Understanding the Nature of Light

Nature of Light

• Visible light is electromagnetic radiation (EMR)

Light Waves

• EMR is energy propagation by periodic variation of electric and magnetic field strengths caused by accelerating charged particles.
• EMR displays similar behavior to mechanical waves and can travel through a vacuum.
• The speed of EMR propagation, or the speed of light, is 3 × 108 m/s in a vacuum (300,000 km/s or 186,000 mi/s).
• Visible light is EMR between 400 and 700 nm (nanometers), which is between blue and red in the spectrum.

Visible Light and Perception

• Sight occurs because eyes pick up reflected energy from objects and send it to the brain.
• Color depends on the light source and the material's atomic structure, where electrons absorb or re-radiate energy.
• Grass appears green because chlorophyll absorbs blue and red light, reflecting green.
• The sky is blue due to atmospheric dust absorbing all colors except blue.
• Black objects absorb all light, while white objects reflect all incoming light.

Wave-Particle Duality

• Light behaves as both a wave and a particle (photon).
• Evidence for particle nature include being affected by gravity, exerting force and generating photoelectric effect.
• Evidence for wave nature include reflection, refraction, dispersion into spectral components, polarization and exhibiting the Doppler effect (red shift).

Reflection of Light

• Reflection occurs when light encounters a boundary and bounces off the surface without energy absorption
• The incoming light is known as the incident wave, and the bounced light is known as the reflected wave.
• The law of reflection dictates that the angle of incidence equals the angle of reflection.
• Angles are measured against the normal, which is perpendicular to the reflective surface.

Refraction

• Refraction occurs when light waves change velocity and direction as they pass from one transparent medium to another.
• The angle of refraction depends on the material's density and how much the light bends through the substance.
• Angle "i "is the angle of incidence, and angle "r" is the angle of refraction with respect to the normal.

Refractive Index

• Refractive index (n) indicates a substance's density and how much it slows down and bends light
• The higher the refractive index, the denser the material.
• Can be determined by the formula: n = (speed of light in a vacuum)/(speed of light in a material)

Snell's Law

• Snell's Law explains the deviation of a light ray's path as it crosses a boundary between two substances
• Expressed as: n = (sin θ1) / (sin θ2)

Refractive Indices of Transparent Substances

• Air: 1.00029
• Diamond: 2.42
• Glass: 1.6 (approx)
• Ice: 1.31
• Water: 1.33
• Jet fuel: 1.45 (approx)

Total Internal Reflection

• As the angle of incidence increases, less energy is refracted, and more is reflected.
• At the critical angle, 100% of light is reflected, leading to total internal reflection.
• Critical angle is a key concept in the design of fiber optic cables.

Dispersion

• The index of refraction varies with the radiation's wavelength.
• White light entering a prism separates into its component colors because each wavelength refracts differently
• Dispersion in raindrops causes rainbows.

Polarization

• Polarization occurs when waves are aligned in a single plane
• Passing a rope through a vertical-slit, only vertical waves pass through
• A second slit with the same orientation has no effect on the polarized wave.

Polarizing Filters

• Two slits at 90° to each other completely absorb all wave motion
• Polarizing sunglasses reduce glare by blocking energy except what is aligned with the filter

Plane Mirrors

• Mirrors create images through reflection off of a plane surface.

Image Creation

• Extended objects create images based on the principle that the angle of incidence equals the angle of reflection
• Binocular vision is required to perceive depth.

Spherical Mirrors

• Spherical mirrors are curved surfaces cut from a sphere

Concave Mirrors

• Different areas of these mirrors reflect the wave according to the local angle of incidence.

Convex Mirrors

• Convex mirrors cause waves to diverge, behaving as if there is a focus behind the mirror
• Driving and security mirrors use convex shapes to increase coverage
• Parabolic mirrors are required for accurate focusing.

Focal Length

• Focal length is the distance where light from infinity is focused
• Approximated by: f ≈ R/2

Lenses

• Lenses use refraction and light is bent passing through transparent material
• The velocity of light changes, creating a refractive index (n)
• Can be determined by: n = (speed of light in a vacuum)/(speed of light in a material)
• Or: n = (sin θ1)/(sin θ2)
• Optical instruments have two surfaces and refract according to Snell's Law

Types of Lenses

• Converging/positive (convex)
• Diverging/negative (concave)

Focal Length of a Lens

• Focal length (f) is the distance from the lens where parallel rays are focused

Image Types

• Real images are points where light converges
• Virtual images are where light appears to have converged
• Real images occur with objects placed outside the focal length of a converging lens or mirror (magnified/inverted)
• Virtual images are formed by diverging lenses (upright, diminished)
• Spherical aberration occurs when a convex lens does not focus light at a single focal point (less expensive lenses)

Fibre Optics

• Fibre optics involves thin strands of high-quality glass that can carry a large amounts of data through internal reflection
• Telecommunications use it to transmit data in light pulses at about two-thirds the speed of light
• Multiple optical fibres form an optical fibre cable.
• Used in medicine for flexible inspection probes with low-heat light sources
• Pure glass cables allow light to be visible over many kilometres, with a thickness comparable to human hair
• Lasers switch on/off to send billions of digital bits per second
• Multiple lasers of different colours can send multiple signals on the same fibre
• Capable of carrying a signal across 100 km and saves weight in aircraft.