About photons & electrons
- Photons are fundamental particles and are considered to be the primary constituent of visible light and all other types of electromagnetic radiation.
- Photons are considered to be pure energy, they have no mass or electric charge and are dimensionless.
- Since photons exist at the subatomic level alongside other fundamental particles such as electrons, quarks, leptons and gauge bosons, they exist outside the range of our everyday perception.
- However, when photons interact with electrons, the effects can be seen by the human eye.
- For example, when light travels through air or water, it interacts with the electrons of the atoms and molecules it encounters, causing different wavelengths to scatter in various directions. This scattering of light is visible to the human eye when we see the blue colour of the sky or the red colour of a sunset.
- If electrons had eyes, they would likely perceive photons as disturbances in the electric and magnetic fields that pervade the space around them.
- In the event of a collision between a photon and an electron, the interaction might lead the electron to absorb the energy of the photon and transition to a higher energy state or to emit a new photon as it returns to a lower energy state.
- These types of interactions are frequently described as “excitation” or “de-excitation” processes and can happen without any significant release of energy or the production of any visible effects.
Contemporary quantum mechanics brings another perspective to the way a photon interacts with an electron.
- Photons are the carriers of electromagnetic force.
- When a photon interacts with an electron, the interaction can take many different forms, depending on the energy and nature of the photon, and the specific properties of the electron and the surrounding environment.
- In some cases, the interaction can cause the electron to take in the energy of the photon and move to a higher energy level, or to give off a new photon as it returns to a lower energy level.
- The interaction between a photon and an electron must be described in terms of the probabilistic nature of quantum mechanics because the precise outcome of the interaction cannot be predicted with certainty. Only the probabilities of different outcomes can be calculated.
- Other useful concepts that quantum mechanics brings to this interaction are:
- Particle-Wave Duality: As both particles and waves, photons and electrons can be described using wavefunctions that give the probability of finding the particle at any point in space. The interaction isn’t deterministic, where you can exactly predict the outcome, but probabilistic, meaning you can calculate the likelihood of different outcomes.
- Energy Levels and Quantum Jumps: The absorption of a photon’s energy by an electron is probabilistic. The electron has a certain probability to absorb the energy and “jump” to a higher level. If the electron is in a superposition of states (a fundamental concept in quantum mechanics), it may or may not absorb the photon depending on how the wavefunction collapses upon measurement.
- Wavefunction: This provides the probabilities of the outcomes of measurements of a quantum system. It essentially describes everything that can be known about a quantum system. The act of measuring these properties causes the wavefunction to collapse to a particular state.
- Wavefunction collapse: When a measurement is made on a quantum system, the wavefunction “collapses” into a state that is consistent with the outcome of the measurement. This state is one of the possible states that the system could have been in before the measurement, according to the superposition principle.
- Superposition: According to quantum mechanics, particles can be in multiple positions and states simultaneously until a measurement is made, a property known as superposition.
- Until a measurement is made, a particle doesn’t have a definite position but rather exists in a spread of possible positions.
- Similar to positions, a quantum particle can exist in a superposition of other states. For example, an electron in an atom can exist in a superposition of spin states (spin up and spin down), or a superposition of energy states.
- Uncertainty Principle: Because of the inherent uncertainty in position and momentum, the exact outcome of an interaction between a photon and an electron can’t be predicted with absolute certainty. We can only talk about probabilities.
- Probability and Wavefunction Collapse: The interaction between the photon and the electron could result in various outcomes, each with its own probability, defined by the wavefunction. The exact outcome isn’t known until the wavefunction collapses upon measurement. This is inherently a probabilistic event.
- Quantum Entanglement: If a photon and electron become entangled, the state of one particle will instantly affect the state of the other, no matter how far apart they are. This correlation is perfectly deterministic, but the outcome of measurements on each individual system is probabilistic, giving rise to the paradoxical nature of entanglement.
A photopic curve is a graphical representation of the sensitivity of the human eye to light under well-lit conditions, such as during the day or in brightly lit environments.
- The photopic curve appears as a line graph that illustrates how sensitive the human eye is to different wavelengths (colours) of light in these bright conditions. This curve is essential for understanding colour perception and visual acuity in bright light. It shows the minimum amount of light required for the eye to detect various wavelengths.
- This information is derived from the response of our cone cells, which are responsible for colour vision and function optimally in bright light.
- Closely related to a photopic curve is a scotopic curve is a graphical representation of the sensitivity of the human eye to light under low-light conditions, such as at night or in very dimly lit environments.
- The scotopic curve also resembles a line graph that shows how sensitive the eye is to light in these low-light conditions. It is an important tool for understanding night vision. The curve illustrates the minimum amount of light needed for the eye to detect different wavelengths (colours) of light.
- This information comes from the response of our rod cells, which are more active in low light compared to the cone cells that dominate in bright conditions.
- It is interesting to note that the scotopic and photopic curves use different units to measure light. The scotopic curve uses units related to light intensity per unit area (such as brightness per square degree), whereas the photopic curve uses units similar to overall brightness.
The retinal pigment epithelium (RPE) is a layer of pigmented cells located between the retina and the choroid of the human eye that supports the photoreceptor cells (rods and cones).
- The RPE plays a critical role in providing nutrients, removing waste products, and regenerating visual pigments needed for photoreceptor function.
- The RPE is firmly attached to the underlying Bruch’s membrane of the choroid on one side, but less firmly connected to the photoreceptor cells of the retina on the other. This weaker attachment can contribute to retinal detachment.
- The choroid is a vascular layer rich in blood vessels and connective tissue that lies between the retina and the sclera. It provides oxygen and nutrients to the outer layers of the retina and parts of the sclera, supporting the function of the retinal pigment epithelium and photoreceptors.
Pigment epithelium
Pigment epithelium is a layer of cells at the boundary between the retina and the eyeball that nourish neurons within the retina. It is firmly attached to the underlying choroid is the connective tissue that forms the eyeball on one side but less firmly connected to retinal visual cells on the other.
A pixel is the smallest addressable element in a digital image that can be uniquely processed and is defined by its spatial coordinates and colour values.
- A pixel, or a picture element, is a physical point in a digital image and the smallest addressable element of a display device.
- During editing, a pixel is the smallest controllable element of a digital image.
- Many digital displays, including LCD screens, contain LEDs arranged in a grid pattern and emit light when an electrical current is passed through them, allowing them to display different colours and brightness levels.
- OLED displays use a different technology that uses organic compounds that emit light when an electrical current is passed through them.
- The RGB colour model is commonly used for still images displayed on digital screens, such as computer monitors and televisions.
- In the RGB colour model, each pixel is composed of three sub-pixels that control the red, green, and blue colour channels.
- By varying the light emitted by an LED, every pixel can display a wide range of colours and shades, allowing for the creation of highly detailed and vibrant images on-screen.
The Planck constant is a fundamental constant of nature that is denoted by the symbol h.
- The Planck constant is a measure of the smallest possible amount of energy that can be carried by a single quantum of electromagnetic radiation (a photon).
- The Planck constant is also related to the wavelength of a photon by the equation E = hf, where E is the energy of the photon, f is its frequency, and h is the Planck constant.
- The equation, energy (E) = Planck constant (h) x frequency (f), allows the quantity of energy associated with electromagnetic radiation to be calculated if the frequency is known.
- The Planck constant is used extensively in modern physics, particularly in the fields of quantum mechanics, atomic physics, and condensed matter physics.
- It plays a crucial role in determining the energy levels of atoms and molecules, as well as the behaviour of subatomic particles such as electrons and photons.
- The value of the Planck constant is approximately 6.626 x 10^-34 joule-seconds (Js).
Polarization of electromagnetic waves refers to the direction in which they oscillate, perpendicular to the direction of the wave’s propagation.
- Polarization can be induced in light waves by various means, such as reflection, refraction, and scattering.
- There are several types of polarization, including circular, elliptical and plane polarization.
- Circular polarization refers to waves that rotate in circles as they propagate, with the electric and magnetic fields perpendicular to each other.
- Elliptical polarization combines linear and circular polarization, in which the wave oscillates in an elliptical pattern.
- Plane polarization (sometimes called linear polarization) refers to waves that oscillate in a single plane, such as waves that are vertically or horizontally polarized.
Polychromatic refers to something that contains or displays multiple colours. In various contexts, this can describe anything from art and design to objects in nature that reflect or emit a variety of colours.
- In the context of light, polychromatic refers to light that contains multiple wavelengths, each corresponding to a different colour in the visible spectrum. For example, white light is polychromatic because it is a combination of all the colours (wavelengths) within the visible spectrum.
- Sunlight is a perfect example of polychromatic light, as it contains all the visible wavelengths that combine to form white light. When this light interacts with the atmosphere or objects, it can be scattered or reflected to produce a range of colours.
- In a rainbow, polychromatic sunlight is dispersed through water droplets, splitting into its constituent colours across the visible spectrum, producing the familiar bands of red, orange, yellow, green, blue, indigo, and violet.
- The opposite of polychromatic is monochromatic, which refers to light composed of only one wavelength, producing a single colour or hue.
Potential energy is energy in storage. When potential energy is released it becomes kinetic energy.
- Potential energy can be converted into other forms, such as kinetic energy, which is the energy of motion.
- Potential energy is not currently being used, but it has the potential to do work in the future.
- Potential energy comes in different forms such as:
- Chemical potential energy is the energy stored in the bonds between atoms and molecules in a substance, such as the energy stored in food.
- Elastic potential energy is the energy stored in an object when it is compressed or stretched, such as a spring.
- Electric potential energy is the energy stored in an electric field due to the position of charged particles, such as the energy stored in a battery.
- Gravitational potential energy is the energy an object has due to its position in a gravitational field, such as a ball held up in the air.
In physics, power is defined as the rate at which work is done. So power describes how quickly energy is transferred from one system to another when work is done.
- In mathematical terms, power is defined as the amount of work done per unit of time.
- Power measures how quickly energy is generated or used.
- When work is done on an object, energy is transferred to or from it, and power measures how rapidly this transfer occurs.
- Essentially, power describes the efficiency or speed of energy conversion processes.
