Light and colors

How do we see light?

• The human eye is like a complicated telescope. We have lenses to focus light with pupils which dilate and contract to control how much light gets through. We also have retinas which turn the light into an electrical signal travelling through neurons to the brain. Retinas have special cells called photoreceptor cells which come in two varieties, rods and cones. Rods are more sensitive to light and are used to see at night, while cones help us distinguish colors. Most humans have three different kinds of cones (color blind people only have two), each of which is sensitive to a different range of wavelengths. Each photon has a chance to be absorbed by the molecules in the rods and cones. This light energy is used to convert molecules in the cell to a higher energy state which sets off a cascade of flowing charges, becoming a signal to the brain.

• We note that there is lots of light that we don't see. Most humans can only see light that has a wavelength approximately between 400 and 700 nanometers. Radiowaves, microwaves, infrared radiation, ultraviolet radiation, x-rays, and gamma rays all have wavelengths outside of this range.

How do we see colors?

• Roses are red and violets are blue, but we only know that thanks to specialized cells in our eyes called cones.

• When light hits an object, e.g. a banana, the object absorbs some of the light and reflects the rest of it. Which wavelengths are reflected or absorbed depends on the properties of the object. For a ripe banana, wavelengths of about 570-580 nanometers bounce back. These are the wavelengths of yellow light.

• When you look at a banana, the wavelengths of reflected light determine what color you see. The light waves reflect off the banana's peel and hit the light sensitive retina at the back of your eye. That's where cones come in. Cones are one type of photoreceptor, the tiny cells in the retina that respond to light. Most of us have 6 to 7 million cones, and almost all of them are concentrated on a 0.3 millimeter spot on the retina called the fovea centralis.

FIREWORKS•

Fireworks

• Fireworks need a source of combustible material for energy such as black powder, a mixture of charcoal and sulfur or smokeless powder such as cellulose nitrate.

• fireworks contain substances that give off bright, colorful light when heated.

This phenomenon has been the basis of flame tests in chemistry laboratories.

The colored flame is a result of electrons in sodium ions absorbing energy and moving up to higher energy levels and then falling back to their ground state, emitting specific amounts of energy that correspond to colors of light

• Chemical ingredients of fireworks are

chosen to produce specific colors.

• Barium compounds produce green colors when heated.

• Copper salts produce green and blue flames.

• Sodium salts are yellow in flame.

• Lithium compounds produce red colors.

• Magnesium metal produces brilliant white light when burned.

• Strontium compounds produce brilliant red colors

• Fireworks consist of a source of energy such as a mixture of a fuel and an oxidizing agent that react to produce high temperatures and some substance that will emit brightly colored light.

Halo

• A halo is an optical phenomenon produced by ice crystals creating colored or white arcs and spots in the sky. Many are near the sun or moon but others are elsewhere and even in the opposite part of the sky. They can also form around artificial lights in very cold weather when ice crystals called diamond dust are floating in the nearby air.

• There are many types of ice halos. They are produced by the ice crystals in cirrostratus clouds high (5–10 km) in the upper troposphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Light is reflected and refracted by the ice crystals and may split up into colors because of dispersion. The crystals behave like prisms and mirrors, refracting and reflecting sunlight between their faces, sending shafts of light in particular directions.

22° halo

• A 22° halo is a halo, one type of optical phenomenon, forming a circle 22° around the sun, or occasionally the moon. It forms as sunlight is refracted in millions of randomly oriented hexagonal ice crystals suspended in the atmosphere. The halo is large; the radius is roughly the size of an outstretched hand at arms length.

AURORA

WHAT ARE THEY?

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

HOW IT HAPPENS?

Aurora is actually the result of collisions between gaseous particles in the Earth's atmosphere with charged particles released from the sun's atmosphere

Blown towards the earth by the solar wind, the charged particles are largely deflected by the earth's magnetic field (the earth's magnetic field is weaker at either pole) → therefore some particles enter the earth's atmosphere and collide with gas particles → These collisions emit light that we perceive as the dancing lights of the north and the south

AURORA BOREALIS

In northern hemisphere the effect is known as the aurora borealis or the northern lights

Auroras seen near the magnetic pole may be high overhead, but from farther away they illuminate the northern horizon as a greenish glow or sometimes a faint red as if the sun were rising from an

unusual direction

The aurora borealis most

often occurs near

the norther hemisphere

AURORA AUSTRALIS

Its southern counterpart, the aurora australis or the southern lights has features that are almost identical to the aurora borealis and changes simultaneously with changes in the northern auroral zone

It is visible from high southern latitudes in Antarctica, South America, New Zealand, and Australia

COLORS

The most common auroral color, a pale yellowish-green is produced by oxygen molecules and very rare, all-red auroras are produced by high-altitude oxygen. Nitrogen produces blue or purplish-red aurora.

AURORA IN FINLAND

The best way to see the Northern lights in Finland is to visit in Lapland

The sky should be clear and cloudless so the lights can be seen well

In the night sky they are beautiful and very well seen

Rainbow

Rainbow

• An optical and meteorological phenomenon

• It appears, when the light refracts

on the front surface of the

waterdrop, reflects on the back

surface of the drop and refracts again on the front

• The waterdrop is disperctive, so the white light breaks into the seven main colours which form the rainbow

The rainbow is seen, when you stand the back to the sun and you point to the rain in front of you

42°

• Rainbow is always a full circle but you see often just part of it

Double rainbow • Sometimes light reflects another time on the

waterdrops before they reach the eye of the spectator

A second, more matt, rainbow appears close to the other and forms the colours in reverse order. It’s called DOUBLE RAINBOW

http://www.youtube.com/watch?v=99E9fDgZZuE

Double rainbow

Monochrome rainbow

• A shower may happen at sunrise or sunset, where the shorter wavelengths like blue and green have been scattered and essentially removed from the spectrum.

• Further scattering may occur due to the rain and the result can be the rare and dramatic. This rainbow is called MONOCHROME RAINBOW

LIGHTNING

Lightning is a massive electrostatic discharge between the electrically charged regions within clouds or between a cloud and the surface of a planet

Lightning happens when the negative charges (electrons) in the bottom of the cloud are attracted to the positive charges (protons) in the ground.

The accumulation of electric charges has to be great enough to overcome the insulating properties of air. When this happens, a stream of negative charges pours down towards a high point where positive charges have clustered due to the pull of the thunderhead.

The connection is made and the protons rush up to meet the electrons. It is at that point that we see lightning and hear thunder. A bolt of lightning heats the air along its path causing it to expand rapidly. Thunder is the sound caused by rapidly expanding air.