Chapter 16

Light

Light - an electromagnetic wave that is visible to the human eye

• History– Newton proposed that light was a particle,

explained reflection and refraction

– Christen Huygens proposed that light was a wave• Wave theory explains reflection, refraction AND

diffraction

• Light electromagnetic spectrum wavelengths from 3.0x10-7 to 7.6x10-7 m

• 300-760nm

• the shorter wavelengths are violet, longer are red - ROY G BIV

DIAGRAM OF ELECTROMAGNETIC SPECTRUM

Red vs Blue Shift• Blue shift object coming towards you– Increase Frequency, Energy– Shorter Wavelength

• Red shift object moving away from you– Lower Frequency, Energy– Longer Wavelength

Light travels in a straight line, which provides the basis for ray optics

– lines representing direction of light waves are called rays

– beam of light - rays traveling in the same direction

• Does not require a medium for transmission

Dual Nature of Light - can be looked as carrying energy as a wave or as a particle

• As a wave - look at Huygen’s Principle: all points on a given wavefront are taken as point sources for the production of spherical secondary waves, called wavelets.

• Quantum Theory of Light - light is emitted from a source in discrete bundles (quantized) called photons

Speed of Light• Today it is a defined value and not possible

to measure, c = 299,792,458 m/s.

• First determined in the “lab” by Albert Michelson. Used a rotating octagonal mirror as a timer and light traveled over a measured path (less than 1 cm in error) between two mountain tops.

• c = f

Transmission and Absorption of Light

• Transparent - objects through which we can see clearly. Transmits light

• Translucent - objects transmit light but cannot be seen clearly.

• Opaque - transmit no light. Absorb or reflect all light that falls on them.

Illumination by a Point Source• Luminous-emits light waves, like the sun

• Illuminated - reflects light waves, like the moon.

• Luminous Intensity (I)– a. Amount of light a source gives out, depends on

energy put into the source and the efficiency of the source.

Luminous Flux (P)• Rate at which light energy is emitted from a

source

• Units of lumens (measures the rate of energy, therefore a power unit)

• Treat object as point source of light

• Luminous flux P = 4 I

E P

4 2r

Illuminance (E)

• Rate at which light energy falls on a unit area some distance from a source.

• Units lumens/m2 or lux (lx). One lux is the illuminance of a surface located one meter from a one candela source.

E I2r

Light and Color• White light is light composed of all colors, whereas black is

the absence of any color or of reflected light.

• Spectrum is the name given by Newton to the ordered arrangement of colors that results from white light being dispersed in a prism

• Primary Additive Colors - red, blue, green– Secondary Light Colors - formed by mixing two of the primary colors.

• Complementary color - the secondary color that when added to a primary color produces white light. Cyan and red = white

Dyes and Pigments

• Dye - a molecule that absorbs wavelengths of light and reflects/transmits others. Explains why an object has a particular color.

• Pigment - a particle larger than a molecule and it can be seen with a microscope.– a. absorbs certain colors and reflects others– b. pigments mix to form suspensions

Polarization of Light• Light is comprised on many electromagnetic

waves moving in all directions. This is because light is emitted by atoms oscillating in many different directions. Such light is unpolarized.– a. waves oriented to a particular plane are said to be

plane polarized.– b. although light contains beams vibrating in every

possible plane, all can be resolved into vertical and horizontal components

When light is reflected it becomes polarized

• One angle at which maximum polarization occurs.

• Light reflected off of a horizontal surface is horizontally polarized, as surface absorbs oscillations perpendicular to it. This reflected light is called glare.

• A polarizer only allows light to pass through in one direction, those parallel to the plane.

• Polarization shows light is a transverse wave. Sound cannot be polarized as it is longitudinal.

Interference in Thin Films

A soap film is held vertically: weight makes if sag and become thicker at the bottom. Since it is thicker, light passing through it will be refracted differently.

Part of light is reflected, part is transmitted and refracted.