Light and Its Properties

Light

  • Visible Light: A component of the electromagnetic spectrum that is perceivable by the human eye.

  • Electromagnetic Spectrum: The range of all types of radiation that exists in the universe.

  • Radiation: A form of energy traveling through physical space.

  • Nature of Light: Light behaves like both a wave and a particle; for this course, it's primarily considered as an electromagnetic wave.

Characteristics of Waves

  • Every wave possesses the following four characteristics:

    • Amplitude (A): The height of the wave,

    • Wavelength (λ): The distance between two consecutive points in phase on the wave (e.g., crest to crest).

    • Period (T): The time it takes for one full cycle of the wave to pass a point.

    • Frequency (f): The number of cycles that pass a point per unit time.

Wave Speed

  • Previous discussions highlighted the four core properties of a wave; namely:

    • Amplitude, Wavelength, Period, and Frequency.

  • Wave Speed: Different types of waves travel at different speeds unless they are electromagnetic waves, which all travel at the speed of light. The basic equation for speed is:

    • v = \frac{\Delta d}{\Delta t}

  • For waves specifically, the speed can be expressed as:

    • v = \lambda f

    • This means that the speed of a wave is equal to its wavelength multiplied by its frequency.

Universal Wave Equation

  • The previous speed equation relates well to kinematics, but it is more common to measure the frequency of a wave rather than its period.

  • Thus the modified speed equation more applicable to wave problems is:

    • c = \lambda f

The Speed of Light

  • Speed of Light: Light is the fastest form of energy known, traveling in a vacuum at:

    • c = 3.0 × 10^8 m/s

  • All forms of electromagnetic radiation travel at this speed.

  • Wavelength and frequency have an inverse relationship; as one increases, the other decreases.

Light in our Solar System

  • Duration for light to reach various celestial objects:

    • Sun: 8 min 20 s

    • Pluto: 5.5 hours

    • Alpha Centauri (closest star): 4.3 years

    • Sirius (brightest visible star): 9 years

    • Betelgeuse: 430 years

    • Orion Nebula: 1500 years

    • Andromeda Galaxy: 2.5 million years

The Electromagnetic Spectrum

  • Represents the entire range of electromagnetic radiation organized by wavelength and frequency:

    • Radio Waves (λ > 1m): Include TV and radio transmission.

    • Microwaves (λ = 1m – 1mm): Used in Wi-fi and microwaves.

    • Infrared (λ = 1mm – 780nm): Associated with heat.

    • Ultraviolet (λ = 380nm – 10nm): High-energy waves; danger to DNA.

    • X-rays (λ = 10nm – 0.01nm): Used for medical imaging.

    • Gamma Rays (λ < 0.01nm): Highest energy waves; used in cancer treatments.

Visible Light

  • Visible Spectrum: λ = 780 nm \text{ to } 380 nm

  • The frequency of a flashlight producing light at 5.65 × 10^{14} ext{ Hz} corresponds to a wavelength of:

    • λ = \frac{c}{f} = \frac{3.00 × 10^8}{5.65 × 10^{14}} = 5.31 × 10^{-7} m = 531 nm

  • The color produced is green light.

Characteristics of Light

  • Propagation:

    • Light travels in a vacuum.

    • Linear propagation means light travels in straight lines from its source.

    • Rays can converge, diverge, or remain parallel.

White Light

  • Examples of white light sources include the Sun and common light bulbs.

  • Newton's Theory of Light and Colors (1672):

    • White light contains the full spectrum of visible colors.

    • A prism separates white light into its constituent colors.

Additive Colour Theory

  • Additive Colour Theory: States that overlapping component colors can predict observed light color.

    • Red + Green + Blue = White

    • Blue + Green = Cyan

    • Green + Red = Yellow

    • Red + Blue = Magenta

Subtractive Colour Theory

  • Involves the manner in which light reflects off surfaces

  • Surfaces may:

    • Absorb wavelengths: Not observed

    • Reflect wavelengths: Observed color

  • If all light is reflected, white is observed; if all is absorbed, black is observed.

Various Light Production Methods

  • Incandescence: Light from heated objects. Example: Sun at ~5600°C.

  • Fluorescence: Produce light via absorbing UV light; used in lamps.

  • Phosphorescence: Absorb UV light and emit it over time.

  • Chemiluminescence: Emit light from chemical reactions (e.g., glowsticks).

  • Bioluminescence: Light produced by living organisms, often for communication or predation.

  • Electric Discharge: E.g., neon lights produced via gas excitation.

  • Triboluminescence: Light emitted via physical contact/friction.

  • Electroluminescence: E.g., LEDs where electricity directly produces light.

Light Amplification: What is a Laser?

  • Laser: Device generating concentrated, coherent light through "Light Amplification by Stimulated Emission of Radiation".

  • Key Properties:

    • Coherence: Light waves perfectly synchronized.

    • Monochromatic: Emits a single color/wavelength; directional.

Light and Surfaces

  • Light may undergo:

    • Transmission: Passes through a surface.

    • Absorption: Completely stopped and absorbed by the surface.

    • Reflection: Redirected without absorption.

Reflection

  • Types of reflection:

    • Specular: Smooth surfaces where light reflects uniformly.

    • Diffuse: Rough surfaces where light reflects irregularly.

  • Law of Reflection: Angle of incidence equals angle of reflection; \thetai = \thetar.

Ray Diagrams

  • Used to predict image characteristics in mirrors:

    • Size: Same, larger, smaller.

    • Attitude: Upright or inverted.

    • Location: Distance relative to reference points.

    • Type: Real or virtual images.

Plane Mirrors

  • Characteristics:

    • Image Size: Same size as object.

    • Attitude: Upright; laterally inverted.

    • Location: Virtual image located behind the mirror at equal distance.

Curved Mirrors

  • Concave: Converging mirrors that reflect from the inner surface. Applications: Cosmetics, telescopes.

  • Convex: Diverging mirrors reflecting from the outer surface; used in safety and rear-view mirrors.

Mirror Equation

  • Use to calculate distances related to object/image formation in concave and convex mirrors:

    • f = \frac{\delta o \cdot \delta i}{\delta o + \delta i}

Applications of Refraction

  • Refraction applications include:

    • Corrective lenses, optics for binoculars and telescopes, microscopes.

  • Optical illusions like mirages occur due to changes in air's index of refraction.

Lenses

  • Lenses: Transparent materials with curved sides causing light to refract.

    • Converging Lenses (Convex): Bring rays to a common focal point.

    • Diverging Lenses (Concave): Cause rays to spread out.

External and Internal Anatomy of the Human Eye

  • External:

    • Eyelids, eyelashes, pupils, iris, tear ducts, sclera.

  • Internal:

    • Retina, cornea, lens, optic disc.

Cornea and Lens

  • Cornea: Initial site of refraction; transparent and convex.

  • Lens: Further focuses rays onto the retina; adjusts shape via ciliary muscles.

  • Vitreous Humour: Provides structural support; excess can lead to conditions like glaucoma.

Vision Correction

  • Common eye conditions; solutions:

    • Myopia (nearsightedness); corrected with concave lenses.

    • Hyperopia (farsightedness); corrected with convex lenses.

    • Astigmatism; affects multiple focal points.

    • Presbyopia; mainly age-related, needing reading glasses.