Visible Light: Waves and Particles

• Visible light is electromagnetic radiation detectable by the human eye.
• Light is considered wave-based but evidence suggests it is composed of particles with mass (photons).

Evidence that Light is Composed of Particles:

• Light is affected by gravity, bending around large planets, implying it has mass.
• Light exerts a force, such as from the sun, causing the deflection of comet tails.
• Light can generate a photoelectric effect.

Evidence that Light is Composed of Waves:

• Light can be reflected and refracted.
• Light can be dispersed.
• Polarization occurs, with Polaroid lenses blocking one plane of light waves.
• The Doppler effect (red shift) can be observed in light.

Reflection of Light:

• Reflection occurs when waves encounter a boundary, do not absorb radiation's energy, and bounce off the surface.
• The incoming wave is the incident wave.
• The wave bounced from the surface is called the reflected wave.
• The law of reflection states the angle of incidence equals the angle of reflection.
• Angles are measured against a line perpendicular to the reflective surface, called the 'normal'.

Refraction:

• When light waves pass from one transparent medium to another, their velocity and direction change.
• The angle of refraction depends on the density of the material through which light passes.
• When light travels from air to water, it slows down and bends towards the normal.
• Substances have a refraction index (n), indicating their density, how much light slows down, and how much light bends.
• A higher refractive index number means the material is denser, and light will slow down and refract or bend more passing through it.

Total Internal Reflection:

• As the incidence angle (i) increases, less energy is refracted and more is reflected.
• At the Critical Angle, 100% of light is reflected, resulting in Total Internal Reflection.
• Total Internal Reflection is important in the design of fiber optic cables.

Dispersion:

• The index of refraction varies with the wavelength of the radiation.
• When white light enters a prism, different wavelengths of the component colors are refracted by different amounts, termed dispersion.
• A rainbow is the cumulative effect of sunlight being dispersed through raindrops.

Images, Real and Virtual:

• Real images are where light actually converges.
• Virtual images are locations from where light appears to have converged.
• If a convex lens does not focus light at a single focal point, the image will not be sharp, termed spherical aberration, common in less-expensive lenses.
• Sometimes the human eye does not focus images well on the retina, needing spectacles, contact lenses, or corrective surgery.

OPTICAL FIBRE CABLE:

• An optical fiber is a thin strand of high-quality glass, where very little light is absorbed.
• Light entering one end is totally internally reflected, even when the fiber bends.
• Optical fibers are used in telecommunication, transmitting vast amounts of information in light pulses at about 2/3rds the speed of light.
• Multiple optical fibers combine to form an optical fiber cable.
• Fiber optics are used in medicine in flexible inspection probes, carrying a low-heat light source and transmitting images back to an eyepiece or video screen.
• An optical fiber consists of three parts: outer coating or buffer, core, and cladding.
• Outer coating or buffer: protects from physical damage and is typically plastic.
• Core: made of glass, light propagates along core.
• Cladding: made of glass or plastic, has a lower index of refraction, reduces light loss into surrounding air, and reduces scattering loss.

Losses in optical fibre cables:

• Attenuation: loss of optical power as light travels along the fiber, induced by imperfections in the structure, impurities, and contamination.
• Attenuation limits the distance an optical signal (pulse) can travel.
• Bending Losses: bending the fiber causes attenuation, classified by bend radius or curvature: microbend or macrobend.
• Microbends: small microscopic bends, likened to dents in the cladding and core.
• Macrobends: bends having a large radius of curvature.
• Macrobend losses occur if fibers are bent too sharply.