Chapter 22: Light and Color - Properties of Light

Fundamental Properties and Behavior of Light

• Propagation Speed and Distance:

  • Light travels extremely fast across vast distances.

  • It carries both energy and information across space.

  • Light travels in straight lines until it interacts with an interface or medium.

• Interactions with Matter:

  • Light bounces (reflects) and bends (refracts) when coming into contact with objects.

  • Most objects in the physical world are visible only through reflected light.

• Visual Characteristics:

  • Light exhibits color.

  • Light displays varying intensities, manifesting visually as bright or dim.

Atomic Generation of Light: Incandescence and Fluorescence

• Atomic Mechanism of Light Generation:

  • Light is primarily produced by atoms.

  • To emit light from an atom, an input of energy into the atom is strictly required first.

• Incandescence:

  • Incandescence uses heat to energize atoms until they release light.

  • Incandescent lighting is less efficient because a significant portion of the supplied energy is lost as heat rather than converted directly into light.

• Fluorescence:

  • Fluorescence uses high energy or electricity to energize atoms directly, causing them to emit light.

  • Fluorescent bulbs are significantly more efficient than incandescent bulbs because all (or nearly all) supplied energy goes directly into light production, whereas incandescent systems dissipate substantial energy as thermal waste (heat).

Speed of Light and Spherical Radiation

• Speed of Light Values:

  • The exact speed of light in a vacuum is $299\,792\,458\,\text{m/s}$.

  • For standard physics calculations, the speed of light is rounded to $300\,000\,000\,\text{m/s}$, expressed in scientific notation as $3 \times 10^8\,\text{m/s}$.

• Radiation Pattern:

  • Light radiates outward in all directions isotropically from its source.

Light Intensity and Inverse-Square Calculations

• Definition and Units:

  • Intensity ($I$) describes how much light energy hits a specific surface per second.

  • Intensity is measured in units of watts per square meter ($\text{W/m}^2$).

• Mathematical Formulation:

  • As light travels further away from its source, its intensity decreases exponentially over distance according to the inverse-square law:     $I = \frac{P}{A} = \frac{P}{4\pi r^2}$

  • $P$ represents the Power emitted by the light source, measured in Watts ($\text{W}$).

  • $A$ represents the surface area over which the light spreads, measured in square meters ($\text{m}^2$).

  • For light radiating spherically in all directions, the surface area is that of a sphere: $A = 4\pi r^2$, where $r$ represents the radius (distance from the light source in meters).

• Intensity Practice Example 1:

  • Problem: A flashlight produces $80\,\text{Watts}$. What intensity does it hit a wall $3\,\text{meters}$ away with?

  • Looking for: Intensity ($I$).

  • Given Values: Power $P = 80\,\text{Watts}$, Radius $r = 3\,\text{meters}$.

  • Relationship: $I = \frac{P}{4\pi r^2}$

  • Solution Setup:     $I = \frac{80}{4\pi (3)^2}$

  • Result: Evaluating gives $I = \frac{80}{36\pi} = \frac{20}{9\pi} \approx 0.707\,\text{W/m}^2$.

• Intensity Practice Example 2:

  • Problem: A sensor or observer feels $2\,\text{W/m}^2$ of intensity from a light source $2\,\text{meters}$ away. How much power does the light have?

  • Looking for: Power ($P$).

  • Given Values: Intensity $I = 2\,\text{W/m}^2$, Radius $r = 2\,\text{meters}$.

  • Relationship:     $I = \frac{P}{4\pi r^2}$     $2 = \frac{P}{4\pi (2)^2}$

  • Algebraic Derivation:     $P = 2 \times 4\pi (4) = 32\pi$

  • Numerical Result:     $P = 100.53\,\text{Watts}$

Physics of Light and the Electromagnetic Spectrum

• Dual Nature of Light:

  • Particle Properties (Photons): Light is composed of tiny individual packets or particles of energy known as photons.

  • Wave Properties: Light exhibits wave properties; it behaves as an electromagnetic wave.

• Wave Frequency and Energy:

  • Like other wave phenomena, the energy carried by light is directly proportional to its frequency.

  • Higher-frequency light waves carry proportionally greater energy per photon.

• Visible Spectrum and Energy Progression:

  • Visible light comprises the entire spectrum of observable colors.

  • As one progresses through the rainbow spectrum from red to violet, the energy increases continuously (red represents lower energy and lower frequency, whereas violet represents higher energy and higher frequency).

Color Perception and Mechanisms

• Color Perception:

  • Color is fundamentally defined as how human vision perceives light.

  • White light is not a single independent wavelength, but rather the combination of all visible light colors combined together.

  • All visible colors collectively make up visible light.

Subtractive Color Process and the CMYK Model

• Subtractive Color Process Definition:

  • The subtractive color process is the physical process by which an object obtains its perceived color by absorbing (subtracting) specific wavelengths of light from an illuminating light source and reflecting the remaining wavelengths to the viewer's eyes.

• Core Physical Mechanisms:

  • Physical objects rely on two primary optical mechanisms to produce perceived color:

  • Absorption (retaining certain wavelengths of incident light).

  • Reflection (bouncing the unabsorbed wavelengths back outward).

• Subtractive Primary Colors:

  • Cyan

  • Magenta

  • Yellow

• The CMYK Color Model:

  • CMYK is an alternate name and technical standard for the subtractive color process.

  • CMYK Acronym:

  • C: Cyan

  • M: Magenta

  • Y: Yellow

  • K: Black

  • Features a CMYK mixing scale and subtractive scale used for printing and pigment applications.

Additive Color Process and Human Vision

• Additive Color Process Definition:

  • The process of creating color by combining emitted light rays directly.

  • Mixing (adding) varying intensities of three primary colors together generates all visible colors.

• Additive Primary Colors:

  • Red

  • Green

  • Blue

• Biological Photoreceptors:

  • Human eyes utilize specialized biological light receptors called photoreceptors located in the retina to perceive color.

  • Human color vision operates via three distinct types of photoreceptor cones sensitive to red, green, and blue light.

  • Because the biological visual system relies on these photoreceptors, the human eye inherently uses the additive color process.

Questions & Discussion

• Why are fluorescent bulbs used more commonly than incandescent bulbs?

  • Response: Fluorescent bulbs are significantly more efficient because all supplied electrical energy is directed into producing light, whereas a substantial portion of the energy in incandescent bulbs is wasted as heat.

• What energy caused the tonic water to glow, and why could we see the energy after it hit the water but not before it got to the water?

  • Response: Ultraviolet (UV) atoms/photons charge and energize the carbonated/quinine atoms within the tonic water, causing them to emit a visible glow. The energy cannot be seen before hitting the water because UV light lies outside the visible spectrum, but once the atoms are charged and energized, they re-emit light in the visible blue spectrum.

• Is the glowing glass of tonic water hot, and what kind of light is being produced from the tonic water?

  • Response: No, the glass is not hot. The light being produced is fluorescence, which does not rely on thermal heat generation to produce illumination.

• What are the atoms in the tonic water doing from the moment the UV beam hits them to the moment the blue glow is seen?

  • Response: The atoms are being charged and energized by absorbing the incident high-energy UV radiation, and subsequently releasing that energy in the form of visible blue light.

• Why can the tonic water glow in every direction even though the flashlight only hits it from one direction?

  • Response: Reflection and omnidirectional re-radiation; once energized, the light scatters and reflects internally, radiating outward in all directions.

• Which process do printers use to make color?

  • Response: Printers use the subtractive color process (specifically CMYK).

• Which process do televisions (TVs) use to make color?

  • Response: Televisions use the additive color process (RGB).

• How are printers and televisions able to use different color processes?

  • Response: Televisions are light sources that actively produce light and mix colors directly using emitted light (additive process), whereas printers apply physical inks and dyes that do not produce their own light; instead, printed surfaces absorb specific wavelengths and reflect the remaining light to show the correct shade (subtractive process).