Light is a Particle Physics 12

What is Light? -most information about the Universe is obtained through analysis of light -a wave is rising and falling (oscillating) disturbance that transports energy from a source to a receiver 2

What is Light? -a light wave is an electromagnetic (EM) disturbance consisting of changing electric and magnetic fields -light waves transport energy from moving electric charges in stars (source) to electric charges in the retina of human eye (receiver) 3

-light waves are distinguished by their lengths -Wavelength (λ) – the distance from any point on a wave to the next identical point (from crest to crest or trough to trough) -measure these waves in billionth of a meter (x 10-9 m) nanometers, nm, or the angstrom unit, Å -visible light has wavelengths of 4000 Å (400 nm) to 7000 Å (700 nm) 4

-visible light of different wavelengths perceived as colors (R-O-Y-G-B-I-V) 5

6 Figure 5-4 Newton’s Experiment on the Nature of Light In a crucial experiment, Newton took sunlight that had passed through a prism and sent it through a second prism. Between the two prisms was a screen with a hole in it that allowed only one color of the spectrum to pass through. This same color emerged from the second prism. Newton’s experiment proved that prisms do not add color to light but merely bend different colors through different angles. It also proved that white light, such as sunlight, is actually a combination of all the colors that appear in its spectrum. 6

Most sensitive Least sensitive Relative sensitivity of the human eye to different colors and wavelengths of visible light. 7

Speed of Light -all EM waves move through empty space at the same speed, c = 300 000 000 m/s -no known object can be accelerated to move faster than “c” -one of the most important and precisely measured numbers in astronomy -a light-year (ly) is the distance light travels through empty space in one year -frequency (f) of a wave motion - the number of waves that pass by a fixed point in a given time, measured in cycles per second or Hertz (Hz) c = λ f 8

What is the wavelength of light with a frequency of 7.5 x 1016 Hz? Example: What is the wavelength of light with a frequency of 7.5 x 1016 Hz?

-light (EM) waves or radiation outside of visible range exist: radio, microwaves, infrared (IR), ultraviolet (UV), X rays, gamma rays -the range of all EM waves ordered according to decreasing wavelength (or increasing frequency) is called the EM spectrum Figure 5-7 The Electromagnetic Spectrum The full array of all types of electromagnetic radiation is called the electromagnetic spectrum. It extends from the longest-wavelength radio waves to the shortest-wavelength gamma rays. Visible light occupies only a tiny portion of the full electromagnetic spectrum. 10

-for example, blue light is more energetic than red light -the lower the wavelength, the higher the frequency  the higher the energy carried in the EM wave -for example, blue light is more energetic than red light -UV light is more energetic than infrared light  this is why UV causes sunburns and cancer Figure 5-7 The Electromagnetic Spectrum The full array of all types of electromagnetic radiation is called the electromagnetic spectrum. It extends from the longest-wavelength radio waves to the shortest-wavelength gamma rays. Visible light occupies only a tiny portion of the full electromagnetic spectrum. 11

12 Figure 5-8 Uses of Nonvisible Electromagnetic Radiation ( a) A mobile phone is actually a radio transmitter and receiver. The wavelengths used are in the range 16 to 36 cm. (b) A microwave oven produces radiation with a wavelength near 10 cm. The water in food absorbs this radiation, thus heating the food. (c) A remote control sends commands to a television using a beam of infrared light. (d) Ultraviolet radiation in moderation gives you a suntan, but in excess can cause sunburn or skin cancer. (e) X rays can penetrate through soft tissue but not through bone, which makes them useful for medical imaging. (f) Gamma rays destroy cancer cells by breaking their DNA molecules, making them unable to multiply. (Ian Britton, Royalty-Free/Corbis, Michael Porsche/Corbis, Bill Lush/Taxi/Getty, Neil McAllister/Alamy, Edward Kinsman/Photo Researchers, Inc., Will and Deni McIntyre/Science Photo Library) 12

Box 5-1 Temperatures and Temperature Scales 13

Radiation Laws -stars, like other hot bodies, radiate electromagnetic energy of all different Wavelengths -energy due to temperature is called thermal radiation  the temperature of a star determines which wavelength is brightest -stars radiate energy almost as a blackbody, or theoretical perfect radiator -the intensity (or amount of energy) of radiation emitted over a range of wavelengths depends only on the blackbody’s temperature  Wien’s law of radiation

Example: What is the peak wavelength of light that emanates from the surface of the Sun, which has a temperature of 5778 Kelvin? What color is the Sun?

The Sun’s thermal radiation spectrum All blackbody radiation spectrums have the same shape Hotter objects emit more energy at all wavelengths, and the peak shifts to shorter wavelengths.

F = σT4 Radiation Laws -where… -Stefan-Boltzmann Law: F = σT4 -where… F  energy flux (joules) per square meter of surface per second (or Watts per m2) σ  a constant 5.67 x 10-8 W/m2/K4 T  temperature in Kelvin

What is the flux at the surface of the Sun? Example: What is the flux at the surface of the Sun? Compare this value to the solar constant 1360 W/m2 at the upper atmosphere of Earth? Why are the two values different?

20 Figure 5-9 Heating a Bar of Iron This sequence of photographs shows how the appearance of a heated bar of iron changes with temperature. As the temperature increases, the bar glows more brightly because it radiates more energy. The color of the bar also changes because as the temperature goes up, the dominant wavelength of light emitted by the bar decreases. (©1984 Richard Megna/Fundamental Photographs) 20

21 Figure 5-11 Blackbody Curves Each of these curves shows the intensity of light at every wavelength that is emitted by a blackbody (an idealized case of a dense object) at a particular temperature. The rainbow-colored band shows the range of visible wavelengths. The vertical scale has been compressed so that all three curves can be seen; the peak intensity for the 12,000-K curve is actually about 1000 times greater than the peak intensity for the 3000-K curve. 21

22 Figure 5-12 The Sun as a Blackbody This graph shows that the intensity of sunlight over a wide range of wavelengths (solid curve) is a remarkably close match to the intensity of radiation coming from a blackbody at a temperature of 5800 K (dashed curve). The measurements of the Sun’s intensity were made above the Earth’s atmosphere (which absorbs and scatters certain wavelengths of sunlight). It’s not surprising that the range of visible wavelengths includes the peak of the Sun’s spectrum; the human eye evolved to take advantage of the most plentiful light available. 22

23 Figure 5-14 The Kirchhoff-Bunsen Experiment In the mid-1850s, Gustav Kirchhoff and Robert Bunsen discovered that when a chemical substance is heated and vaporized, the spectrum of the emitted light exhibits a series of bright spectral lines. They also found that each chemical element produces its own characteristic pattern of spectral lines. (In an actual laboratory experiment, lenses would be needed to focus the image of the slit onto the screen.) 23

24 Figure 5-15 Various Spectra These photographs show the spectra of different types of gases as measured in a laboratory on Earth. Each type of gas has a unique spectrum that is the same wherever in the universe the gas is found. Water vapor (H2O) is a compound whose molecules are made up of hydrogen and oxygen atoms; the hydrogen molecule (H2) is made up of two hydrogen atoms. (Ted Kinsman/Science Photo Library) 24

25 Figure 5-13 The Sun’s Spectrum Numerous dark spectral lines are seen in this image of the Sun’s spectrum. The spectrum is spread out so much that it had to be cut into segments to fit on this page. (N. A. Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF) 25

26 Figure 5-16 Continuous, Absorption Line, and Emission Line Spectra A hot, opaque body (like a blackbody) emits a continuous spectrum of light (spectrum a). If this light is passed through a cloud of a cooler gas, the cloud absorbs light of certain specific wavelengths, and the spectrum of light that passes directly through the cloud has dark absorption lines (spectrum b). The cloud does not retain all the light energy that it absorbs but radiates it outward in all directions. The spectrum of this reradiated light contains bright emission lines (spectrum c) with exactly the same wavelengths as the dark absorption lines in spectrum b. The specific wavelengths observed depend on the chemical composition of the cloud. 26

Figure 5-17 Iron in the Sun The upper part of this figure is a portion of the Sun’s spectrum at violet wavelengths, showing numerous dark absorption lines. The lower part of the figure is a corresponding portion of the emission line spectrum of vaporized iron. The iron lines coincide with some of the solar lines, which proves that there is some iron (albeit a relatively small amount) in the Sun’s atmosphere. (Carnegie Observatories) 27

Quantum Theory Max Planck was able to determine an empirical mathematical relationship between the intensity and frequency of blackbody data In order to develop a theory that described his relationship, Planck was required to use discrete mathematics

Quantum Theory This lead to the idea that there was a minimum amount of energy that could be exchanged This minimum amount of energy lead to the idea that energy was quantized meaning that it can only exist is specific “packets” E = nhf where n = 0, 1, 2, 3, …

Quantum Theory E = hf Einstein improved on Planck’s ideas Einstein concluded that to conserve energy a blackbody radiator must emit light with “packets” of energy or photons or… E = hf

Calculate the energy of light with a frequency of 7.5 x 1016 Hz. Example: Calculate the energy of light with a frequency of 7.5 x 1016 Hz. Estimate how many visible light photons a 40-W light bulb emits per second. Assume λ = 500 nm and the efficiency of the bulb is 10%.

Quantum Theory Using Boltzmann’s statistical models Planck was able to model the behaviour of blackbody radiation exactly This was published in 1900 and was the birth of modern physics However, these results were not immediately accepted (even by Planck) as they were contrary to previous work

Photoelectric Effect The photoelectric effect eventually provided support for the idea of quantization of energy The photoelectric effect occurs when photoelectrons are emitted from a metal when exposed to certain frequencies of light

Photoelectric Effect Experiments with the photoelectric effect led to two key conclusions: When the intensity of light increases, the number of electrons emitted increases The maximum kinetic energy of the electron ejected from the metal is determined only by the frequency of light and is not affected by intensity

Einstein and the Photoelectric Effect Einstein saw the link between Planck’s quantization of energy and the photoelectric effect He proposed that not only would light be emitted as quanta but must also be absorbed as quanta By considering these quanta (or photons) he was able to explain the photoelectric effect

Einstein and the Photoelectric Effect By using the concept of the photon (and its associated energy), Einstein proposed the following: hf = W + Ek(max) Despite the fact this equation worked, it was not widely accepted (even by Planck)

Millikan and the Photoelectric Effect Because the charge on the electron was not known when Einstein published his paper on the photoelectric effect, his result could not be proven Millikan, having determined the charge on the electron, improved on the photoelectric effect experimental design and was able to confirm Einstein’s assumptions

Millikan and the Photoelectric Effect Using his experimental design, Millikan was able to produce data that supported Einstein’s equation: Ek(max) = hf – W Ek – kinetic energy of photoelectron h – Planck’s constant f – frequency of EM radiation W – work function of metal

Example: Light with a wavelength of 571 nm strikes a cesium metal surface inside a vacuum tube. What is the maximum kinetic energy of the emitted photoelectrons? What is the threshold frequency, fo, for cesium?

The electron volt (eV) Since the energies involved in Quantum Physics are so small, instead of using joules to describe energy, the electron volt is used instead One electron volt is equal to 1.60x10-19J