Propagation of Light

c

• Symbol that represents the constant speed at which light travels in a vacuum.

• Central to the theory of relativity

• Basically represents how fast light travels in an empty space.

Theory of Relativity

• A theory that explains how space, time, and motion behave at very high speeds

• Says that the speed of light is the same for everyone

• Time can pass differently for fast moving objects

• Gravity is actually the bending of space and time around massive objects.

atoms, crystal lattices, structures

Light travels slower in materials because it interacts with _____, _______________, and __________ within the medium.

Ole Rømer

Danish Astronomer who first measured the speed of light in 1675 by studying variations in the orbital period of Io.

Io’s Eclipses

• Events when Jupiter casts shadow on its moon Io

• Causing Io to periodically disappear from the view

• Was used by Ole Rømer to measure the finite speed of light

• Because their observed timing changes as Earth moves closer to or farther from Jupiter.

Light Travel-Time Effect (Romer’s Method)

The idea that changes in Earth’s distance from Jupiter alter how long Io’s eclipse signals take to reach Earth, revealing light’s finite speed.

Armand Fizeau

French physicist who made the first successful Earth-based measurement of the speed of light in 1849 using a rotating toothed wheel.

Fizeau’s Toothed-Wheel Method

An experiment where a rotating wheel, chopped a beam of light, the wheels rotation rate revealed the light travel time to a distant mirror and back.

Jean Bernard Léon Foucault

French physicist who refined Fizeau’s method using a rotating mirror, achieving a more accurate measurement of light speed.

Foucault’s Rotating Mirror Method

A technique where light reflects off a rapidly spinning mirror. The amount of deflection reveals the light’s travel time.

Albert Michelson

American physicist who improved Foucault’s method and obtained most precise pre-modern value of the speed of light (around 1926)

Index of Refraction

A number describing how much light slows in a material. Higher value means greater slowing.

n

Symbol to represent the Index of Refraction.

n = c/v

Mathematical representation of the index of refraction.

Light Rays

• A simple geometric model where the light is represented as straight lines.

• Only valid when objects are much larger than light’s wavelength.

v

Symbol that represents the speed at which light travels through a material. (Is always less than c)

Geometric Optics

The branch of optics using geometry and straight-line behavior to describe how light travels, reflects, and refracts.

Reflection

The change in direction when light bounces off a surface such as a mirror.

Refraction

The bending of light as it passes from one material to another due to a change in speed.

Index of Refraction Condition (n ≥ 1)

Because light can only slow down (not speed up) in materials, the index of refraction is always equal to or greater than 1

Law of Reflection

• A rule describing how light bounces off surfaces

• Angle of incidence = Angle of reflection

Law of Refraction

• A rule describing how light bends when passing from one material to another due to changes in speed

Light-Matter Interaction

The process by which atoms in a material slow light down, changing its speed but not its central frequency

Light Wavelength

The distance between the light waves

Visible light

__________ has wavelengths less than a micron, making the ray behavior dominant in everyday objects.

Straight-Line Propagation

The behavior of light moving in straight paths unless affected by reflection or refraction.

Direct Light Transmission

• Light traveling from a source to a location through empty space

• Example: Sun to Earth

Light Transmission Through Media

Light passing through substances like air, water, or glass, slowing down depending their atomic structure.

Reflected Light Transmission

Light reaching an observer after bouncing off something, like a mirror or reflective object.

Ray Approximation Validity

• Light behaves as rays when interacting with objects much larger than its wavelength.

• Visible light < 1 μm