Medical Biophysics Lecture Notes PDF

 There are a dual nature of light:  Particle nature.  Wave nature  The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight.  Light is electromagnetic radiation within a certain portion of...

 There are a dual nature of light:  Particle nature.  Wave nature  The word usually refers to visible light, which is the visible spectrum that is visible to the human eye and is responsible for the sense of sight.  Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum.  visible light occupies only a narrow band from about 400- 700 nm.  Reflection Light waves reflect when they strike the surface that separate between two medium.  the law of reflection says very simply that when light is reflected the angle of incidence equals the angle of reflection.  Reflection of light may be specular or diffuse  The first occurs on a blank mirroring surface that retains the geometry of the beams of light.  The second occurs on a rougher surface, not retaining the imaging geometry, only the energy.  Reflectance is the ratio of reflected power to incident power.  Lambert's cosine law says that the total radiant power observed from a "Lambertian" surface is directly proportional to the cosine of the angle θ between the observer's line of sight and the surface normal.  The law is also known as the cosine emission law or Lambert's emission law  Refraction Light waves usually changes directions when it goes from one medium to another because of changes there velocity in the different medium. There is most easily observed for objects partially submerged in water.  When an electromagnetic wave strikes a new medium, they may be partially absorbed and produce an effect (Heat), and the amount of rays absorbed depends on:  1. Wavelength & Frequency  2. Angle of incidence  3. Nature of a medium  4. Intensity of radiation.  The light that is reflected by an object determines the color(s) of that object. Color is NOT caused by the absorption of light by an object.  Law of Grotthus Draper Describes the inverse relationship between the penetration and absorption.  If The Energy Is Not Absorbed It Must Be Transmitted To The Deeper Tissues  The Greater The Amount of Energy Absorbed, the Less Transmitted and Thus The Less Penetration  Simply put, Arndt-Schult law states for every substance, small doses stimulate, moderate doses inhibit, large doses kill.  Put another way, a small stimulus might not have a biological effect, a moderate stimulus may have a biostimulatory effect, whereas a large stimulus may have an bioinhibitory effect.  “Intensityof radiation striking the surface is inversely proportional to the square of the distance” from the source.  Inverse square law can be employed in practice; The closer the source of radiation , the greater the intensity of radiation being received by the skin, the further away, the less the intensity.  The light in medicine is beneficial in diagnostic purposes and therapeutic purposes.  An obvious use of visible light in medicine is to permit the physician to obtain visual information about the patient regarding.  but when she wishes to look into a body opening she is faced with the practical problem of getting light into the opening without obstructing the view. Like a lot of tricks, this one is done with mirrors.  More sophisticated instruments, such as the ophthalmoscope for looking into the eyes and the otoscope for looking into the ears, use basically the same principle.  A number of instruments, called endoscopes, are used for viewing internal body cavities.  Special purpose endoscopes are often given names indicating their purpose.  For example, cystoscopes are used to examine the bladder, proctoscopes are used for examining the rectum, bronchoscopes are used for examining the air passages into the lungs and colonoscopes are used for examining the colon.  Flexible endoscopes can be used to obtain information from regions of the body that cannot be examined with rigid endoscopes, such as the small intestine and much of the large intestine.  Some flexible endoscopes are over a meter in length.  The image obtained with a flexible endoscope is not as good as that obtained with a rigid endoscope, but often the only alternative to a flexible endoscopic examination is exploratory surgery.  Flexible endoscopes usually have an opening or channel that permits the physician to take samples of the tissues (biopsies).  Is the transmission of the light through the tissues of the body.  Transillumination is used clinically in the detection of hydrocephalus (water-head) in infants.  Transillumination is also used to detect pneumothorax (collapsed lung: air leaks into the space between lung and chest wall ) in infants.  Visible light has an important therapeutic use.  Since light is a form of energy and is selectively absorbed in certain molecules, it should not be surprising that it can cause important physiological effects.  Many premature infants have jaundice, a condition in which an excess of bilirubin is excreted by the liver into the blood.  Relatively recently (1958) it was discovered that most premature infants recover from jaundice if their bodies are exposed to visible light (phototherapy)  The wavelengths adjacent to the visible spectrum also have important uses in medicine.  Ultraviolet photons have energies greater than visible photons, while IR photons have lower energies.  Because of their higher energies, UV photons are more useful than IR photons.  1-Ultravioletlight with wavelengths below about 290nm is germicidal, that is, it can kill germs-and it is sometimes used to sterilize medical instruments.  2- Ultraviolet light also produces more reactions in the skin than visible light. Some of these reactions are beneficial, and some are harmful. One of the major beneficial effects of UV light from the sun is the conversion of molecular products in the skin into vitamin D.  4-Ultraviolet light from the sun affects the melanin in the skin to cause tanning. However, UV light can produce sunburn as well as tan the skin.  5- Lens and cornea can be damaged by ultraviolet (UV), it can developed cataracts which destroy the clarity.  The warmth we feel from the sun is mainly due to the IR component.  Heat lumps that produce a large percentage of IR light are often used for physical therapy purposes.  The IR rays are not usually hazardous even though they are focused by the cornea and lens of the eye onto the retina.  However, looking at the sun through a filter (e.g., plastic sunglasses) that removes most of the visible light and allows most of the IR wavelengths through can cause a burn on the retina.  Some people have damaged their eyes in this way by looking at the sun during a solar eclipse.  Dark glasses absorb varying amounts of the IR and UV rays from the sun.