Light

Chapter 16 - 19Chapter 16 - 19

Socrates and Plato

• 5-6th c. BC

• Light traveled from a person’s eyes to the objects one saw.

Pythagorus

• 6th c BC

• Light was made up of particles emitted from the objects.

Aristotle

• 4th C. BC

• Light moved as a wave, like ripples on water.

Leonardo da Vinci

• 1500’s

• Sound and light had similar properties, so they both were waves.

Galileo

• 1564 - 1642 •The first to hypothesize that light had a specific speed.

• Determined that light was too fast to measure due to human reaction time being to slow.

Olaus Roemer

• 1644-1710•He was the first to determine that light did travel with a measurable speed.

• It took 22min. to cross a diameter of the Earth’s orbit giving a speed of light of 2.2*10^8m/s1675 was the year of the first demonstration that light travels at a finite speed (a definable number)

Newton

• 1642-1727– 1666: colors produced when a beam of

sunlight passes through a prism is called a spectrum. White light is made up of colors.

– Light moved as a stream of particles, accounting for refraction, reflection, and color.

– Wrote a book called “Optiks”

Christian Huygens

• 1629-1695

• Calculated the speed of light to be 2.3 x 108 m/s. Error was based on the time estimate, which was limited by equipment.

• His work was rejected by fellow scientists as being too large.

Christian Huygens

•  1629-1695

• Proposed that wave theory also explained light’s properties.

Thomas Young

• 1773-1829

• Light exhibited interference, which is NOT accountable by the particle theory.

Augustin Fresnel

• 1788-1827

• Proposed a comprehensive mathematical wave theory which accounted for all observable behaviors of light.

James Clerk Maxwell

• 1831-1879• Light is a wave produced

when electric forces accelerate charge particles within atoms.

• Called electromagnetic radiation.

• He was able to predict the speed of light.

Albert Michelson

• 1852-1931

• In 1905 he precisely measured the speed of light using the Michelson-Morley apparatus.

• The speed of light is equal to the product of its frequency and wavelength.

• The speed of light: c = 3 x 108

PRESENT 

• Neither the wave model nor the particle model alone can explain all of the properties and behaviors of light. The present theory unifies the wave and particle natures of light.

The Nature of Light

• Wave model of light: light behaves as a wave, showing properties of reflection, refraction, diffraction, and interference.

• Particle model of light: light behaves as a particle, showing properties of reflection and refraction.

Electromagnetic Radiation

Light is a member of a large family of radiating waves.

The result of the acceleration of charged particles. They can travel through empty space. Visible light is only one type of electromagnetic wave.

The entire range is called the Electromagnetic Spectrum.

The Electromagnetic Spectrum

• Is the entire range of electromagnetic waves, roughly divided into regions by how wavelengths interact with matter.

• All travel at 3 x 108 m/s. • Frequency ranges from 101 to 1025 Hz.• Visible light is small portion at about 1015 Hz, 10-

7m.

• wavelengths from 400nm to 700nm• shortest are seen as violet light• longest are seen as red light• travels in a straight line

Light is the range of frequencies of electromagnetic waves that stimulates

the retina of the eye.

Energy and Electromagnetic Waves

The energy of electromagnetic waves depends on wavelength.

In the standard wave model, energy depends on amplitude. For electromagnetic waves, the shorter the wavelength(l), the greater its energy.

Types of Electromagnetic Waves

Gamma rays – Have the shortest wavelength and the greatest energy (wavelengths of gamma rays are shorter than the width of an atom) These are used to destroy cancer cells.

X rays – penetrate all but the very dense part of the body.

Types of Electromagnetic Waves

Ultraviolet light – sun/sunlight/tanning beds. Sunburns occur when ultraviolet light damages skin cells.

Visible light – ROYGBIV Shortest wavelength is violet, longest wavelength is Red.

White light – A mixture of all visible colors of the spectrum.

Types of Electromagnetic Waves

Infrared light – Wavelengths longer than visible light. Responsible for warming the earth and heat.

Microwaves.

Radio waves – longest wavelengths of the electromagnetic spectrum.

Medium

Light travels in a vacuum.

No transmitting medium is necessary, but if a medium is present, it may pass through that as well. 

Speed of Light (c) = λf

What is the frequency of yellow light, λ = 556nm?

f = c / λ

= 3 x 10^8 m/s .Convert nmm 556 x 10^-9 m

= 5.4 x 10^14 Hz

c = 3.00 x 10^8 m/s

The Speed of Light

• The speed of light is dependent on the medium through which light travels

• The speed of light determines the wavelength. 

Fun Facts about the speed of light. 

Light is so fast that if a beam of light could travel around the earth it would take 7.5 trips in 1 second.

It takes 8 minutes for light from the sun to reach our eyes.

It takes 4 years for light from the star Alpha Centauri (nearest star next to the sun) to reach earth.

A light year is the distance light travels in 1 year.

• A ray is a straight line that represents the path of a very narrow beam of light

Represented by a solid line with an arrow indicating direction of motion 

A BEAM

• A beam is a collection of rays.

• A luminous body emits light waves.

• A illuminated body reflects waves.

• An incandescent lamp (light bulb) is luminous because electrical energy heats a thin tungsten wire in the bulb and causes it to glow.

Luminous Vs. Illuminated

Luminous objects are themselves a source of light.

Illuminated objects are visible only when light reflects off of them.

• Luminous Flux (P) is the flow of light from source measured in lumens (lm).

• Illuminance (E) is the rate at which electromagnetic wave energy falls on a surface (illumination surface).

• Measured in lumens per square meter, lm/m², or lux (lx).

• A candela (cd) is a unit of luminous intensity.

• The candela is the official SI unit from which all-light intensity units are calculated.

E (illumination) = P (luminous flux) 4Пd²(distance)

Light Illuminance varies as the inverse square of the distance.

Illumination drops off quickly with distance from source.

Light and Matter

• Light that shines upon materials can be reflected, absorbed, or transmitted.

• Materials are classified as transparent, translucent, or opaque.

• This depends on the material.

Transparent, translucent and opaque

• Transparent objects allow light to be fully transmitted.

• Translucent objects allow light to be partially transmitted. Some light is absorbed and/or reflected.

• Opaque objects absorb all light. They cast dark shadows. 

SHADOWS

• formed when an object blocks all of the light from a light source.

• have a darker part on the inside called the umbra where all of the light is blocked.

• have a lighter part around the edges called the penumbra. – formed when light from one source is blocked with

light from another source filling in some of the shadow or when light from a broad source is only partially blocked.

Examples of Shadows

Regions of a Shadow

Polarization

Light contains electromagnetic waves vibrating perpendicular to the direction of travel, with half the waves vibrating in the x-y plane, and half in the x-z plane.

Polarizing filter – allows light traveling on the same plane to pass through. All other light waves are absorbed.

EM Wave

Polarization

Placing a polarizing filter in a beam of light will reduce the intensity by ½, as the filter only allows the waves vibrating in the same direction to pass.

Placing a second filter perpendicular to the first will eliminate all of the light.

For this reason, polarized sunglasses can help reduce glare.

Are the horizontal lines parallel or do they slope?

Can you see the three faces?

How many legs does this elephant have?

What does the sign say? Are you sure?

3D Viewing

• The glasses have two Polaroid filters. Each filter has a different polarization axis - one is horizontal and the other is vertical. There are two projectors also having two different polarizing filters. The result of this arrangement of projectors and filters, is that the left eye sees the movie which is projected from the right projector while the right eye sees the movie which is projected from the left projector. This gives the viewer a perception of depth.

View with 3D Glasses

View with 3D Glasses

View with 3D Glasses