ISNS 3371 - Phenomena of Nature The Doppler Effect - Wavelength Shift Due to Motion. Each circle represents the crests of sound waves going in all directions from the train whistle. The circles represent wave crests coming from the train at different times, say, 1/10 second apart. If the train is moving, each set of waves comes from a different location. Thus, the waves appear bunched up in the direction of motion and stretched out in the opposite direction.

Hearing the Doppler Effect Animation ISNS 3371 - Phenomena of Nature Hearing the Doppler Effect Animation

Doppler Shift vs Velocity Animation ISNS 3371 - Phenomena of Nature Doppler Shift vs Velocity Animation

ISNS 3371 - Phenomena of Nature Sonic Boom As the airplane moves, it pushes air molecules out of its way, continuously creating waves of compressed and uncompressed air. These air pressure waves move away from the airplane in all directions at the speed of sound. Next, break the sound barrier by increasing the airplane's speed to supersonic - the air pressure waves (called shock waves) cannot precede the plane, and so accumulate in a cone behind the plane. The shock waves will move out and back from the plane, towards the ground. There is a sudden change in pressure when the shock wave hits your eardrum. You hear this as a loud sonic boom.

ISNS 3371 - Phenomena of Nature There are actually two shocks - the first shock forms at the nose of the aircraft and the second near the tail - so you actually hear two sonic booms. They start out very close together (one body length apart, which means they are separated by much less than 0.1 sec), but, as they propagate the long distance to the Earth's surface, they spread apart, and the time between them gets as large as 1 sec. That's why you can actually hear two closely-spaced booms. The longer the aircraft's fuselage and the higher it is flying, the easier it is to distinguish the two shocks. To increase the intensity of a sonic boom, increase the size of the airplane. The larger the aircraft, the more air it displaces and the stronger the shock waves become.

F/A-18 Hornet photographed just as it broke the sound barrier. ISNS 3371 - Phenomena of Nature F/A-18 Hornet photographed just as it broke the sound barrier. A leading theory is that a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets.

ISNS 3371 - Phenomena of Nature Light

In space everyone can see you scream! (they just can’t hear you) ISNS 3371 - Phenomena of Nature Light What is light? - A vibration in an electromagnetic field through which energy is transported. Since it is an electromagnetic field, it does not require a medium - In space everyone can see you scream! (they just can’t hear you) The dual nature of light or wave-particle duality - light can be treated as a wave or as a particle: Light as a wave f  = c Light as a particle E = hf photon

ISNS 4371 Phenomena of Nature Dual Nature of Light Light may show properties of a wave or of a particle, called a photon. Newton (1680) explained light as a particle of energy. In reflection and refraction, light behaved as a particle. Young, (circa. 1800) showed that light interfered with itself. Therefore, it must be a wave. Reflection and refraction could be explained by light being a wave. Maxwell (1850) showed that light was a form of high frequency electromagnetic wave.

ISNS 4371 Phenomena of Nature Dual Nature of Light Einstein (1905) showed that in the photoelectric effect (light causing electrons to be emitted from a metal surface) light must act as a particle. Planck (1900) developed a model that explained light as a quantization of energy. Energy of a light wave is present in bundles of energy called photons; the energy is said to be quantized into the photons. Therefore, light must be regarded as having a dual nature; in some cases light acts as a wave; in others it acts like a particle.

The Photoelectric Effect ISNS 3371 - Phenomena of Nature The Photoelectric Effect First observed by Heinrich Hertz in 1887 - light shining on a metal plate causes electrons to be knocked loose (ejected) from the metal plate. Several aspects of the phenomena could not be explained in terms of an electromagnetic wave: Increasing the brightness of the light did not eject faster electrons - think of light as a wave - brighter light (bigger amplitude wave) should eject more energetic (faster) electrons. Energy and number of ejected electrons depends on color of light - for some metals, red light would not eject any electrons at all - blue lights ejects very fast electrons even if very dim. The electrons were emitted immediately - no time lag - if light is dim, expect a delay while the waves wiggle the electrons and break them loose.

The Photoelectric Effect ISNS 3371 - Phenomena of Nature The Photoelectric Effect In 1905 Einstein gave a very simple interpretation of the results - the incoming radiation should be thought of as quanta of energy hf - a photon - with f the frequency. - one of the crucial landmarks in the development of modern physics. - introduced in one of his three papers, published in 1905 (called the Remarkable Year of Physics) - helped to initiate the fundamental revolution in science that we now call Quantum Physics. - it was for this work that he was awarded a Nobel Prize in 1921 - not relativity! Idea used in a number of technologies today including photovoltaic cells - solar cells.

Light as a Particle (Photon) ISNS 3371 - Phenomena of Nature Light as a Particle (Photon) Light propagates as quanta of energy called photons Photons move with speed of light have no mass are electrically neutral Energy of a photon or electromagnetic wave: E = hf = h c/ l where h = Planck’s constant f = frequency of a light wave - number of crests passing a fixed point in 1 second c = velocity of light l = wavelength of a light wave - distance between successive crests

ISNS 3371 - Phenomena of Nature Light as a Wave Remember: Light is a vibration in an electromagnetic field through which energy is transported. So electrons can be manipulated by light. Electrons wiggle up and down as light passes by. It is a transverse wave - the vibration of particles is perpendicular to the propagation of the wave.

ELECTROMAGNETIC WAVES (LIGHT WAVES) ISNS 3371 - Phenomena of Nature ELECTROMAGNETIC WAVES (LIGHT WAVES) Velocity 186,000 miles/second 300,000 kilometers/second 3 x 106 m/second • It takes 1 1/3 second for light to travel from the earth to the moon. • It takes 8 1/3 minutes for light to travel from the sun to the earth.

Light Travel Time From Earth to the Moon our natural satellite ISNS 3371 - Phenomena of Nature Light Travel Time From Earth to the Moon our natural satellite 1.25 seconds From Earth to the Sun the centre of our Solar System 8.3 min From the Sun to Jupiter the largest planet 41 min From the Sun to Saturn the furthest naked eye planet 85 min From the Sun to Pluto the furthest of the Sun's planets 5.5 hr From the Sun to Alpha Centauri the nearest star to us 4.3 yr From the Sun to Sirius the brightest star in our sky 8.6 yr Distance where the Sun would no longer be visible to naked eye 60 yr From the Sun to Polaris the north pole star 650 yr From the Sun to the Galactic centre the centre of our Galaxy 31,000 yr Galactic diameter the diameter of our Galaxy 81,500 yr To the Andromeda Galaxy the nearest large galaxy 2,200,000 yr Extinction of the dinosaurs 65,000,000 yr To Q0134+329 typical quasar 4,500,000,000 yr Formation of the Earth and Sun 4,700,000,000 yr To remotest quasars discovered in 1998 14,000,000,000 yr Edge of Universe limit of observable Universe 15,000,000,000 yr

ISNS 3371 - Phenomena of Nature Light as a Wave For a wave, its speed: s = f ( is wavelength) But the speed of light is a constant, c. For light: f  = c The higher f is, the smaller  is, and vice versa. Our eyes recognize f (or ) as color!

Visible Light Waves Animation ISNS 3371 - Phenomena of Nature Visible light ranges through 7 major colors from long wavelengths (low frequency - red) to short wavelengths (high frequency - violet) - Red, orange, yellow, green, blue, indigo, violet (Roy G Biv) Visible Light Waves Animation

The Electromagnetic Spectrum ISNS 3371 - Phenomena of Nature The Electromagnetic Spectrum Most wavelengths of light can not be seen by the human eye. The visible part of the electromagnetic spectrum lies between ultraviolet and infrared light (between about 400 and 700 nm). The higher the frequency (shorter the wavelength), the higher the photon energy. Radio waves are at the long wavelength end of the spectrum and gamma rays are at the short wavelength end of the spectrum.

ISNS 3371 - Phenomena of Nature Color Additive primary colors - adding light of additive primary colors produces complementary colors - all colors produces white Red Green Blue Red + Green  Yellow Red + Blue  Magenta Green + Blue  Cyan Red Magenta Yellow Blue Green Cyan White

ISNS 3371 - Phenomena of Nature Color Subtractive primary colors - mixing pigments (that absorb light) of various subtractive primary colors produce complementary colors - added all together, produce black. Yellow Magenta Cyan Yellow + Magenta  Red Yellow + Cyan  Green Magenta + Cyan  Blue

ISNS 3371 - Phenomena of Nature Only four colors are used to print color illustrations and photos: (a) magenta, (b) yellow, (c) cyan, (e) black. (d) is with all but black, (f) is with all.

Four Ways in Which Light can Interact with Matter ISNS 3371 - Phenomena of Nature emission – matter releases energy as light absorption – matter takes energy from light transmission – matter allows light to pass through it reflection – matter repels light in another direction

ISNS 4371 Phenomena of Nature Properties of Light Law of Reflection - Angle of Incidence = Angle of reflection Law of Refraction - Light beam is bent towards the normal when passing into a medium of higher Index of Refraction. Light beam is bent away from the normal when passing into a medium of lower Index of Refraction. Index of Refraction - Inverse square law - Light intensity diminishes with square of distance from source.

ISNS 3371 - Phenomena of Nature Mirror reflects light at angle equal to incoming angle - Most materials reflect light randomly - scattering. Movie screen scatters light into array of beams that reaches every member of the audience

ISNS 3371 - Phenomena of Nature Materials that transmit light are transparent Materials that absorb light are opaque In general - some combination of reflection, absorption, and transmission Red glass transmits red light - absorbs all other colors Green grass reflects green light - absorbs all other colors

Atmospheric Light Scattering ISNS 3371 - Phenomena of Nature Atmospheric Light Scattering Atmospheric gases are largely transparent to visible light Most photons penetrate to the ground, warming it as the light is absorbed Small portion of light is scattered - why our sky is bright - light is not scattered on Moon, Mercury - their skies are dark - stars are visible during day - shadows extremely dark Gas molecules scatter blue light more effectively than red light

ISNS 3371 - Phenomena of Nature Why is the Sky Blue? Sunlight is scattered off the molecules of the atmosphere - called Rayleigh scattering - more effective at short wavelengths (the blue end of the visible spectrum). Little of the red, orange and yellow light is affected. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white.

Why are Sunsets Red or Orange? ISNS 3371 - Phenomena of Nature Why are Sunsets Red or Orange? The blue light has been scattered out and away from the line of sight. As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes. Dust particles can scatter red and orange light in all directions leading to spectacular sunsets - why sunsets in LA are so beautiful - pollution.

ISNS 3371 - Phenomena of Nature Moonrise Earthrise On the moon, there is no atmosphere to scatter light and the sky is black Atmosphere on Mars too thin to scatter light effectively - sky is reddish from presence in the atmosphere of reddish dust from surface. On Venus, almost all blue light scattered away - atmosphere dimly lit and appears reddish orange.