Dictionary tag: O

• The optical density of a material is not directly related to its physical density.
• The higher the optical density of a material, the slower light travels through it.
• The lower the optical density of a material, the faster light travels through it.
• A vacuum is not a medium and has zero optical density.
• Light travels through a vacuum at the maximum possible speed of light which is 299,792 kilometres per second.
• Optical density and refractive index are related properties.
  • In general, materials with higher optical density tend to have higher refractive indices and vice versa.
  • The greater the difference in refractive index between two materials, the more they will bend light when they come into contact.

Physical illusions

Physical illusions result from the limitations and assumptions of the human visual system when interpreting the external world. Examples include:

• The Sun and Moon appear larger near the horizon as a result of the brain’s interpretation of distance cues.
• Rainbows are composed of a continuous range of wavelengths across the visible spectrum but appear to be formed from a series of bands of colour.

Physiological illusions

Physiological illusions are often connected with the different attributes of visual perception and occur when visual stimuli are beyond our brain’s processing ability.

Physiological illusions arise due to the way that the human eye and visual system process information from the outside world, such as lighting, contrast, and colour. Examples include:

• After-images occur when the eye’s photoreceptor cells become fatigued due to overstimulation, resulting in an image appearing after the stimulus is removed.
• Moiré patterns occur when two similar patterns with slightly different frequencies overlap, creating a new pattern that appears to move or vibrate.

Cognitive illusions

Cognitive illusions result from the brain’s inability to correctly interpret visual information, leading to uncertainties or errors in perception. Examples include:

• Ambiguous illusions are images that can be read in more than one way, depending on contextual cues and the viewer’s past experiences. They often cause a perceptual “switch” between alternative interpretations.
• Geometrical illusions occur when the brain uses contextual cues and assumptions to interpret visual stimuli, leading to distortions in size, length, position, or curvature.
• Paradox illusions occur when visual stimuli contain conflicting information that cannot be resolved by the brain, leading to a perceptual paradox.
• Fictions are created when the brain fills in missing visual information based on contextual cues and past experiences, leading to the perception of additional content that is not actually present.

• Optics is the branch of physics that studies the behaviour and properties of light, including visible, ultraviolet, and infrared light.
• Visible, ultraviolet, and infrared light, along with X-rays, microwaves, and radio waves, are all examples of electromagnetic radiation.
• Many optical phenomena can be explained using the classical electromagnetic theory that describes light in terms of waves.
• Geometric optics describes light as travelling in straight lines and changing direction when passing through or reflecting from surfaces. These phenomena can be analysed using ray diagrams.
• Ray diagrams are useful when explaining the workings of everyday objects such as mirrors, lenses, telescopes, microscopes, lasers, and fibre optic devices.
• Some optical effects such as diffraction and interference can be explained in terms of the particle-like properties of photons and with reference to the field of quantum mechanics.
• About photons:
  • A photon is a fundamental particle of light.
  • It is the smallest unit of electromagnetic radiation.
  • It has no mass, but it does have energy and momentum.
  • Photons travel at the speed of light in a vacuum (299,792,458 meters per second).

• Optics studies the behaviour of electromagnetic radiation in the visible, ultraviolet, and infrared regions of the electromagnetic spectrum.
• Some fields of optics also study the behaviour and properties of other forms of electromagnetic radiation such as X-rays and microwaves.
• The observation and study of optical phenomena offer many clues as to the nature of light.
• Optical phenomena include absorption, dispersion, diffraction, polarization, reflection, refraction, scattering and transmission.
• Optics explains the appearance of rainbows, how light reflects off mirrors, how light refracts through glass or water, and why light separates into a spectrum of colours as it passes through a prism.

Contemporary optics

• Most optical phenomena can be accounted for using the classical electromagnetic description of light (wavelength, frequency and intensity) but they can also be modelled as particles called photons.
• Optics is both a field of physics and an area of engineering. It has been used to create many useful devices, including eyeglasses, cameras, telescopes, and microscopes. Many of these devices are based on lenses, which can focus light and produce images of objects that are larger or smaller than the original.
• New discoveries are being made in the field of optics For example, The first working fibre-optic data transmission system was demonstrated in 1965. Less than 60 years later, fibre optics are now used to send vast amounts of data through thin optical fibre around the world.
• Contemporary specializations within the field of optics include:
  • Geometrical optics is a branch of optics that deals with the behaviour of light as a collection of rays that propagate in straight lines and are subject to reflection and refraction.
  • Physical optics is a branch of optics that describes the behaviour of light as both a wave and a particle and includes wave phenomena such as diffraction and interference that are not explained by geometrical optics.
  • Quantum mechanics is a branch of physics that describes the behaviour of light as both a wave and a particle and investigates the interactions between light and matter.

• Oscillation is a characteristic of waves, including electromagnetic waves.
• Examples of oscillation include the side-to-side swing of a pendulum and the up-and-down motion of a spring with a weight attached.
• Electromagnetic waves oscillate due to the transmission of energy by their electric and magnetic fields.
• An oscillating movement is typically around a point of equilibrium and the motion repeats itself around an equilibrium position.