EA Notes (Scen 101), Tillery Chapter 7. Light

Transcription

1 EA Notes (Scen 101), Tillery Chapter 7 Light Introduction Light is hard to study because you can't see it, you only see it's effects. Newton tried to explain the energy in a light beam as the KE of a particle stream, and failed. About 1800, some experiments proved that light acts like waves. About 1900, other experiments proved that light acts like particles. Present idea: sometimes like waves, sometimes like particles. Waves when Interference (as with sound waves) is important. Particles when Mass (momentum) is important. "Quantum Theory" lets us use both descriptions. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

2 Sources of Light Luminous Source: actually produces light. Light bulbs in ceiling fixtures and projector are luminous. Everything else in room merely reflects that light. Incandescent Source: produces light by high temperature. Fluorescent tubes are not incandescent. Standard light bulbs are. So is the SUN. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

3 Electromagnetic (Wave) Spectrum: ( Introduced end of last chapter. ) Propagating Perpendicular E & M Fields shown in Fig.7.2. There is a very large frequency range for this EM Radiation. Propagation Speed: ENTIRE range has SAME speed. (Later in Chap). Production Method: DIFFERENT at low and high ends. 11 f 10 Hz production is by Antenna (last chapter). Otherwise by the Quantum method described later. Blackbody Radiation: The EM radiation from any object at ANY temperature. Experiment (ca. 1880): Intensity varies with wavelength as CORRECTLY shown in Fig.8.2, 5th Ed. (Shown as next slide.) As temp. increases: peak gets higher and moves to shorter. Fig.7.4, 7th Ed has an unacceptable X-Axis: 0 at far right, at origin. Sun's Radiation: The Sun's center is actually at 5700 C. Surface is 5200 C. The Sun's spectrum in Fig.7.5 has the correct frequencies for limits, but the arrow points in the wrong direction. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

4 Note that BOTH Wavelength & Intensity go properly to ZERO at origin. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

5 Properties of Light Light Ray: { BOARD } A line drawn to represent a thin beam of light. Sources emit rays in all directions. We use an ARROW to show their direction. They re STRAIGHT LINES, except at material boundary. Light Interacts with Matter When light meets a material boundary, its energy is split into 3 parts: Reflected Transmitted through material, where some usually is -- Absorbed How much energy into each depends on the material, the surface roughness, and the Ray Angle. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

6 Diffuse Reflection From a Rough surface. (Scattered Light) Rays leave object in MANY random directions. Most light entering our eye is from diffuse reflections. Regular Reflection From a Smooth surface. Rays leave object in ONE predictable direction. DEMO Draw: Incident Ray: The ray coming INTO the surface. Normal: Line drawn perpendicular to surface where incident ray hits. Angle of Incidence: ( i ) Between incident ray and normal. Reflected Ray: The ray coming FROM the surface. Angle of Reflection: ( r ) Between reflected ray and normal. Law of Reflection: i r EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

7 Flat Mirror Images (Fig.7.12) Our eye tells us the direction from which light entered it. Our brain thinks objects are behind mirrors. They aren't, ONLY their Image is. DEMO Draw a Plane mirror image location. Pick a point on the Object a distance P from mirror. Pick Two Rays from the point that are incident onto mirror. The reflected rays extended backward intersect at a distance Q behind mirror. Brain believes the object is at point Q. Accurate drawing shows: Q = P and Line QP is perpendicular to the mirror. Construction can be repeated for other points. Image point is ALWAYS on the NORMAL and Q = P. This is called a Virtual Image: because NO light actually comes from the image point. It just appears to. The other possibility is a Real Image: in which light actually comes from image point (like a projector screen image). EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

8 Refraction DEMO Refraction: Bending of a ray passing through a material boundary. (Recall Sound Waves) Refraction depends on speed of light difference between materials. Draw: Incident Ray: at Angle ( i ) to the normal: Refracted Ray: Bent ray on OTHER SIDE of the surface. Angle of Refraction: ( r ) Between refracted ray and normal. Speed of Light: 8 Speed in vacuum (and air) = c m/s. SLOWER in everything else. Just as with sound waves: Ray refracted toward normal going to slower speed. Away from normal going to faster speed. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

9 Index of Refraction: Dimensionless ratio easier to use in tables and formulas than the speed of light for each material. c Index of Refraction: n [no units] v v is always less than c, thus n > 1.00 (Tab.7.1, p.187) NOTE: Rounded value for air is Rule for ray bending at boundary in terms of n: Ray is refracted toward normal going to higher index. Away from normal going to lower index. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

10 DEMO Total Internal Reflection: (Abbreviated "TIR") For light moving from higher to lower index material. Rule: Ray bent away from normal going to lower index. Draw: Critical (Incident) Angle: Refracted ray at 90. For Incident angles greater than Critical, % reflection. Today's BIG use for TIR: Keeps light energy from leaking out the walls of FIBER OPTICS. {DRAW Fiber Construction }(see p.214) Fiber Optics uses: Medicine: First developed for Endoscopes to see inside body. Communication: (much use now, tremendous future growth): Developed for long distance (digital) telephone. Potential for any DIGITAL high capacity communication. Earlier TIR uses: "Sparkle" in faceted diamond gemstones. 90 bends with no light loss using 45 glass prism. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

11 Visible Light, Color and Dispersion Wave Equation (reminder): Visible Light: c f Electromagnetic waves of a particular frequency range that can be detected by the human eye. Physicists usually define light with: Wavelength, range: Color: to 8 10 m. The colors we observe fall into specific smaller ranges. ( Tab.7.2, p.202 for reference. ) Most Important: Blue & Violet are shorter than Red & Orange. Spectrum: Light spread out according to wavelength. Done with triangular "prisms" of glass using dispersion (below). c f EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

12 Visible Light, Color and Dispersion (Continued) Dispersion: For most materials (but not for air or vacuum), the index of refraction varies slightly with wavelength. This means refracted angle varies with wavelength. A CLOSER LOOK: Optics p.204 top 206: A lot of interesting information in this extra section. Evidence for Waves The observations discussed so far COULD be explained by either particles or waves. The ones in this section REQUIRE light to be a wave. Early Reasoning on Nature of Light ( about 1670 ) Newton advocated a Particle Theory (for reasons discussed next slide.) Huygens proposed Longitudinal Waves, (as part of a more complete theory that used WAVEFRONTS to explain how lenses focus.) EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

13 Diffraction Important info from Ed.5 that was dropped. DEMO Diffraction: The spreading of a wave as it passes through a hole (or the edge of an opaque object). Draw: Consider a Particle beam passing the Edge of a Wall: Particles would go straight past it. Sharp shadow. Light appears to make sharp shadows, like a particle beam. This is why Newton tried to use particles in his explanation of light. Consider a Water or sound wave passing the Edge of a Wall: Energy bends around the edge. Shadow not sharp. Which is why you can hear around corners. Waves always diffract, but detecting it is sometimes difficult. Much Spreading: d Easy Observation Little Spreading: d Hard Observation for LIGHT 7 ~ 6 10 m and normal objects Hard. for SOUND ~ 03. m or WATER with normal objects Easy. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

14 Double Slit Interference Experiment (Young, 1801) is proof that light is not particles. Start with 2 narrow slits illuminated by the same monochromatic (one wavelength) source. If light were particles, there would be only two bright lines, one above, one below the midpoint of a screen behind the slits. Young saw bright line at midpoint and many lines above and below. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

15 Double Slit Interference DEMO Here's how waves explain the experiment: Light diffracted through slits spreads out and overlaps on screen. Consider the two rays reaching the mid-point of the screen. The distance from the two slits is EQUAL, Thus there are the same number of wavelengths on each ray, Thus the two rays are always in phase. This is Constructive interference. (A BRIGHT SPOT). Now move up from the mid-point. When distance from slits differs by HALF wavelength, two rays arrive completely out of phase This is maximum Destructive interference. (A DIM SPOT). Continue moving up: When distance differs by a WHOLE wavelength there's Constructive interference again. (ANOTHER BRIGHT SPOT). Move further up, dark and bright areas continue to alternate. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

16 Polarization DEMO Def: The selection of one vibration direction from many. Only TRANSVERSE WAVES have more than one vibration direction. The effect is seen with very few materials, the newest being "Polaroid" polarizing film, developed by that company s founder for Military Pilots. Polarizing sunglasses have this film in them. Draw or Show: Place one piece of this material so light passes through it. Rotate the piece. Intensity of transmitted light is constant. Repeat rotation with a second piece of material. Intensity of transmitted light still constant. Place both pieces so light passes through them. Rotate either piece. Intensity of light through second piece varies from bright to zero and back to bright with 180 rotation. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

17 Polarization (Continued) TRANSVERSE Wave Explanation: (Fig.7.21, p.209) Blackbody and Fluorescent light is produced unpolarized, with its Electric (and Magnetic) Field vibrating in Random directions in the plane perpendicular to its propagation direction. First "Polarizer" transmits only one vibration direction. Second "Polarizer" blocks vibrations not parallel to its polarizing direction. Polarization by Reflection: Light reflected from a surface at an angle greater than 0 is partly polarized with the electric field vibration parallel to surface. (Fully polarized at one specific angle that depends on index of refraction.) Polarizing sunglasses have their polarizing direction vertical, thus partly block reflected light. Fig.7.22, p.209: This drawing tries to show that reflected light is parallel to the blue "lake," which is horizontal. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

18 Scattering of Sunlight Scattering: Absorption and re-radiation of light by particles that are smaller than the wavelength of light: All atmospheric gas molecules. Very small dust particles. Particle Volume (Rayleigh Scattering: Scattered Intensity 4 Scattering gets stronger with shorter wavelength (blue - violet). DEMO Draw: Blue sunlight is scattered more than red. Blue sky color is all scattered light. If there's no atmosphere, the sky around Sun is black. 2 ) EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

19 Maxwell's Wave Theory (1865) (See end of Chap 6 Notes) Proposed that radiation is a transverse wave of periodic electric and magnetic fields. Using this theory he: Described how EM radiation could be emitted from vibrating electrons and then travel through empty space. (End of Chap. 6: Vibrating electrons in an antenna create a varying circular magnetic field, which in turn creates a varying electric field.) Calculated the correct speed of light from two basic physical constants. Provided theoretical basis for Rayleigh's sunlight scattering. But it failed to explain some important experimental observations on Emission & Absorption of light discussed in the next Section. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

20 Evidence for Particles Blackbody Radiation Re-show the Blackbody Radiation shown in Fig.8.2, 5th Ed. Maxwell's Wave Theory didn't explain Intensity going to zero at zero wavelength. (It said it should continue to increase to infinity.) EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

21 Blackbody Radiation & Planck's Quantum Proposal Quantization of Energy (1900) Max Planck found a fit to the Experimental Data with a RADICAL idea: Assume a radiating blackbody has atomic vibrators whose energies can have only whole multiples of discrete amounts: nlhc Planck's Discrete Energy Levels: Eq nlhf ; n L 1, 2, 3, 4, where f is the frequency, h J s, (fit Blackbody Curves) = wavelength, and c = speed of light. Assume atom radiates energy when its level decreases one step: Planck's Radiated Quantum: 34 hc Eq hf Assume the Intensity at each frequency depends on the number of atoms ( n ) able to emit that energy. atoms Since emitted energy can't be infinite, n must go to zero at zero wavelength. atoms EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

22 Photoelectric Effect Fig.7.23, p.211: When light hits a clean metal surface, electrons are sometimes ejected. Three experimental observations differed from Maxwell's Wave Theory predictions: Below a threshold light frequency there s no electron ejection. The KE of an ejected electron depends only on the light frequency, not intensity as predicted by wave theory. The Current (Num. of electrons/sec) depends only on the light intensity, not frequency as predicted by wave theory. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

23 Einstein's Photoelectric Explanation (Photons) Einstein (1905, led to Nobel Prize) extended Planck's emitted quantum to describe propagating light. He said: Light exists only as bundles or "particles" of energy called Photons whose energy equals Planck's quantum jump E q : hc Einstein's Photon: Ep hf Energy hitting metal surface depends on frequency. From chemical Ionization: Each material has a minimum energy (Work Function) for losing an electron. Must be a threshold frequency for Ejection, (Maxwell Missed) Ejected electron's KE = Photon Energy Work Function. (Maxwell predicted Intensity) Assume (along with Planck): the intensity at each frequency depends on the number of photons at that frequency. Rate of electron ejection (current) will depend on intensity. (Maxwell predicted Frequency) EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM

24 The Present Theory Light is an Extremely small-sized phenomenon compared to the world that Galileo and Newton knew, and this is what makes it different. We are forced to accept that: Sometimes light acts like waves with a frequency and sometimes like particles with a KE. We switch between the concepts as needed, and Quantum Theory Ep hf provides the bridge. Chapter 8 of the Current Edition of Tillery (the first chapter of the Chemistry section) explores the Extremely small-sized phenomena of Electrons and Atoms, again using Quantum Theory. EA Lec Notes (Scen 101) Til 6 Ed-Chap Printed 11/8/2008 9:04 PM