is only different from x-rays, radio waves, etc. in its frequency.

The Nature of Light The Electromagnetic Spectrum

Answer Cards Do waves in the electromagnetic spectrum need a medium? A. Always B. Never C. Sometimes D. IDK What makes a radio wave different from a light wave? A. Velocity B. Frequency C. Wavelength D. More than one of the above

The Nature of Light Order of increasing frequency of visible light: Red- lowest frequency Orange Yellow Green Blue Violet- highest frequency

Indigo and blue have shorter frequencies, than red and orange.

Label one wave as red and the other as violet.

Color Color we see depends on frequency of light ranging from lowest (red) to highest (violet). In between are colors of the rainbow. “Hues” in seven colors: red, orange, yellow, green, blue, indigo, and violet. Grouped together, they add to appear white.

Color Most objects don’t emit light, but reflect light. Selective Reflection Most objects don’t emit light, but reflect light. A material may absorb some of the light and reflect the rest. The color we see is made of the light waves being reflected.

Side note: I hate indigo and here’s why…

Pigments A pigment is a chemical that absorbs some frequencies of light and reflects others. So a blue t- shirt is absorbing the colors: red, orange, yellow, green and violet- and is reflecting blue.

Answer Cards What colors of light are being reflected in the following: A red t-shirt: A. All except red B. Only red C. All colors D. No colors A white piece of paper: A black t- shirt: absorbed in

Color Mixed Color Lights (Integrated) Three types of cone receptors in our eyes perceive color—each is stimulated by only certain frequencies of light: Light of lower frequencies stimulates the cones sensitive to low frequencies (red) Light of middle frequencies stimulates the cones sensitive to mid-frequencies (green) Light of high frequencies stimulates the cones sensitive to high frequencies (blue) Stimulation of all three cones equally, we see white light

Color Additive primary colors (red, blue, green): red + blue = magenta red + green = yellow blue + green = cyan

Color Opposites of primary colors: opposite of green is magenta opposite of red is cyan opposite of blue is yellow The addition of any color to its opposite color results in white.

To which is the human eye blind? Color Answer Cards To which is the human eye blind? A. Infrared radiation. Ultraviolet radiation. Both A and B. Neither A nor B. C. Both of the above.

To which is the human eye blind? Color CHECK YOUR ANSWER To which is the human eye blind? A. Infrared radiation. Ultraviolet radiation. Both A and B. Neither A nor B. C. Both of the above.

The Nature of Light A situation to ponder… A photographer wishes to photograph a lightning bolt by setting his camera so that it is triggered by the sound of thunder. Put the correct answer here including the letter designation.

Is this a good idea or a poor idea? A situation to ponder… Is this a good idea or a poor idea? A. Good idea for nearby lightning strikes. Good idea for all strikes. Poor idea for nearby lightning strikes. Poor idea for all strikes. D. Poor idea for all strikes.

Is this a good idea or a poor idea? A situation to ponder… CHECK YOUR ANSWER Is this a good idea or a poor idea? A. Good idea for nearby lightning strikes. Good idea for all strikes. Poor idea for nearby lightning strikes. Poor idea for all strikes. Explanation: Light travels about a million times faster than sound. By the time the sound of thunder arrives, the lightning bolt is long gone. D. Poor idea for all strikes.

The Speed of Light The speed of light waves is : 299,792,458 m / s Or approximately 300,000 kilometers per second. That’s 186,400 miles per second!

Reflection Reflection: the returning of a wave to the medium through which it came when encountering a reflective surface Law of reflection: angle of incidence = angle of reflection

Reflection Diffuse Reflection When light is incident on a rough surface, it is reflected in many directions.

Absorption When a photon of EM wave hits a matter and some energy is taken up by electrons. May release energy in another form (heat for example)

Absorption lines Some frequencies of EM spectrum may be missing. Using this information is called spectroscopy.

Transparent and Opaque Materials Transparent materials: glass and water—light passes through in straight lines, with atoms undergoing a series of absorptions and reemissions

Transparent and Opaque Materials colored glass is opaque to much of white light. In blue glass what color can go through the glass? What must happen to the rest?

Even though glass is transparent to visible light, it is not transparent to ultraviolet and infrared light.

Refraction Refraction: the bending of a wave due to a change in the medium and/or speed of the wave

Refraction Examples of refraction: When light slows down in going from one medium to another, as when going from air to water, it bends toward the normal. When light speeds up in traveling from one medium to another, as when going from water to air, it bends away from the normal.

Refraction Sound waves refract when parts of the wave fronts travel at different speeds. Refraction occurs when sound waves are affected by uneven winds, or when air near the ground is warmer than air above.

Refraction Lenses are classified into two kinds: Converging lens incoming parallel light rays refract and converge to a focal point Diverging lens incoming parallel light rays refract in such a way that extended rays diverge to a focal point in front of the lens

Diffraction Diffraction: Any bending of light by means other than reflection and refraction Smaller openings produce greater diffraction (greater bending of the waves at edges) Amount of diffraction depends on the wavelength of the wave compared to the size of the obstruction that casts the shadow

Interference Interference is the combined effect of two or more overlapping waves.

Interference Two types of interference: Constructive interference crest of one wave overlaps crest of another wave  individual effects add, resulting in a wave of increased amplitude Destructive interference crest of one wave overlaps the trough of another  individual effects are reduced

Interference is a property of CHECK YOUR NEIGHBOR Interference is a property of A. sound. light. both of these. neither of these. C. both of these.

Interference is a property of CHECK YOUR ANSWER Interference is a property of A. sound. light. both of these. neither of these. Explanation: See Figure 8.47 to see illustrations of both light and sound interference. Interestingly, the presence of interference tells a physicist whether something is wavelike or not. All types of waves can interfere. C. both of these.

The Doppler Effect The Doppler Effect: a change in frequency as measured by an observer due to the motion of the source or listener Named after Austrian physicist and mathematician, Christian Johann Doppler

The Doppler Effect Example of Doppler Effect: The frequency of waves received by an observer increases as a sound source moves toward the observer. The wave frequency decreases as the source moves away.

When a fire engine approaches you, the The Doppler Effect CHECK YOUR NEIGHBOR When a fire engine approaches you, the A. speed of its sound increases. frequency of sound increases. wavelength of its sound increases. all increase. B. frequency of sound increases.

When a fire engine approaches you, the The Doppler Effect CHECK YOUR ANSWER When a fire engine approaches you, the A. speed of its sound increases. frequency of sound increases. wavelength of its sound increases. all increase. Comment: Be sure you distinguish between sound, speed, and sound frequency. B. frequency of sound increases.

The Doppler effect occurs for CHECK YOUR NEIGHBOR The Doppler effect occurs for A. sound. light. both A and B. neither A nor B. C. both of the above.

The Doppler effect occurs for CHECK YOUR ANSWER The Doppler effect occurs for A. sound. light. both A and B. neither A nor B. Explanation: As the text states, the Doppler effect occurs for sound (Figure 8.50) and for light (see the Integrated Science—Astronomy feature). Astronomers measure the spin rates of stars by the Doppler effect. C. both of the above.

The Wave–Particle Duality In ancient times, Plato, other Greek philosophers, and Isaac Newton thought that light was composed of tiny particles. 100 years after Newton, Thomas Young demonstrated the wave nature of light with interference experiments. 25 years later, the wave view was confirmed by Heinrich Hertz. Later in 1905, Albert Einstein challenged the wave theory and stated that light was confined in tiny particles of energy called photons. His particle model of light was verified by the photoelectric effect.

The Wave–Particle Duality Today, light is acknowledged to have both a wave nature and a particle nature— wave–particle duality: light reveals itself as a wave or particle depending on how it is being observed light behaves as a wave when traveling from a source to a place where it is detected and behaves as a stream of photons when it interacts with a detector

The Wave–Particle Duality The Photoelectric Effect When light shines on certain metal surfaces, electrons are ejected from those surfaces. Ultraviolet and violet light impart sufficient energy to knock electrons from those metal surfaces while lower-frequency light does not, even when very bright.

The Wave–Particle Duality CHECK YOUR NEIGHBOR Why is unexposed black and white photographic film not “exposed” when in red light, but is when in white light? A. The red light in a dark room is usually too dim. Red light has insufficient energy per photon to “expose” the film. Red light is low-temperature light. None of the above. B. Red light has insufficient energy per photon to “expose” the film.

The Wave–Particle Duality CHECK YOUR ANSWER Why is unexposed black and white photographic film not “exposed” when in red light, but is when in white light? A. The red light in a dark room is usually too dim. Red light has insufficient energy per photon to “expose” the film. Red light is low-temperature light. None of the above. B. Red light has insufficient energy per photon to “expose” the film.

The Wave–Particle Duality Findings: 1. ejection of electrons depended only on the frequency of light the higher the frequency of the light, the greater the kinetic energy of ejected electrons Explanation: Electrons in the metal are bombarded by “particles of light”—photons. The energy of each photon is proportional to its frequency: E  .

Light travels as a wave and is absorbed as a The Wave–Particle Duality CHECK YOUR NEIGHBOR Light travels as a wave and is absorbed as a A. wave. particle. both of the above. none of the above. B. particle.

Light travels as a wave and is absorbed as a The Wave–Particle Duality CHECK YOUR ANSWER Light travels as a wave and is absorbed as a A. wave. particle. both of the above. none of the above. Explanation: Light is wavelike as it travels but particle-like when it encounters a surface. B. particle.

EM Waves Gamma Rays X-rays : Soft X-Rays, Hard X-Rays UV rays: UVA, UVB, UVC Infrared rays: Near Infrared, Far Infrared Microwaves Radio Waves: TV, AM, FM