Announcements. One more bit of vocabulary about waves

Transcription

1 Announcements Homework 5 now available; due on Sunday, Nov. 16 before midnight No class on Tuesday, Nov. 11 (Veteran s Day) Midterm 2 on Wednesday, Nov. 12 Study materials now available on class web page; practice exam up on OWL this afternoon One more bit of vocabulary about waves Amplitude is the extent of the excursion away from the midpoint between the peak and trough of a wave Sea or sound waves can have various amplitudes Interestingly, amplitude isn t really a meaningful characteristic of a light wave 1

2 Sound vs. Light Density and Pressure Density = number of particles per volume, e.g., number of air molecules/cubic cm Pressure = force per area pushing on an object λ 2

3 Sound vs. Light Both are waves, and therefore speed = frequency x wavelength For sound: higher frequency (shorter wavelength) implies higher pitch. This is why flutes, piccolos, and violins are small while a bass or a tuba is big. For sound, the higher the pressure, the louder the sound. For light: higher frequency (short wavelength) implies bluer color; lower frequency (long wavelength) implies redder color. The bluer the light, the more energy it carries: energy = h x frequency (h is Planck s constant ) Note that the brightness of a light source only depends on the number of photons. More photons means more energy, which means brighter light. This is different from sound. You can t blow harder to make brighter light! 3

4 The electroscope, a device for detecting charged particles Opposite charges attract; like charges repel! The like-charges push each other away Light waves How does light propagate through empty space? Light is like electrons on a rope. Electrons have a negative charge and therefore exert an electromagnetic force; the closer the electron is, the Charges close together causes strong repulsive force e- e- Charges farther apart leads to weaker repulsive force stronger the force. We call the force field from an electric charge the electric field. A change of the electric field creates a magnetic field. A change of the magnetic field, in turn, creates a new electric field. Light carries itself along by its own bootstraps! Light is also know as an electromagnetic wave. It is a series of changing electric and magnetic fields that propagate through space. 4

5 Electromagnetic Radiation Something must vibrate to transmit energy in a wave. In the case of light, the wave is caused by vibrating electric and magnetic fields. Radiation is something that carries energy through space. Energy carried by particles of matter is called particle radiation. Energy carried by light is called electromagnetic radiation. Electromagnetic radiation comes in many flavors! Visible light: Frequency Energy Wavelength 5

6 The electromagnetic spectrum: the many flavors of light 1. The analog of the full electromagnetic spectrum, but in sound 2. The sound analog of only visible light. 3. The sound analog of only infrared light. 4. The sound analog of only radio light. 5. The sound analog of only X-ray light. Suppose that we compare light to sound. If we could hear light, this is what it would sound like: Sun seen in visible and ultraviolet Optical Ultraviolet 6

7 Sun seen in X-rays X-ray The Andromeda Galaxy in different parts of the electromagnetic spectrum. Visible light (i.e., that our eyes Ultraviolet light can see) 7

8 Four Ways in Which Light can Interact with Matter 1. emission matter releases energy as light 2. absorption matter takes energy from light 3. transmission matter allows light to pass through it 4. reflection/scattering matter repels light in another direction Specular reflection vs. diffuse reflection (aka scattering) In specular reflection, the angle of incidence = angle of reflection In diffuse reflection, photons are reflected in semi-random directions 8

9 PRS Question: A movie theater works on the principle of 1. Specular reflection 2. Diffuse reflection Reflection vs. Scattering Reflection: angle of incidence = angle of reflection Scattering: angle of scattering is random 9

10 reflection will redirect the Reflection vs. Specular light in a specific direction only. Thus, Scattering while a person in one seat might see Reflection: angle of incidence =this light, someone somewhere else in the theater will not see it. angle of reflection Scattering: angle of scattering is random On the other hand, diffuse reflection (scattering) will redirect the photons in all directions, and consequently, the light will be seen anywhere in the room. What about refraction? Or diffraction? Diffraction is either a Refraction is a special special case of reflection case of transmission (e.g., grooves on a mirror) or transmission (grooves on glass/plastic) 10

11 How to make an electromagnetic wave? If you can move charged particles (electricity) back and forth in a wire, this should generate an electromagnetic wave. This is the classical model of light, i.e., in the late 1800s e - How to detect an electromagnetic wave? Likewise, if an EM wave impinges on a metal rod, the electric field would be expected to move the charges back and forth, and this could be detected as electricity. 11

12 Line emission vs. thermal emission There are two ways to produce light emission: Spectral Line Emission Thermal Radiation Every dense object emits a continuum spectrum, and the properties of the object s continuum emission depend only on its temperature. Hence, this continuum emission is called thermal radiation (also known as blackbody radiation). Properties of an opaque object, a blackbody Called a blackbody because it can absorb any color (wavelength) of light Typically a higher density object, often (but not necessarily) a solid As a blackbody is heated up, it is observed to emit light, which is called thermal radiation The color of thermal radiation changes with temperature 12

13 Light has difficulty escaping from highdensity objects that are opaque - it is emitted and reabsorbed many times In matter in a box, huge numbers of collisions randomize the kinetic energy of the particles. This gives the particles a characteristic average kinetic energy called the temperature. The same thing happens with light in an opaque object. The huge number of interactions between the light and matter randomizes the radiative energy of the light. The light that eventually emerges is a continuum spectrum that depends only on the temperature of the object. Why is a dense, opaque object so different from a thin, transparent gas? A thin gas (like the discharge tubes we studied) only emits the specific colors of its constituents. However, as matter becomes more complicated, so too do its allowed energy levels become more complicated. For example, consider simple molecules: Molecules can vibrate or rotate Oddly, the rotational and vibrational energies are also quantized (like the energy levels of the electron in an atom), i.e., only certain energies are allowed. Consequently, the spectra of molecules are much Notice: more lines than an atom richer and more complex 13

14 Why is a dense, opaque object so different from a thin, transparent gas? Similarly, dense objects like liquids and solids are even more complex. The electrons in a dense gas or solid are disturbed by neighboring atoms, which has the effect of smearing out the energy levels that the electron is allowed to take on. At increasingly higher temperatures, electrons have access to higher energy levels (collisions occur at higher velocities and have more energy) and thus can produce bluer light. Also, at higher temperatures collisions are also more frequent, which leads to more light being emitted than at cooler temperatures Notice: more lines than an atom The spectra of molecules are much richer and more complex Rules for Thermal Emission by Dense and Opaque Objects 1. Hotter objects emit more total radiation per unit surface area. Stephan-Boltzmann Law Energy per second (power) = σt 4 2. Hotter objects emit bluer photons (with a higher average energy.) Wien Law λ max = 2.9 x 10 6 / T(K) [nm] 14

15 Thermal (aka Blackbody) Radiation 15