Medical PhysicsModule 1st year medical student
Dr. Nadia H. Sahib
Project: Light in Medicine
Aims:Knowledge of the properties and applications of light also laser as therapeutic techniques.
Introduction:
The speed of light when it goes from one material into another given by equation:
n= c / v ------(1)
c: Speed of light in vacuum
v: Speed of light in material
n: Refractive index
Light behaves both wave and particle:
Wave = interference + diffraction
Particle; absorbed by single molecule.
Light has a number of attractive properties, many of which are used in medicine:
1- Light behaves both as a wave and a particle. As a wave it produces interference and diffraction. As a particle it can be absorbed by a single molecule.
2-When light is absorbed, its energy generally appears as heat. This property is the basis for the use IR light to heat tissues. Also the heat produced by laser beams is used to weld attached retina to the back of eyeball and to coagulate small blood vessels in retina.
3-Some time when a light photon is a absorbed, a lower light energy light photon is emitted. This property is known as fluorescence.
4-The speed of light changes when it goes from one material into another. The ratio of the speed of light in a vacuum to its speed in a given material is called the index of refraction.
5- Light is reflected to some extent from all surfaces. There are two types of reflection:
A-Diffuse reflection: occurs when rough surface scatter the light in many direction.
B- Specular reflection: it is obtained from very smooth surface such as mirrors.
Measurement of Light and Its Unit:
The three general categories of light (UV- Visible- IR) are defined in terms of their wavelengths. Wavelength of light used to be measured in:
Microns 1 μ =10-6 m
Angstroms 1 Aᵒ = 10- 10 m
Nanometer 1 nm = 10-9 m
-Ultraviolet light has wavelengths from 100 to 400 nm.
-Visible light has wavelengths from 400 to 700nm.
-IR light has wavelengths from 700 to 104 nm.
Visible light has energy (2-4 ev).
K.E of molecule in air (0.025 ev).
Visible light is measured in photometric units:
IIIuminance; the quantity of light striking a surface
Luminace; the intensity of a light source
UV and IR radiation can be measured in radiometric units.
Irradiance; the quantity of light striking a surface
Application of Visible Light in Medicine:
- Endoscope: a number of instruments are used for viewing internal body cavities.
- Gyroscopes: are used to examine the bladder.
- Bronchoscope: are used for examining the air passages into lungs. Some endoscopes are rigid tubes with a light source to illuminate area of interest.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 large intestine.
Application of UV and IR light in Medicine:
-UV photons have energies than visible and IR light.
Because of their higher energies, UV photons are more useful than IR photons.
-UV can kill germs and used to sterilize medical instruments.
-UV produces more reaction in the skin some of these reactions are beneficial, and some are harmful.
Beneficial effects of UV light from the sun is the conversion of molecular products in the skin into vitamin D.
Harmful effects of UV light can produce sunburn. Solar UV light is also cause of skin cancer in humans.
Two types of IR photography are used in medicine:
- Reflective IR photography, which uses wavelength of 700 to 900 nm to show patterns of viens just below the skin.
- Emissive IR photography uses the long IR heat waves emitted by the body that give an indication of the body temperature, is usually called thermograph.
Laser in Medicine:
Laser is “light amplification stimulated emission by radiation”.
A laser is a unique light source that emits a narrow beam of light of single wavelength in which each wave is in phase with others near it.
When an electron makes a transition from higher energy to lower energy state, a photon is emitted. The emission process can be one of two types, spontaneous emission or stimulated emission.
- In spontaneous emission, the photon is emitted spontaneously, in a random direction, without external provocation.
- In stimulated emission, an incoming photon stimulates the electron to change energy levels.
To produce stimulated emission, however, the incoming photon must have energy that exactly matches the difference between the energies of two levels, namely.
Laser energy that has been stored in the laser material. The output of pulsed laser is usually measured by the heat produced. Since in medicine lasers are used primarily to deliver energy to tissue, laser energy directed at human tissue causes a rapid rise in temperature and can destroy the tissue.
The amount of damage to living tissue depends on how long the tissue is at the increased temperature.
There are many different types of medical lasers. The two main groups are:
- High Power (Heat)Lasersare used in many surgical procedures to cut, coagulate and evaporate tissues. These lasers are often called surgical lasers because they are used in surgery instead of a scalpel.
- Low Level (Therapeutic) Laserscan be used for the stimulation of cell function. Their biological effect is not thermal, as is the case with surgical lasers.
Questions:
1. List three ways light can interact with tissue.
2. Give the wavelengths of visible, IR and UV light.
3. List three biological effects of UV light.
4. Why does the eye not see IR, UV light?
5. Which type of light responsible for tan the skin and why?
6. List the properties of laser.
7. Give the examples of applications for two types of laser in medicine. What are three ways light interacts with matter?
QUICK ANSWER
- Light interacts with matter through absorption, transmission and reflection.When light strikes the leaf of a plant, the leaf absorbs the light energy, and through the process of photosynthesis, transforms the energy into the food glucose. The leaf reflects the green wavelengths of visible light, giving it the green color.
According to How Stuff Works, reflection occurs when waves are bent back after hitting a surface. Reflection is necessary to vision. All objects reflect some wavelengths of light. The precise wavelengths reflected and absorbed determine the colors of objects.
According to Dictionary, refraction waves are also bent, but they travel through the substance as well as being bent back. In this way, a pool of water that reflects light and therefore shows the image of a face also refracts that light, causing it to give the bottom of the pool a speckled look. The reason for refraction is that waves pass at different speeds through different substances. The wavelengths of light that determine color are broken up during refraction, which creates rainbows.
2.Eyeglass lenses correct refractive errors by focusing light directly on the retina. The type of lens depends on the type and severity of the refractive error.
The strength of a lens (i.e., its refractive power) is measured in optical units called diopters. This measurement indicates how much refractive power the lens must have to focus images directly on the retina. The higher the diopter, the stronger the lens.
The type of refractive error determines the lens's shape. A concave (minus) lens is used to correct myopia (nearsightedness). In myopia, light rays fall in front of the retina rather than on it. Because a concave lens is thin in the center and thicker on the edges, it diverges (spreads out) light rays so that the eye's lens focuses them directly on the retina.
A convex (plus) lens is used to correct hyperopia (farsightedness). In hyperopia, light rays fall behind the retina. The lens is thickest in the center and thinnest on the outer edges. The convex lens converges (concentrates) light rays so that the eye's lens focuses them on the retina.
To correct astigmatism, which is caused by distortions in the shape of the lens or cornea, a cylinder lens is frequently used. The cylinder lens has two refractive powers on one lens. One power is placed over the entire lens and the other is oriented in one direction. This corrects the scattered pattern in which light enters the eye and creates one focal point on the retina.
3.
Small group discussion:
- List three ways light can interact with tissue.
- Man has far point 200 m, what is the type of this refractive error and how we can correct this defect.
- Why does the eye not see IR, UV light?
- Which type of light responsible for tan the skin and why?
- What is the difference between a transmission electron microscope and a scanning electron microscope?