Light & Optics

ELECTROMAGNETIC

RADIATION

Part 1

ELECTROMAGNETIC RADIATION

Radiation: The transfer of energy by waves; the

transfer of heat by waves.

Electromagnetic radiation (EMR): Light. The waves

by which radiant energy (light) moves and carries

energy.

Parts of the electromagnetic wave

• Photon: Compact packet of energy (quanta)

• Electric field: invisible electric force.

• Magnetic field: invisible magnetic force.

As the photon moves in space from the source, the

perpendicular magnetic and electric fields travel with it.

Light waves and the forces move in straight lines. The

forces originate from the light source that is influenced

by a vibrating or oscillating electrical charge.

“Wave-particle duality”

Light waves and electrons have characteristics that are

“hybrids” between energy (waves) and particles (matter).

• Light (photons) are waves and are energy, but

sometimes behave like particles (matter).

• Electrons (subatomic particles) are matter, but

sometimes behave like waves and energy.

Light waves behave like particles because…

• Light waves move in straight lines through space

• Light reflects from surfaces at symmetrical angles

• Shadows are cast with crisp edges

Light waves behave like waves/energy because

• Light waves diffract after passing through openings,

and light bends around edges of objects.

• Light waves refract through lenses and prisms

• Light waves interfere and pass through each other

without losing or changing their energy

• Light is transformed to heat when absorbed by matter

Electromagnetic Spectrum

Ionizing radiation

Increasing frequency

Decreasing wavelength

Increasing energy

Electromagnetic Spectrum

• Gamma rays have the highest frequency, greatest energy,

and shortest wavelengths.

• Radio waves have the lowest frequency, lowest energy,

and longest wavelengths.

Humans can only see visible light. The other classes of

EMR are invisible to our eyes, we cannot see them.

For example, the dark black regions of space have

infrared and microwave light which we cannot see.

Gamma rays, xrays, and ultraviolet light are ionizing

radiation. They have enough energy and high enough

frequency to break chemical bonds in molecules. They

can energize electrons and remove them from atoms.

EMR Spectrum

EMR Wavelengths (λ)

Gamma 1x10-13 to 1x10-12 m

X-Ray 1x10-12 to 1x10-9 m

Ultraviolet 1x10-9 to 4x10-7 m

Visible 4x10-7 to 8x10-7 m

Infrared 8x10-7 to 1x10-5 m

Microwaves 1x10-5 to 1x10-2 m

Radiowaves 1x10-2 to 20 m

Least energetic,

lowest frequency,

greatest wavelength

Most energetic,

greatest frequency,

lowest wavelength

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InfraredUltravioletX-Ray

Note: all light except for visible is invisible to the eye.

Special filters and lenses create false-color representations

of EMR emitted from the Sun so we can see thos classes

of light.

The Sun emits all classes of EMR. Earth’s atmosphere

blocks the most energetic of the EMR spectrum.

Sunlight that makes through

space to the Earth has all

EMR.

Our atmosphere blocks

gamma, x-rays, and ultraviolet

light (harmful radiation)

Our atmosphere allows

visible, infrared, microwave,

and radio waves to pass

through (harmless radiation)

Speed of light

• All forms of EMR travel at the speed of light.

Radio waves travel at the same speed as gamma rays

• 3.00x108 m/s (300,000 km/s) through a vacuum/space.

• Upper most limit in the universe—no matter or energy

can travel faster than the speed of light.

• In one second, light travels the same distance as 7.5

times the distance around the Earth’s equator.

• 1 light year = distance light travels through space in 1

calendar year. 1 LY = 9,460,000,000,000 km.

• 8.33 minutes for sunlight to travel from sun to the earth.

Speed of Light

Vacuum: 300,000,000 m/s

Air: 299,000,000 m/s

Water: 225,000,000 m/s

Glass: 200,000,000 m/s

Diamond: 124,000,000 m/s

Fastest

Slower

• More dense medium (solids) = move slower

• Less dense medium (gases) = move faster

• Through a vacuum (space) = fastest

• Electrons around atoms become excited (gained energy)

from an outside source (electricity, heat, or absorbing

light).

• The excited electrons “relax” and return to their normal

state because they emit that excess energy in the form of

a photon (light wave).

Most slight is formed by the release of photons (quanta energy)

from excited electrons.

Gamma rays are released

during nuclear reactions.

The unstable nuclei of very

large atoms (like uranium,

thorium, and plutonium).

According to Einstein’s

Theory of Relativity: E = mc2

The energy holding protons and neutrons together, plus a

very small quantity of mass, is released as gamma rays.

Photons are pure energy that move as pulses in light

waves. If photons interact with matter, and if those

photons have exactly the correct amount of energy, they

can energize electrons and cause the electrons to break

away from their atoms causing chemical reactions.

Luminous objects generate and emit their own visible

light or white light. Stars, fire, and light bulbs are

luminous. Those object generate and emit their light.

Emission: The release of radiant energy (EMR, visible

light, photons) by matter. Light is given off (not

reflected).

The Moon is not a

luminous object. It does not

generate and emit its own

visible light. Moonlight

forms from the reflection of

sunlight off of the lunar

surface onto the Earth.

The Sun is a luminous

object. The Sun generates

and emits its own light. The

Sun appears yellow because

the temperature of the Sun’s

photosphere is ~ 5800ºC.

The sun makes its heat

and light by fusion. 4

atoms of hydrogen, under

extreme pressure in the

sun’s core, are fused into

1 atom of helium.

The fusion (a nuclear

process) releases photons

that bounce from atom to

atom upward to the Sun’s

surface.

Incandescence: Objects have temperatures that are so hot

that they emit visible light. (fire, surface of stars,

incandescent light bulbs, forged metals).

• Incandescence begins around 800 K (527ºC) as a pale

red glow (like a metal poker in a fire).

• At much hotter temperatures, the color of incandescing

matter changes from red to blue as temperature

increases.

The color of stars can range from red (cool) to blue (very hot). The

wavelength of the light emitted from the corona of the star is

inversely proportional to the star’s surface temperature.

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Visible Spectrum

White light: Total collection of all wavelengths of the

visible spectrum (all colors ROYGBIV collectively).

Double refraction of white light through a prism yields

the separation of the visible spectrum into bands of color.

Shorter wavelengths (blues) refract more than longer

wavelengths (reds).

Splitting white light into the visible colors of light by

diffraction on a grating—you can see the rainbow on a

compact disk or in the film of a bubble.