Colour, optical fibres and lasers
Colour describes light and perception
Visible light spans a range of frequencies, and its perceived colour depends on the spectrum entering the eye and the observer's visual response. White light can contain many wavelengths rather than being one special . A prism separates components because refractive index depends on , producing dispersion. In an introductory visible-light comparison, red has a longer than violet in the same medium. All electromagnetic waves travel at the same vacuum , although and differ. Do not infer that red photons travel faster through vacuum because their wavelengths are larger.
Adding coloured light
Additive mixing combines light reaching the eye from different sources. The conventional additive primaries are red, green and blue. Red plus green can appear yellow, green plus blue cyan, and red plus blue magenta. Suitable balanced contributions of all three can appear white. This describes perception of combined spectra, not the creation of one new monochromatic . A display produces a wide range of appearances by adjusting its subpixels. The actual result depends on spectral output and relative intensity, so school mixing diagrams are idealised models rather than guarantees for arbitrary lamps or pigments.
Subtracting components from illumination
Pigments and filters work mainly by absorbing some spectral components and transmitting or reflecting others. The conventional subtractive primaries are cyan, magenta and yellow. An ideal cyan filter removes red while transmitting green and blue; magenta removes green, and yellow removes blue. Combining ideal cyan and yellow filters transmits their shared green component. Combining nonoverlapping ideal red and blue filters in series can transmit almost no light. Real filters have imperfect transmission bands. Do not apply the rules for adding projected light to layers of absorbing paint without explaining which physical process is involved.
Objects do not every observed colour
An opaque object's appearance depends on what wavelengths illuminate it and what it reflects. An ideal red object under white light reflects red components and absorbs much of the rest. Under an ideal blue-only source, it can look dark because the incident light lacks the wavelengths it efficiently reflects. A white object reflects a broad range of incident visible light and can appear coloured under coloured illumination. A black object reflects relatively little visible light. These descriptions are simplified: surface texture, fluorescence and actual spectra can affect observed results, so identify any ideal assumptions in an examination question.
prepares the fibre model
When light crosses a stationary boundary, its remains unchanged while its and can change. Refractive index is. Snell's law uses angles from the normal. For a ray moving from higher to lower refractive index, the refracted ray bends away from the normal. The critical angle is the incidence angle for which the refracted ray runs along the boundary. If n₁ >. At incidence greater than c, total internal reflection occurs under the ordinary transparent-boundary model.
Conditions for total internal reflection
Two conditions matter: the ray must approach a boundary from the higher-index medium, and its angle from the normal must exceed the critical angle. A large incidence angle from lower to higher index is not enough. At exactly the critical angle, the ideal refracted ray is along the boundary; reserve total internal reflection for incidence beyond it. For glass of index 1.50 beside air of approximately 1.00, about 41.8°. A question may give an angle with the surface instead; convert it to its complement before using the formula.
Guiding light inside a fibre
An optical fibre has a transparent core surrounded by lower-index cladding. Appropriate rays can undergo repeated total internal reflection at their interface and remain guided. The cladding also helps define the optical boundary and protects the core environment. For core index 1.50 and cladding index 1.46, the internal critical angle is about 76.7° from the interface normal. This is not the external acceptance angle at the fibre end. The entry geometry determines which rays can remain guided, and a full wave description distinguishes allowed modes. The ray model is useful when its limits are recognised.
Practical fibre advantages and losses
Fibres can carry information through modulated light and can guide illumination for inspection. They are electrically insulating and can offer high information capacity, but they are not lossless. Absorption, scattering, imperfect connections, bending and pulse spreading can reduce performance. Excessively tight bends can increase leakage from the guided path. An endoscope uses optical guidance for illumination and imaging or associated camera systems, while communication systems encode information into light signals. Do not assume one introductory diagram explains every modern fibre instrument. State the relevant advantage in relation to a specific problem, such as electrical interference or access to a confined space.
From atomic transitions to amplification
An can absorb a photon whose matches an allowed transition to a higher state. In spontaneous emission, an excited system releases a photon without an incoming photon triggering that event. In stimulated emission, suitable incoming radiation triggers emission with a corresponding , phase and propagation relationship in the ideal description. A laser uses amplification by stimulated emission together with an and an optical arrangement that supports selected modes. Pumping establishes conditions for amplification, commonly described with population inversion. A laser does not create without input, and ordinary fluorescence is not automatically laser action.
What laser light is like
Lasers can produce highly directional, relatively narrow-spectrum and coherent light. Coherence concerns a stable phase relationship, not simply brightness. Real lasers have finite linewidth and beam divergence; they are not infinitely thin, perfectly single- rays. Their output may be continuous or pulsed, and laser radiation can also lie outside the visible range. Applications include optical communication, measurement, manufacturing and selected medical procedures. The useful property depends on the task: directionality can support precise aiming, while coherence can support interference measurements. A label saying laser does not specify the , output or safe operating conditions.
Compare mechanisms and work safely
A colour filter selects from existing light; it does not add missing wavelengths as an ideal passive component. A fibre guides light using its refractive-index structure; it does not require every internal ray to travel along the axis. A laser supplies amplified radiation from an energised system; it is not defined merely by red colour. For numerical work, label indices, angle references and the medium from which incidence occurs. For qualitative answers, connect cause to observation. Never look into a laser beam or fibre output, including an apparently dark one, and use only supervised approved classroom demonstrations. Invisible optical radiation can still present a hazard.