Physics 1230: Light and Color

Transcription

1 Physics 1230: Light and Color Instructor: Joseph Maclennan TOPIC 3 - Resonance and the Generation of Light How do we generate light? How do we detect light? Concept of resonance

2 In vacuum speed = c = distance time = λ T = λf

3 Static magnetic fields Magnets generate magnetic fields that we can draw as field lines Magnetic field lines form continuous loops Bar magnet The Earth

4 Static electric fields Positive and negative charges generate electric fields that we can draw as field lines Electric field lines from charges behave like stretched strings Electric fields generate static sparks: first radio transmissions Positive charge Negative charge

5 How do we generate a wave on a rope? A) Drop one end B) Raise and lower one end quickly C) Drop both ends D) Twang it in the center

6 How do we generate EM waves? How do we make waves in the electromagnetic field? Let s wiggle the charges... Light waves are disturbances in the electromagnetic field, a non-material physical entity whose equilibrium state is vacuum Heinrich Hertz (1888)

7 Electromagnetic Wave Propagation Electromagnetic waves can be generated by a variety of methods, such as a discharging spark or by an oscillating electrons in an atom or molecular. As the current oscillates up and down in the spark gap, at a characteristic circuit frequency, a magnetic field is created that oscillates in a horizontal plane. The changing magnetic field, in turn, induces an electric field so that a series of electrical and magnetic oscillations combine to produce a formation that propagates as an electromagnetic wave.

8 Sources of electromagnetic waves So electrons moving up and down will emit electromagnetic waves because the electrons make an electric field, and the lines are wiggled when the electrons move What makes electrons move? 1. Heat 2. An oscillating electric field due to current changes (e.g. radio, TV, microwave oven) 3. Electrons in an excited atom (e.g. neon sign, or a fluorescent light) 4. Chemical excitation (e.g. firefly, phosphorescence)

9 Concept Quiz: How does your cell phone transmit your voice? A) Generating sound waves B) Generating heat C) Generating electromagnetic waves (high frequency radio) D) Generating neural activity

10 How does your cell phone generate electromagnetic waves? A) Heat B) Voltage in a circuit C) Excited atoms D) Neural activity/heat

11 Generating electromagnetic waves SOURCE MECHANISM Radio stations Microwaves Human body Cell phones Neon light Laser Warm stove Voltage in a circuit Voltage in a circuit Neural activity/heat Voltage in a circuit Excited atoms Excited atoms Heat

12 Sources of electromagnetic waves What are the frequencies of these sources? Radio stations Microwaves Human body Cell phones Neon light Laser Warm stove

13 Creating Light from Atoms Electrons can absorb energy from external sources, such as lasers, arcdischarge lamps, and tungsten-halogen bulbs, and be promoted to higher energy levels. Light energy is absorbed by an electron to elevate it into a higher energy level and the energy can subsequently be released, in the form of a lower energy photon, when the electron falls back to the original ground state. The precise difference between the energy levels determines the resonance frequency or color of light that is emitted or absorbed.

14 Detecting Electromagnetic Waves Resonance The emission and reception of electromagnetic waves is not equally efficient for all waves, but depends on the nature of the emitter and receptor. One phenomenon, called resonance, is responsible for this selectivity. Resonance effects cause molecules to vibrate - for example in your eye. Light of certain frequency (wavelength) drives the molecules to vibrate, so that eventually a signal is transmitted to your brain via the optic nerve.

15 Detection of electromagnetic waves - Resonance and Selectivity Why do we see only certain colors? How does tuning of a radio signal work?

16 Tacoma Narrows Bridge (State Route 16 spanning the Tacoma Narrows) In 1940, the Tacoma Narrows Bridge failed from windinduced torsional oscillations. Research of design flaws in the bridge led to the use of aerodynamic testing as a standard procedure in suspension span structural analysis. Can you think of other examples of resonance?

17 Resonance Every material (such as glass, steel, concrete) has a natural frequency at which it vibrates, called a resonant frequency. If you put energy into the substance at its resonant frequency, you will force it to vibrate or resonate (resonance is a forced vibration). In the case of the wine glass, your finger slides and sticks along the surface of the glass as you rub the rim (a wet fingertip has no oil and makes a better contact with the glass). The rubbing imparts energy to the glass molecules and causes them to resonate. The motion of your hand sets up a wave of vibration traveling through the glass. The vibrating glass causes air molecules to vibrate at the same frequency. The vibrating air molecules are the sound wave that you hear (the frequency or pitch of the sound wave is the same as the resonant frequency of the glass). So, how does the water change the pitch of the singing wine glass? As the resonant wave moves around the glass, it drags the water molecules with it, creating a wave of water that you can see near the edge of the glass. The dragging water molecules effectively increase the mass (both the water and the glass molecules) and reduce the energy of the wave traveling through the glass. When the energy is reduced, so is the frequency of the wave in the glass, which is reflected in the pitch of the sound wave that you hear. If you impart enough energy to the glass at its resonant frequency, you can cause the glass to shatter. However, this takes more energy than you can provide by rubbing the rim. Some singers can sing a note equal to the resonant frequency of a wine glass and cause it to shatter

18 RESONANCE IN SOUND If you look at a guitar string under a strobe light (or even a fluorescent light) you can see it that makes a standing wave. Another experiment you can do is to stand in the shower (they reflect sound well) and start singing while changing the pitch slowly. At certain pitches the sound will suddenly amplify, because the sound waves fit an even number of times between the walls.

19 Examples of Resonance Guitar string - can be tuned by changing the tension Electrical circuits in radios Nerve cells in eye - sensitive to red, green Chemical dyes and blue

20 Using Resonance to Shatter a Wineglass If a singer can match the natural frequency of the wine glass she can put more energy into the wine glass than it can handle. As the energy builds, the glass begins to deform beyond what its bonds can sustain. The trick is to sing with the right frequency and being able to sustain that note. It is not about singing loudly or horribly. It is also not possible for one note to shatter all glasses as each glass would have its own natural frequency.

21 Another Example of Resonance Using Resonance to shatter a Kidney stone. By tuning ultrasound waves to the natural frequency of a kidney stone, we can rely on resonance to pulverize the stone

22 Concept Test on Resonance You are swinging back and forth on a swing at the natural frequency. If a friend (or your cat) joins you on the swing, the new natural frequency will be: A. greater B. the same C. smaller D. zero - you won t be able to swing any more

23 Concept Test on Resonance If you rub the rim of a wineglass you can make it sing. This is because: A. vibrations from your finger excite a resonant response in the glass B. you need to sing the same note to get the resonance C. the glass is a mechanical system in resonance D. the table top transmits a musical tone to the glass E. otherwise it would shatter

24 RESONANT ABSORPTION OF LIGHT - Photosynthesis Green plants absorb water and carbon dioxide from the environment, and utilizing energy from the sun, turn these simple substances into glucose and oxygen. With glucose as a basic building block, plants synthesize a number of complex carbon-based biochemicals used to grow and sustain life. This process is termed photosynthesis, and is the cornerstone of life on Earth. In the applet, water molecules are converted to molecular hydrogen and oxygen as a result of photon absorption in the granum. Subsequently, the hydrogen molecules react with carbon dioxide in the stroma to produce oxygen and carbohydrates.

25 Color of Plants- Photosynthesis Examples of photosynthetic organisms: leaves from higher plants flanked by colonies of photosynthetic purple bacteria (left) and cyanobacteria (right). Chlorophylls absorb blue and red light and carotenoids absorb blue-green light, but green and yellow light are not effectively absorbed by photosynthetic pigments in plants; therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green.

26 COLOR OF PLANTS - Photosynthesis Chlorophylls absorb blue and red light and carotenoids absorb bluegreen light, but green and yellow light are not effectively absorbed by photosynthetic pigments in plants; therefore, light of these colors is either reflected by leaves or passes through the leaves. This is why plants are green. Absorption spectrum of isolated chlorophyll and carotenoid species. The color associated with the various wavelengths is indicated above the graph.

27 What we see

28 What is a resonance? Many objects oscillate or vibrate at special frequencies called resonant frequencies or resonances When these objects are hit or "shaken" by an external agent at a frequency = to their resonant frequency they will oscillate at their resonant frequency. Hand moving back and forth at same frequency as pendulum s resonant frequency (or hit) Tacoma narrows bridge in the wind Car on a dirt road with regular bumps (washboard effect) The oscillations of the object are largest when the "shaking" occurs at the object s resonant frequency. We then say that a resonance has occurred e.g. girl on swing being pushed by her mother (mother s push frequency = swing frequency) Energy is transferred from an external agent to the object during resonance. Wineglass broken by an opera singer s voice due to resonance between voice sound frequency and natural frequency of wineglass

29 Effect of resonance produced by military helicopter blade going around at frequency resonant with the helicopter body

30 What do resonances have to do with light? When light is absorbed by atoms we can think of this as a resonance The light frequency may match a certain frequency of resonant vibration in the atom. When this happens, the energy of the light is transferred to the atom and the light disappears. For example, we see light rays of 470 nm coming into our eyes because this light excites a resonance in certain atoms inside our eyes When light is emitted by atoms we can think of this as a resonance For example when an electron hits an atom the atom can gain energy in the form of resonances. This energy in the atom can then be released by another resonant interaction in which light is emitted and the atom loses energy. Each color of light emitted corresponds to a particular atomic resonance.

31 Resonance and the Creation of Light Absorption of light Emission of light