What is Light?

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Note to teachers and students.

If you're studying for the SAT (US) or GCSE's (UK), the following posts are either not essential, or are already covered on the Waves board (http://pindex.com/b/00122).

The electromagnetic spectrum.
Light in the Universe.
Light in a vacuum.

Light waves are waves in the electromagnetic field, which can exist in a vacuum (like space).

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.


1 light year = 9 460 730 472 580 800 meters ≈ 5 878 625 352 016 mi

Light Nanosecond.

A radar antenna sends out a short radio pulse and waits for it to echo off an airplane or other target. Radio waves travel at the speed of light, so each nanosecond (a billionth of a second) represents 30cms. This is divided by two (the outward and return journeys of the waves), to calculate the distance of objects.

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.

Refraction / Dispersion.
Why is the sky blue?
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.

Northern Lights / Aurora Borealis.

What are Northern Lights?

The bright dancing lights of the aurora are actually collisions between electrically charged particles from the sun that enter the earth's atmosphere. The lights are seen above the magnetic poles of the northern and southern hemispheres. They are known as 'Aurora borealis' in the north and 'Aurora australis' in the south..
Auroral displays appear in many colours although pale green and pink are the most common. Shades of red, yellow, green, blue, and violet have been reported. The lights appear in many forms from patches or scattered clouds of light to streamers, arcs, rippling curtains or shooting rays that light up the sky with an eerie glow.

What causes the Northern Lights?

The Northern Lights are actually the result of collisions between gaseous particles in the Earth's atmosphere with charged particles released from the sun's atmosphere. Variations in colour are due to the type of gas particles that are colliding. The most common auroral color, a pale yellowish-green, is produced by oxygen molecules located about 60 miles above the earth. Rare, all-red auroras are produced by high-altitude oxygen, at heights of up to 200 miles. Nitrogen produces blue or purplish-red aurora.


How Invisibility Cloaks Work.