Light and Color

Ancient Theories

• Earlier views of light believed that something like a streamer went out from the eye and gave us sight.

• Newton came up with a theory of light that said it was particle in nature and came to the eye from a source.

• A contemporary of Newton’s, Christian Huygen’s, believed that light traveled in waves.

• In 1905, Einstein published a paper called the Photoelectric Effect, stating that light traveled in massless bundles of electromagnetic energy called photons.

Current view

• Today it is agreed upon that light has a dual nature, part particle and part wave. This idea is called the “duality of light”.

The speed of light

• The speed of light was a mystery up until recent times.

• Olaus Roemer discovered a discrepancy with the time it took for one of the moons of Jupiter to reappear as it orbited.

• Albert Michelson used a setup of mirrors and measured the speed of light in 1880.

• He became the first American to receive the Nobel Prize in science.

• The speed of light is 300,000,000 m/s in a vacuum and is slightly less through transparent materials.

• A light year is the distance light travels in one year(9.5 x 1012 km.

• It takes light 8 min to reach us from the sun and 4 years from the nearest star, Alpha centauri.

• Light travels as energy waves that come from vibrating electrons in atoms.

• These waves are called electromagnetic waves and are the same as x-rays and radio waves. The only difference is their frequency.

The electromagnetic spectrum

• Light is transferred through matter in much the same way that sound is, by vibrations.

• A transparent material is one that allows light to travel in through in straight lines ( ex. glass and clear plastic)

• An opaque material is one that absorbs light and turns it into heat energy.

• A translucent material allows light through but cannot be seen through (Ex. frosted glass)

• A ray is a thin beam of light. Blocked light rays cause shadows.

• An umbra is a total shadow and a penumbra is a partial shadow.

• Light travels in transverse waves that vibrate in all directions.

• When a wave vibrates in only one direction, it is said to be polarized.

Polaroid filters

• Certain filters are capable of polarizing light and are useful in sunglasses since they greatly reduce glare

• 3-D movies also use this phenomenon. They are filmed and projected with two lenses to give increased depth.

Color

• The color of an object is the result of the frequencies of light it emits or reflects.

• A red rose reflects red light. It will absorb all others colors of the spectrum.

• If placed in green light the rose reflects no light and will be black.

White light

• A white object will reflect all colors and will be white in white light.

• White light (such as sunlight) contains all the colors of the visible spectrum.

• A black object absorbs all the colors of the spectrum.

• Black is the absence of light.

• Light is absorbed when its frequency matches a natural vibration frequency of electrons in the material. This is a form of resonance.

Combining colors

• Color mixing occurs when light or pigments get combined. Combining colors of light is called color addition.

• Combining colors of pigment is called color subtraction.

• The eyes sees an equal combination of red, green, and blue light as white.

• Red, green, and blue are the additive primary colors.

• Magenta, cyan and yellow are the subtractive primary colors.

• The sky is blue due to the random scattering of the high frequency blue and violet light by the oxygen and nitrogen molecules in the atmosphere.

Sunrises and sunsets

• The low frequency yellow, orange and red light gets transmitted.

• If the sun is at a low angle virtually all the high frequency colors get scattered out and the sun appears deep orange or red.

Bright line spectra

• Each element has a characteristic line spectra that is like its fingerprint.

• This is how atoms can be identified without directly examining an object.

• Ex. Composition of planets and stars