WAVES

www.oceanweather.com/datahttp://www.fluent.com/about/news/newsletters/04v13i2/a6.htmhttp://www.worldofteaching.com/powerpoints/geography/erosion.ppt#257,3,Attheshoreline http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=16 http://www.siskiyous.edu/shasta/env/cloudshttp://geophysics.ou.edu/solid_earth/notes/seismology/seismo_interior/seismo_interior.htmlhttp://www.geology.wisc.edu/~geo302/spring2008page/Lectures/Lec3&4_SeismicWaves+Structure.pdfhttp://www.ig.uit.no/webgeology/english/earthsinterior.html http://web.ics.purdue.edu/~ecalais/teaching/eas450/seismology3.pdf

http://www.weathervortex.com/sky-ribbons.htm

Recall that wave clouds form at the crests of otherwise invisible waves in the air that become visible when the air rises above and sinks below the condensation level

Wave Clouds over the Sea of Japan

A cloud wave train moves off the SE coast of the USA

Wavelength, : Distance between adjacent crests or troughs.Amplitude, a: Half the maximum difference of the wave quantity (e. g., height, pressure, temperature, electric field) between the trough and crest. The amplitude of an ocean wave 6 m high is 3 m.Speed, c: Distance the wave moves divided by the time interval.Period, T = 1/f: Time interval (of a cycle) between successive crests or troughs.Frequency, f = 1/T: Number of waves that pass a given point per second or number of cycles at a given point per second.

fc

General equation for wave speed, c, is the number of waves that pass a given point per second times the length of the waves.

When wave speed is constant, frequency is inversely proportional to wavelength.

Crest

Trough

Tf

1

General Properties of WavesWaves are periodic variations of height, pressure, electric and magnetic fields, etc. They are omnipresent in Nature. Sound and light are transmitted by waves. Even winter storms are troughs of waves. Because waves repeat regularly, they can be predicted.

Wavelength, Wave Speed, and Frequency: Some Units

Wavelength (L or ) is the distance between two successive crests or troughs. One meter (m) is about 3.29 feet long. Typical ocean waves are about 100 feet (30 m) long. Visible light waves are incredibly short, ranging from 0.4 m (micrometers) or 400 nm (nanometers) for violet light to 0.7 m for red light. Each capital letter on a printed page is as long as 3000 red light waves lined up end to end! (1 meter = 106 m = 109 nm)

Wave speed (c) is the distance a wave travels divided by the time. The speed of light waves in empty space is, c 300 million m/s (ms-1) or about 186,000 miles per second. At this rate light will travel from New York to Denver in 0.01 s and from Sun to Earth in 8 min. Sound waves travel through air at a speed about 330 m/s or 700 mph (faster as temperature, T increases), so that it takes about 3 s to travel 1 km and 5 s to travel 1 mile. This is why we hear thunder after we see lightning.

Frequency (f) is the number of wave cycles experienced at a point every second. One cycle per second is called one hertz or 1 hz after Heinrich Hertz, who discovered radio waves. The frequency of violet light is 7.5(10)14 hz because every second, 750 trillion violet waves pass by. The typical frequency of ocean waves is 0.1 hz because one wave passes roughly every 10 s. For each wave speed the shorter the wavelength the higher the frequency. Click to see that the shortest waves (violet) have the highest frequency.

Travelling Transverse Waves

Even though the wave moves from left to right the ball only moves up and down. This is a transverse wave. Light waves are transverse.

Standing Transverse Waves

Crests and troughs of waves in tanks or bathtubs, or vibrations of a violin string remain in place.

Animation of Waves

Travelling Longitudinal Waves

Waves whose particles move in the same direction as the wave motion such as sound waves.

Light Waves and Light Particles (Photons)Light has a dual nature, sometimes acting like waves and sometimes like solid particles.

Visible light waves range in length from the shortest violet waves (about 0.4 m) to blue, green, yellow, orange and the longest red waves (about 0.7 m). This constitutes the visible light spectrum in order of increasing wavelength. Visible light waves form a tiny part of the spectrum of electromagnetic waves. Shorter waves include gamma rays, X-rays, and ultraviolet (UV) radiation. Longer waves include infrared (IR) radiation, microwaves and radio waves. The Sun radiates over a wide range of wavelengths, but with a sharp peak at = 0.5 m, right in the center of the visible light spectrum. Solar radiation contains about 45% visible light, 10% UV, and 45% short IR waves from 0.7m to 4m. Earth radiates mostly long IR waves.

When light strikes atoms or molecules, they act as if they had been struck by a particle (called a photon). Einstein found that the shorter the electromagnetic wave (and the higher its frequency) the greater the energy of its photons and the more dangerous and even lethal it is. If the photon has enough energy it will smash the atom or molecule apart. With less energy it can knock the electrons into more energetic (outer) orbits. With even less energy it can only make molecules with 3 or more atoms spin faster or vibrate more vigorously. Thus, X-rays can penetrate skin and smash atoms, UV radiation can smash DNA molecules, but IR radiation can only change the rotation or vibration of molecules such as CO2 and H2O. Even so, when CO2 and H2O absorb the IR waves radiated by the Earth, they keep the planet warm through the Atmospheric Greenhouse Effect. A good rule is, the hotter an object the more rapidly it radiates and the greater the percentage of high frequency, short waves it emits.

Visible Light covers a small part of the electromagnetic spectrum with waves that range from violet ( = 0.4 m = 400 nm) to red ( = 0.7 m = 700 nm)

The Nature of Light

High Grade Energy More per Photon

Low Grade Energy Less per Photon

Increasing energy and frequency

Increasing wavelength Decreasing frequency

Radiation and Temperature: As molten steel cools, it radiates less energy at lower grade (lower frequency, longer waves). Color changes from brightest white to yellow to orange to dullest red.

Energy of photons also affects the colors of the Aurora

The Aurora is produced when high energy particles (mainly electrons) from the Sun smash into the Atmosphere about 80 km above sea level. They energize oxygen and nitrogen atoms and molecules, which then emit light. The more energetic particles penetrate deeper into the atmosphere and produce green light with higher frequency than red light produced by less energetic particles that cannot penetrate the atmosphere as deeply.

SOLAR RADIATION TRAVELS DIRECTLY TO EARTH

SOLAR WIND PARTICLES (not waves) ARE DEFLECTED BY EARTH’S MAGNETIC FIELD TO PRODUCE AURORAS

The Auroral Oval over Antarctica is the ring of Auroras that forms around Earth’s South Magnetic Pole

For Animation of this Image see Auroras under Wikipedia

Diffraction pattern of waves passing through 2 slits.

Diffraction pattern of X-rays passing

through a crystal (see Bragg’s Law)

Scattering: Intensity varies with Angle

Refraction pattern as

wave passes obliquely into

slower medium

Waves are disturbed when they encounter obstacles. They can then be Reflected, Refracted, Diffracted, Scattered, or Absorbed

i = r i

r

Reflection Refraction

i > r

Law of Reflection:

Angle of Incidence, i = r, Angle of Reflection

Law of Refraction:

nc

c

r

i

r

i )sin(

)sin(

Air

Water or Ice

Waves refract or bend toward the region where they move the slowest.

Refraction of Light Waves that enter a Prism Obliquely.

The prism reflects some light when the light strikes the left side (from the outside) and when it strikes the right side (from the inside).

Violet waves travel slowest of all visible light waves in most materials such as glass or water so that they refract the most. Red waves travel fastest and refract the least.

Wavelength Change: A Fundamental Difference between Reflection and Absorption followed by Emission

When light strikes an object and is reflected, there is no change in wavelength.When someone throws pie in your face and it bounces off your face it is still pie.

When light strikes an object and is absorbed, the object will almost surely emit radiation with a different wavelength.When you eat pie, your body will almost surely eliminate the pie in a different form.

RUN REFLECT-ABSORB-EMIT

The Equal Color Illusion We do not always see in absolute terms, but are affected by relative contrasts of light and color. What square is darker – A or B?

Square, ‘B’ is just as dark as Square, ‘A’.

The Gradient Illusion: Which side of the Gray Rectangle is Darker?

The right side of the gray rectangle appears darker by contrast with the lighter part of the graded gray background. When the background fades to white, the gray rectangle finally appears to have uniform color but they eye plays such tricks on us that we think that as the background is changing the gray rectangle is also changing.