Dictionary tag: C

• The principal task of rod and cone cells alike is photo-transduction.
• Photo-transduction enables pigmented chemicals to sense light and convert it into electrical signals.
• Cone cells are most concentrated towards the macula and densely packed in the fovea centralis, but reduce in number towards the periphery of the retina.
• There are believed to be about 120 million rod cells and 6 million cone cells in the human retina.
• Cone cells in human eye take three forms, long-wavelength cones (L = red), medium-wavelength cones (M = green) and short-wavelength cones (S = blue) .

• A continuous spectrum of light is produced by a light source that emits photons over a continuous range of wavelengths.
• The spectral colour model deals with a continuous spectrum, it presents colours in a strip, arranged according to their wavelengths, from red at one end to violet at the other.
• Sunlight is usually described as a continuous spectrum of colours that make up the visible spectrum with red at one end and violet at the other.
• In reality, the spectrum of sunlight is not entirely continuous, but has dark lines called absorption lines. These lines correspond with specific wavelengths at which light is absorbed by elements in the Sun’s atmosphere.
• The component colours of white light become visible to an observer when the light is dispersed by a prism or a raindrop.
• The colours produced by the RGB colour model and the CMY colour model are usually displayed in the form of a colour wheel rather than a strip of colours.

• The CMB is the oldest known form of radiation and is considered to be evidence for the Big Bang theory of the formation of the Universe.
• With a standard optical telescope, the background space between stars and galaxies is almost completely dark. However, a sufficiently sensitive radio telescope can detect the CMB as a faint glow that is not associated with any star, galaxy, or other object.
• The CMB was initially composed of extremely high-energy gamma rays. However, as the universe expanded and cooled, these gamma rays have been red-shifted, meaning that their wavelengths have been stretched. Today, the CMB appears as microwave radiation.
• The CMB is detected as a faint glow of uniform thermal energy coming from all parts of the sky.
• The CMB is a relic of the Big Bang, dating back to about 13.8 billion years ago in look-back time.
  • The phrase look-back time refers to the time it takes for light to travel from its point of origin to our here-and-now.
  • This means that the radiation that we see today from the CMB was emitted from a point in time that is 13.8 billion light-years away.
  • However, the light from the CMB may have taken far longer than 13.8 billion years to reach us, because we believe that the universe is expanding.
  • However, we do not know the exact rate of expansion of the universe during the time since the CMB was emitted.
  • It is therefore not possible to know for sure how long the light has taken to reach us or for that matter, the distance it has travelled.

• Coulomb’s law states that the magnitude of the electrostatic force between two point charges, q1 and q2, is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them. In simpler terms, the stronger the charges (either positive or negative), the closer they are and the stronger the force between them.
• The expressions in Coulomb’s law are:
  • Force: It describes the electrostatic force, which can be either attractive or repulsive depending on the charges involved. Like charges repel each other, while opposite charges attract.
  • Charge: The force is directly proportional to the product of the magnitudes of the two charges (q1 and q2). In simpler terms, the greater the charges, the stronger the force.
  • Distance: The force is inversely proportional to the square of the distance (r) between the charges.
• So, as the distance between the charges increases, the force weakens rapidly, following an inverse-square relationship.
• Formula: This relationship can be expressed mathematically by the following equation:
• F = k * q1 * q2 / r^2
  • F:  Represents the electrostatic force, measured in Newtons (N). It signifies the push or pull exerted between two charged particles due to their electrical interaction. The direction of the force depends on the signs of the charges: like charges repel each other, while opposite charges attract.
  • k: This is Coulomb’s constant, a fundamental constant in electromagnetism. It has a value of approximately 8.99 x 10^9 N m^2/C^2 and serves as a proportionality constant relating the force to the charges and distance.
  • q1 and q2: TRepresent the magnitudes of the two charges, measured in Coulombs (C). The force increases as the magnitude of either charge increases. Remember, charge can be positive or negative, and the signs matter for determining the attractive or repulsive nature of the force.
  • r:  Represents the distance between the two charges, measured in meters (m). The force weakens with increasing distance, following an inverse-square relationship. As the distance doubles, the force decreases by a factor of four, and so on.
  • r^2: This is the square of the distance. The force is inversely proportional to this value, emphasizing the rapid decrease in force with increasing distance.

Coulomb’s law and electric fields

• Coulomb’s law deals with electrically charged particles and the force they exert on other objects, whilst the electric field describes the resulting influence of that force on surrounding space.
• The strength and direction of the electric field produced by one or more charged particles can be calculated from Coulomb’s law using a mathematical calculation.
• The following information can be gathered:
  • Magnitude: This refers to the strength of the electric field at a specific point in space. It reflects the intensity of the force experienced by a test charge placed at that point. The calculation typically results in a value with units of Newtons per Coulomb (N/C).
  • Direction: The electric field is a vector field, meaning it has both magnitude and direction. The calculation based on Coulomb’s law allows you to determine the direction of the electric field at a specific point. This direction points away from a positive charge and towards a negative charge.
  • Distribution: Depending on the complexity of the charge arrangement, you might be able to understand the spatial distribution of the electric field. For example, for a single-point charge, the field weakens with the inverse square of the distance from the charge. More complex configurations might require advanced calculations or numerical methods to visualize the field distribution.
  • Force on other charges: Once you know the strength and direction of the electric field at a point, you can use that information to calculate the force that would be exerted on another charge placed at that point. This is achieved by simply multiplying the field strength by the magnitude of the other charge.

What is a test charge?

• A test charge is:
  • Tiny and neutral: They have negligible mass and no inherent electric charge, minimizing their influence on the field.
  • Respond easily: They are designed to easily react to the electric field, indicating its presence and characteristics.
  • Examples: In reality, test charges can be extremely small charged particles like electrons or protons, but in the real world are more likely to be specialized instruments designed to detect electric fields, such as electroscopes.

• On a wave at sea, the crest of a wave is a point where the displacement of water is at a maximum. A trough is the opposite of a crest, so a trough is a point where the displacement of the water is at a minimum, meaning the lowest point of the wave.
• In an electromagnetic wave, the terms “crest” and “trough” refer to points of maximum and minimum values of the wave’s oscillating fields. A crest corresponds to the maximum value (in the positive or negative direction), and a trough corresponds to the minimum value (in the opposite direction).
• So electromagnetic waves consist of oscillating electric and magnetic fields that are perpendicular to each other and to the direction of wave propagation.

• Wavelength (λ): It is the distance between two consecutive points that are in phase on the wave, such as from crest to crest or trough to trough. This distance represents one complete cycle of the wave.
• Frequency (f): It is the number of wave cycles that pass a specific point per unit of time, typically measured in hertz (Hz), where one hertz equals one cycle per second.
• Energy: The energy carried by a wave is related to both its amplitude and frequency.
  • For mechanical waves (like water waves or sound waves), the energy is proportional to the square of the amplitude. This means if you double the amplitude, the energy increases by a factor of four.
  • For electromagnetic waves, the energy is directly proportional to the frequency. Higher-frequency waves (like X-rays) carry more energy than lower-frequency waves (like radio waves).

• Internal reflection is a common phenomenon that occurs with all types of electromagnetic radiation, including visible light.
• Internal reflection takes place when light reaches the boundary between a medium with a higher refractive index and a medium with a lower refractive index.
• So, internal reflection takes place when light travels from glass to air at an angle greater than the critical angle, but not when it travels from air to glass.
• The amount of internal reflection depends upon the angle of incidence as light approaches the boundary. Here are the different outcomes that result from different angles of incidence:
  • At 0 degrees angle of incidence, there is no internal reflection; the light passes straight through the boundary without deviation.
  • As the angle of incidence increases, more and more light is internally reflected at the boundary. This means that less is refracted and so progressively less crosses the boundary into the medium with the lower refractive index.
  • At the critical angle, the light grazes the boundary, and all of it is internally reflected, resulting in no refraction into the second medium.
  • Beyond the critical angle, total internal reflection occurs, and the light is entirely reflected back into the first medium.
• Here is an example. If light travels from water to air where the critical angle is about 48.6 degrees.
  • This means that if light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of less than 48.6 degrees, the angle of incidence determines how much light is internally reflected.
  • If light reflects off a fish in a fish tank and then strikes the surface of the water at an angle of 48.6 degrees or greater, it will be totally internally reflected and no light will pass out of the water and into the air.
• In reality, light is usually partially refracted and partially reflected because of irregularities in the surface at the boundary. This causes differences in the angle of incidence at different points across the boundary.

• Crown glass is produced from a mixture of sand, soda ash, and lime.
  • The sand provides the silica, which is the main component of glass.
  • The soda ash provides the sodium oxide, which lowers the melting point of the glass.
  • The lime provides the calcium oxide, which strengthens the glass.
  • The potassium oxide is added to give the glass its characteristic optical properties.
• Crown glass has a relatively low refractive index, which means that it bends light less than other types of glass. This makes it ideal for lenses that need to transmit a lot of light, such as camera lenses and microscope lenses.
• Crown glass also has low dispersion, which means that it bends different colours of light by the same amount. This makes it ideal for lenses that need to produce sharp images, such as telescope lenses and binoculars.
• The term crown glass is sometimes used to refer to other optical glasses with similar properties, such as flint glass and borosilicate glass.
  • Flint glass has a higher refractive index and produces more dispersion than crown glass.
  • Borosilicate glass has a low coefficient of thermal expansion, which makes it resistant to shattering when it is heated.