Total
internal reflection.
When a ray
of light travelling in denser medium enters into a rarer medium, it bends away
from the normal. If the angle of incident increases, the angle of refraction
incidence, the angle of refraction also increases. For a particular value of
the angle of incidence, the angle of reflection becomes 90 degree. The angle of
incidence that causes the refracted ray in the rarer medium is bend through 90
degree is called critical angle. When the angle of incidence becomes larger than
the critical angle, no refraction occurs. The entries light are reflected back
into the denser medium. This is known as total internal reflection of light.
Applications
of total internal reflection
·
Total
internal reflecting prism: many optical instruments use right- angled prism to
reflect a beam of light through 90 degree or 180 degree, such as camera,
binoculars, periscope and telescope. One of the angles of right- angled prism
is 90 degree. When a ray of light strikes a face of prism perpendicularly, it
enters the prism without a deviation and strikes the hypotenuse at an angle of
45 degree. Since the angle of incidence45 degree is greater than critical angle
of glass which is 42 degree, the light totally reflected by the prism through
the angle of 90 degree. Two such prisms are used in periscope. The light is
totally reflected by angle of 180 degree. Two such prisms are used in
binoculars.
·
Optical
fibers: total internal reflection is used in fiber optics which has number of
advantages in telecommunication field. Fiber o [optics consist of hair size
threads of glass or plastic through which light can travel. The inner part of
the fibre optics is called core that carries the light and an outer concentric
shell is called cladding. The core is made of glass or plastic of relatively
high index of refraction. The cladding is made of glass or plastic but of
relatively low strikes the core- cladding boundary at the angle of incidence
greater then critical angle and is reflected back into the core. In this way light
travels many kilometers with small loss of energy.
In Pakistan
optical fibres were used in telephone and advanced telecommunication systems.
Now we can listen thousands of phone calls without any disturbance.
·
Light
pipes: light pipe is a bundle of thousands of optical fibres bounded together.
They are used to illuminate the inaccessible places by the doctor and
engineers. For example, doctors view inside the human body. They can also be
used to transmit images from one place to another.
·
Endoscope:
an endoscope is a medical instrument used for exploratory diagnostics, and
surgical purposes. An endoscope is used to explore the interior organs of the
body. Due to its small size, it can be inserted through the mouth and thus
eliminates the invasive surgery. The endoscope used to examine the cyst scope
and bronchoscope respectively. An endoscope uses two fabric- optics tubes
through a pipe. A medical procedure using any type of endoscope is called
endoscopy.
· Doctors can see the image of organs in the screen monitors through the help of camera at the end of fabric- optics.
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