Light optics and LESAR By Prof Ahmed Mostafa
Light, optics and LESAR By Prof. Ahmed Mostafa Professor of anesthesiology and ICU
• The spectrum of electromagnetic radiation covers many different forms of radiation, ranging from lowfrequency radio waves (as used in MRI) at the lower end of the spectrum to γ rays at the high-frequency end. • Light is a type of electromagnetic wave forming a narrow band of frequencies that are defined as those frequencies detectable by the human retina (it is in a range of 390 – 750 nm).
• Wavelength of light: The lowest frequencies of visible light are dark red, with wavelengths of around 700 nm, while the highest frequencies visible are violet, with wavelengths of around 400 nm. • Speed of light: - The speed of light in a vacuum is a fundamental constant in physics, which has a defined value of 300000 km per second. - The speed of light decreases with the density of the medium. Thus light travels more slowly in air and is even slower in glass.
Properties of light Refraction Reflection Diffraction Dispersion
Refraction of light
Refraction of light • A consequence of the changes in the speed of light between different media is that the path of light is ‘bent’ when it travels across a boundary from one medium to another. • The refraction of the light can be quantified using two angles, the angle of incidence (i) and the angle of refraction (r). The deviation produced is dependent on the ratio of the speeds of light in air and glass (c 1 and c 2 respectively), which will be a constant.
Snell’s law • States that for light travelling between two given media sin i / sin r = c 1/ c 2 • The refractive properties of a medium are measured by its absolute refractive index (n), which is defined by Refractive index = speed of light in a vacuum / speed of light in medium n = c / c 1
Snell’s law • When considering light passing from air to a medium, the value of the refractive index relative to air is virtually the same as its absolute value, because the refractive index of air is 1. 0003 (i. e. the speed of light in air is almost the same as in a vacuum). Snell’s law then becomes sin I / sin r = 1 / n • Refraction results in the apparent distortion of images and distances, when viewing objects in one medium from another. This is why a pool of water appears shallower than it actually is. Refraction is also responsible for the actions of lenses, including that in the eye. The refractive index for glass is typically 1. 5, for water n = 1. 33.
Reflection of light
Reflection of light When a light ray is reflected from a boundary between two media or from a surface, the geometry is again defined by the angles made with the normal to the surface. In this case the angle of incidence (i) is always equal to the angle of reflection (r). Reflection is used in mirrors, and in the design of dish aerials or mirrors to focus light or other waves.
Total internal reflection
Total internal reflection • When light passes from a dense medium to a less dense medium (e. g. glass to air) Snell’s law will only apply over a range of angles. This is because the angle of refraction is greater than the angle of incidence, the light being deviated away from the normal. • As the angle of incidence increases, a value is reached when the angle of refraction becomes 90◦. This value of the angle of incidence is called the critical angle (C). In this case, applying Snell’s law: sin C / sin 90◦= 1/ n
Total internal reflection • When the angle of incidence exceeds C, total internal reflection occurs. This is used in the construction of prisms to guide light in optical equipment, and also in the use of optical fibres to conduct light in fibreoptic equipment. The critical angle for glass is approximately 42◦.
Diffraction of light • It is the bending of waves around the corners of an obstacle or aperture into the region of geometrical shadow of the obstacle. • Diffraction occurs with all waves, including sound, water, electromagnetic waves.
Diffraction of light
Dispersion of light • It is the splitting of light into different colours when it passes through a lens or a prism.
LASER Laser is an acronym derived from Light Amplification by Stimulated Emission of Radiation and is applied to devices which emit a special form of light radiation.
Characteristics of LESER Monochromatic All radiated waves are in phase Narrow beam (Not diverge) High light energy intensities
Laser construction Source of energy A suitable LESAR substance to raise the electrons from Capable of the ground state stimulated to an excited one emission (pumping) A system of mirrors To reflect light repeatedly backwards and forwards through the laser substance
Laser construction
Many substances can act as laser materials
LASER safety • Lasers are classified according to their degree of hazard from class 1 (least dangerous) to class 4 (most dangerous). • Domestic lasers (CD players, laser printers) are safe because of the wavelengths used and their low power. • However, all surgical lasers are class 4, being inherently hazardous as they are specifically designed to damage tissue.
LASER safety Safety precautions when working with lasers include: - Appropriate training for all staff. - A designated suitably equipped area with all exposed surfaces matt finished. - All instruments with matt finish. - No inflammable material in the vicinity of the patient or in the operating field.
LASER safety Safety precautions when working with lasers include: - All theatre staff must wear protective eye glasses, and the patient’s eyes and skin must be protected against stray laser light. - The laser theatre must be well ventilated with a suitable smoke extraction system. - Precautions against use of inflammable or explosive anaesthetic gases.
Thank you Prof. Ahmed Mostafa
- Slides: 26