Lecture 20: Properties of Light

What is light?

• form of electromagnetic energy with oscillating electric field and magnetic field at right angles to each other

• pure energy

Light is a:

wave and particle

How does light travel?

propagates through a vacuum and does not require a medium

What is speed of light relative to other things in the universe

Fastest thing in universe

Why does light travel fastest in a vacuum

nothing to impede or slow it down

Why does light slow down in materials like glass?

The atoms in the material interact with photons, slowing light propagation.

How are photons generated?

When electrons drop from a higher energy level to a lower energy level, they release energy as a photon.

What determines the color of light emitted by an atom?

The energy difference between electron energy levels.

Light makes what kind of patterns?

Interference patterns

Why do electrons return to lower energy states?

Lower energy states are more stable.

What is photon generation?

atoms ho to same energy level to produce light color

What is the relationship between light, frequency, and wavelength?

Frequency = speed of light / wavelength

What makes objects transparent?

resonance mismatch

Energy of a photon equation

E=λ/hc​

• E = energy of the photon

• h = Planck’s constant

• c = speed of light

• λ (lambda) = wavelength of light

What does the color black mean?

Fully absorbed, only other photons released

White light is what?

combination of all wavelengths of visible light

What is light refraction

the bending of a light ray as it passes through a medium with different speed of light

Lenses do what to light

they refract light

Whales and dolphins do what to speed of sound

bend

Difference between diffraction and refraction

• Diffraction: bending around edges/objects

• Refraction: bending when entering a different medium

How do different wavelengths refract?

shorter wavelengths refract more strong

What is polarization?

Filtering light so electric field lines align along a single plane

• filter’s out sun’s light

What do bees use polarization for?

sun’s position for navigation

What type of light do animals see?

visible light

What light is easily absorbed?

• radio waves

• microwaves

• infrared

• visible

What light passes through tissue?

• UV

• x rays

• gamma rays

What happens to light that is not reflected

it’s absorbed

What determines an object’s color

the wavelength of light it reflects

Translation and Rotation

Microwaves

Bond Vibrations

Infrared

Electron Rearrangements

UV

Break bonds and ionize

between soft x rays and gamma rays

Molecules can oscillate in what four ways?

• Translational oscillations

• Rotational Oscillations

• Bond Oscillations

• Electron Oscillations

Translational oscillations

molecules move to new location

Rotational Oscillations

molecules rotate at the same location

Bond Oscillations

vibration of bonds between atoms in a molecule

Electron Oscillations

electrons within an atom change orbital levels

Reflection off solid objects (makes vision possible)

light will bounce off of most objects in an orderly manner

Straight line transmission (makes vision possible)

allows receiver to make a spatial map of reflected rays

Refraction (makes vision possible)

allows ficusing and image formation in the eye

Rayleigh scattering

light scatters in all directions

• happens with tiny particles like gas molecules, scatters blue light more than red

Mie scattering

scattering in primarily one direction = diffraction

• larger particles like water droplets

What field of physics explains light’s wave-particle duality

Quantum mechanics

What is resonance match

photon energy matches electron energy levels, photons absorbed

color is opaque

What is structural color

color produced by physical interactions of light with structures, not pigments

Example of structural color in animals?

blue feathers in birds

Why do diamonds produce intense colors?

have very strong refraction due to their dense atomic structure, dispersing light stronlgy

Why is sky blue?

rayleigh scattering scatters shorter blue wavelengths more strongly

Why are clouds white?

mie scattering scatters all wavelength equally, producing white light

• also contributes to mist and fog