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Waves

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Narrated by Stephen Fry.

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Types of waves.
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Sound waves.

Speakers vibrate to cause waves in air molecules. When these waves reach our ears they vibrate our ear drums. The brain interprets this as sound.

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Frequency.

Frequency is the number of occurrences of a repeating event per unit of time.

The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency.

The hertz (Hz) is the unit of frequency and is defined as one cycle per second.

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Frequency hearing range.

Human ear frequencies range from 20 Hz (lowest pich) to 20,000 Hz (highest pitch). Some animals, such as elephants, can hear sounds below 20 Hz (infrasounds) and others can hear over 20,000 Hz (ultrasounds). Cats and dogs hear up to 40,000 Hz, while dolphins hear up to 160,000 Hz!

Illustration via cochlea.org

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Echo.

An echo is the reflection of a sound. You can only hear an echo if it comes back more than 0.1 seconds after the original sound.
Sound travels 34 m in 0.1 seconds, so you only hear echoes from surfaces that are at least 17 m away.
Smooth surfaces give the best echoes because they break up the sound waves the least.
See more at: http://www.houseandhome.org

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Speed of sound.

The speed of sound in air is 1,237 kilometres per hour (343.59 m/s).
A plane creates a 'sonic boom' as it crosses the sound barrier. The white halo formed by condensed water droplets results from a sudden change in air pressure around the aircraft.

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Sonic booms in nature.

The sound of thunder is produced by rapidly heated air surrounding lightning which expands faster than the speed of sound.
The loud noise created by cracking a whip can also be a sonic boom, as the tip can break the speed of sound.

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Relating velocity to frequency and wavelength.
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New wave of energy.
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How speakers and microphones work.
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The electromagnetic spectrum.
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Light in the Universe.
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Light in a vacuum.

Light waves are waves in the electromagnetic field, which can exist in a vacuum (like space).
Sound is a pressure wave caused by particles (atoms or molecules) hitting each other and passing on their energy. If you are in a vacuum, there are not particles, so no energy can pass between them. Therefore sound waves can not exist in a vacuum.

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Speed of light.

The speed of light in vacuum (299,792,458 m/s), commonly denoted c, is a universal physical constant. According to special relativity, c is the maximum speed at which matter and information can travel. It is the speed at which all massless particles and changes of the associated fields (including electromagnetic radiation such as light and gravitational waves) travel in a vacuum. Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer. In the theory of relativity, c interrelates space and time, and also appears in the famous equation of mass–energy equivalence E = mc².

Text via Wikipedia

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Light absorption.

Lakes and oceans appear blue for several reasons. One is that the surface of the water reflects the color of the sky.

When materials absorb light, the light's energy is transferred to the material, often as heat. Some of the light hitting the surface is reflected back directly but most of it penetrates the surface, interacting with its molecules. The water molecule can vibrate in three different modes when light hits it. The red, orange, yellow, and green wavelengths of light are absorbed and the remaining light we see is composed of the shorter wavelength blues and violets. This is the main reason why the ocean is blue.

Text via Wikipedia

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Reflection.
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Refraction / Dispersion.
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Why is the sky blue?
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The Physics of Sunsets.

If you see a green sunset, run.
Full text: http://scienceblogs.com/startswithabang/2013/02/13/the-physics-of-sunsets/

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Why are trees green?

Trees are green because the cells that make up the leaves contain little packets (called chloroplasts) of the pigment chlorophyll. This pigment absorbs red and blue light, but reflects green light. Trees and plants use chlorophyll to absorb energy from sunlight and transform it into energy.
When we eat plants we are getting energy that originally came from the sun, and the same is true if we eat animals that eat plants or animals.

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water exposed to sound waves

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Radar uses radio waves to detect planes. How do stealth planes hide from it?

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A fun and insightful way to think about waves.

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