Photomultiplier Tubes & Refraction

• Photomultiplier tubes (PMTs) are vacuum tubes used to detect and multiply weak light signals.
• PMTs utilize the photoelectric effect, where photons strike a photocathode, releasing electrons.
• These electrons are then accelerated and multiplied through a series of dynodes, creating a measurable electrical signal.
• PMTs are highly sensitive detectors, capable of detecting single photons.
• PMTs are used in various applications: astronomy, medical imaging, and high-energy physics experiments.

Refraction

• Refraction is the bending of light (or other waves) as it passes from one transparent medium to another.
• The change in speed of light causes the bending.
• The refractive index of a medium determines how much light will bend when entering/exiting it.
• Snell's Law quantifies refraction: n1sinθ1 = n2sinθ2, where n is the refractive index and θ is the angle of incidence/refraction.
• Refraction is responsible for phenomena like lenses focusing light.
• Refraction is responsible for mirages.

Reflection

• Reflection is the change in direction of a wave at an interface between two different media, returning the wave back into the original medium.
• Specular reflection occurs on smooth surfaces, where the angle of incidence equals the angle of reflection.
• Diffuse reflection occurs on rough surfaces, scattering light in many directions.
• The law of reflection states that the angle of incidence equals the angle of reflection.
• Mirrors utilize specular reflection to create images.

Interference

• Interference occurs when two or more waves overlap in space.
• Constructive interference happens when waves are in phase, resulting in a wave with a larger amplitude.
• Destructive interference happens when waves are out of phase, resulting in a wave with a smaller amplitude or complete cancellation.
• Interference patterns are observed in phenomena like thin films (e.g., oil slicks) and diffraction gratings.
• Young's double-slit experiment demonstrates the interference of light waves, showing the wave nature of light.

Diffraction

• Diffraction is the bending of waves as they pass through an aperture or around an obstacle.
• Diffraction is more pronounced when the size of the aperture or obstacle is comparable to the wavelength of the wave.
• Diffraction gratings use diffraction to separate light into its component wavelengths.
• Single-slit diffraction creates a central bright fringe with weaker fringes on either side.
• Diffraction limits the resolution of optical instruments.

Absorption

• Absorption is the process by which a substance takes in energy from electromagnetic radiation (light) or other forms of energy.
• During absorption, the energy of the photon is transferred to the absorbing material, usually exciting electrons to higher energy levels.
• The absorbed energy can be re-emitted as heat or as light of a different wavelength (fluorescence or phosphorescence).
• Absorption spectra show which wavelengths of light are absorbed by a substance.
• Different materials have different absorption characteristics, which gives them their color.

Transmittance

• Transmittance is the fraction of incident electromagnetic radiation (light) that passes through a substance.
• Transmittance is often expressed as a percentage.
• Transmittance is the opposite of absorbance.
• Transparent materials have high transmittance.
• Opaque materials have low transmittance.
• Transmittance depends on the wavelength of the light and the properties of the material.

Scattering

• Scattering is the process by which particles or irregularities within a medium redirect electromagnetic radiation (light) in various directions.
• Rayleigh scattering is scattering of electromagnetic radiation by particles of a wavelength much smaller than the wavelength of the radiation.
• Rayleigh scattering is responsible for the blue color of the sky.
• Mie scattering occurs when the size of the particles is similar to the wavelength of the radiation.
• Scattering can cause blurring in images and reduce the intensity of light.

Polarization

• Polarization is a property of transverse waves that describes the direction of the oscillations.
• Light is typically unpolarized, meaning its electric field oscillates in random directions.
• Polarizing filters selectively transmit light with a specific polarization direction.
• Polarization can be achieved through reflection, refraction, scattering, or absorption.
• Polarized light is used in sunglasses to reduce glare and in LCD screens to create images.

Emission

• Emission is the process by which a substance releases energy in the form of electromagnetic radiation (light).
• Emission occurs when an electron transitions from a higher energy level to a lower energy level.
• The energy difference between the levels determines the wavelength of the emitted light.
• Emission spectra show the wavelengths of light emitted by a substance.
• Examples of emission: incandescent light bulbs, lasers, and light-emitting diodes (LEDs).

Coherence

• Coherence describes the correlation between the phases of waves.
• Coherent waves have a constant phase relationship in time and space.
• Lasers produce highly coherent light.
• Incoherent light, such as that from an incandescent bulb, has random phases.
• Coherence is essential for interference and holography.