opacity (original) (raw)

Author: the photonics expert (RP)

Definition: the property of a substance or an object not to transmit light or at least to attenuate it

Category: general optics

• optical properties of materials
  • absorptance
  • absorption coefficient
  • absorption length
  • birefringence
  • chromatic dispersion
  • dichroism
  • emissivity
  • group delay dispersion
  • group index
  • group velocity
  • nonlinear index
  • opacity
  • optical activity
  • optical density
  • polarization beat length
  • principal dispersion
  • propagation losses
  • refractive index
  • Sellmeier formula
  • thermo-optic effect
  • transparency
  • (more topics)

Related: absorptionscatteringtransparency

Opposite term: transparency or translucency

Units: 1 or m2/kg

Formula symbol: ($\kappa$)

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DOI: 10.61835/80o Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is Opacity?

Opacity is the property of a substance or an object to block light, i.e., to prevent light from getting through it. In contrast, an object transmitting light can be called transparent or at least translucent.

Perfect and Partial Opacity

There can be perfect opacity, i.e., complete blocking of light, or some degree of opacity, meaning that light is more or less attenuated. Partial opacity is in most cases substantially dependent on the considered optical frequency. An object or a material may be completely opaque in certain spectral regions, while well transmitting light in other spectral regions. Light outside the visible spectral range may be considered. For example, semiconductors are opaque in the visible spectral regions while substantially transmitting light in certain regions of the infrared.

Opacity is often understood as a qualitative term, but in some cases it is taken as a quantity. However, there is not a generally agreed definition of what opacity means in a quantitative sense:

• In some cases, the opacity of an object can vary between 0 and 1, the latter meaning complete blocking of light, and 0.8 would mean that 80% of the light is prevented from passing through.
• In other cases, e.g. in astronomy or plasma physics, opacity is considered as a property of the substance, and defined such that the exponential attenuation coefficient is the opacity times the density of the substance.

Physical Origins of Opacity

Opacity can have different physical origins. A typical origin is absorption, but light may also be reflected back or strongly scattered. This shows that blocking of light does not necessarily mean making it disappear; it may only be sent to other directions.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is opacity?

Opacity is the property of a substance or object that prevents light from passing through it. An object can be perfectly opaque, completely blocking light, or partially opaque, where it only attenuates the light to some degree.

What causes opacity?

Opacity has several physical origins. Light can be blocked through absorption, where its energy is converted into other forms within the material. It can also be caused by reflection or strong scattering, which redirect the light away from its original path.

Is a material opaque to all kinds of light?

No, opacity strongly depends on the optical frequency. A material may be opaque in one spectral region (e.g., visible light) but transparent in another. For example, semiconductors are often opaque to visible light but can transmit infrared radiation.

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