CHAPTER 14- REFRACTION OF LIGHT AND OPTICAL INSTRUMENT
Refraction of Light
"The change in the direction and velocityof light as it enters from
one medium to another is known as Refraction of Light."
Laws of Refraction
*.The incident ray, refracted ray and the normal at the point of
incidence all lie in the same plane.
*.The ratio of sine of angle of incidence (i) to the sine of angle of
refraction (r) is constant for all rays of light from one medium to
another. This constant is known as Refractive Index (u). This ratio is
also equal to the ratio of the speeds of light in one medium to
another.
Refractive Index = sin<i/sin<r = Speed of light in first medium/Speed
of light in second medium
Refractive Index
The ratio between the sine of the angle of incidence to the sine of
angle of refraction is known as Refractive Index.
Refractive Index = sin <i/sin<r
Snell's Law
The refractive index between two particular mediums is equal to the
ratio of speed of light in first medium and speed of light in second
medium equal to the ratio between sin <i and sin <r.
Refractive Index = sin<i/sin<r = Speed of light in first medium/Speed
of light in second medium
1. Emergent Ray:
The ray after passing the second mediumcomes again in the first
medium. It is called emergent ray.
2. Emergence Angle:
The angle formed by the emergent ray and normal is called emergence
angle denoted by <e.
3. Optical Center:
The middle point of the lens is called optical center. The ray passing
through this point does not bend.
4. Accommodation:
The ability of the eye to change the focal length of its lens so as to
form a clear image of an object on its retina is called is power of
accommodation.
5. Persistence of Vision:
When an object is seen by an eye, its image forms on retina. If the
object is removed, the impression of image persists in the eye for
about 1/10 second.This interval is called Persistence of Vision.
6. Power of Lens:
The power of the lens is the reciprocal of the focal length measured
in meter. Its unit is Dioptre.
Prism
"Prism is a transparent piece of glass. It has three rectangular sides
and two triangular sides.
Refraction Through a Prism
(Diagram)
where,
*.<i = angle of incidence
*.<r = angle of refraction
*.<e = angle of emergence
*.<D = angle of deviation
Total Internal Reflection:
(Diagram)
If the value of angle of incidence is increased so much so that it
becomes greater than tht of the critical angle then no more refraction
occurs but on the other hand refracted ray again comes back in the
denser medium. Actually at that time, the surface of denser medium
acts as a plane mirror and the incident ray bends in the same medium.
This phenomenon is called Total Internal Reflection. It is used in
Periscope, Optical Fibers and other instruments.
Total Reflecting Prism:
Total internal reflection is used in prism. In prism the angle between
two opposite sides is 90 and other two angles are 45 each. If we
arrange a ray sothat it falls perpendicular to the AB side then it
will refract without bending and strike the side AC with angle 45.
Then it totally reflects to the side BC.
Conditions for Total Internal Reflection
*.The ray of light should travel from denser to rarer medium.
*.The angle of incidence should be greater than the critical angle.
Lenses
A transparent and smooth glass or any refracting medium surrounded by
two spherical surfaces is known as lens.
Types of Lenses
There are two types of lenses:
1. Convex Lens:
If the glass is thick at the center and thin at the edges then it is
known as convex lens. It is a converging lens.
(Diagram)
It has three types:
*.Double Convex Lens
*.Plano Convex Lens
*.Concavo Convex Lens
2. Concave Lens:
If the lens is thinner in the center and thicker at the edges then it
is known as aconcave lens. It is a diverging lens.
(Diagram)
It has three types:
*.Double Concave Lens
*.Plano Concavo Lens
*.Convex Concave Lens
Formation of Image by Convex Lens
1. Object at Infinity:
When object is placed at infinite distancefrom convex lens the rays
coming from the object are parallel to each other and they meet after
refraction at the focus.
Details of Image:
*.Formed at Focus
*.Real
*.Inverted
*.At opposite side
*.Highly diminished
2. Object Beyond 2F:
When object is placed at some distance from 2F then image is formed
between the focus and center of curvature (2F).
Details of Image:
*.Between F and 2F
*.Opposite side of Lens
*.Real
*.Inverted
*.Small in size
3. Object at 2F:
When object placed at center of curvature, image is formed at center
of curvature at the opposite side.
Details of Image:
*.Real
*.Inverted
*.At 2F
*.Same in size
*.At the opposite side of the Lens
4. Object between F and 2F:
When object is placed between the focusand center of curvature then
the image isformed on opposite side beyond the center of curvature.
Details of Image:
*.Real
*.Inverted
*.Large in size
*.Opposite side of lens
*.Beyond 2F
5. Object at F:
When object is placed at focus the refracted rays are parallel to each
other and meet at infinity.
Details of Image:
*.Real
*.Inverted
*.Extremely Large
*.Opposite side of Lens
*.At infinity
6. Object between F and O:
When object is placed between the lens and principal focus, then the
refracted rays does not meet at opposite side but image is formed at
the same side where the object is placed.
Details of Image:
*.Virtual
*.Erect
*.Large
*.Same side of lens
*.Beyond the object
Optical Instruments
1. THE EYE:
(Diagram)
Functions of the Parts of Eye
1. Sclera Sclerotic:
It is a layer enclosed in cavity filled with afluid called Vitreous
Humor. It is the outer coating of eye.
2. Choroids:
It is a dark membranous coating. This is coated with black pigments.
It keeps the inner parts of the eye ball light proof.
3. Retina:
It is semi-transparent membranes of nerve fibers forming the innermost
coating of the eye and sensitive to light. It is a screen on which
image is formed.
4. Cornea:
It allows light into the eyes. It is transparent and bulging in shape.
5. Iris:
It is like diaphragm of a camera. It has a tiny opening at its center
called pupil, which regulates the quantity of light entering the eye.
6. Crystalline Lens:
This is a lens that automatically contracts and expands, alters the
focal length of eye lens.
7. Ciliary Body:
It holds crystalline lens in the proper position.
8. Aqueous Humor and Vitreous Humor:
The place between cornea and the lens isfilled by a transparent liquid
called Aqueous Humor. The large chamber of the eye between the lens
and the back ofeye is filled with a jelly like substance called
Vitreous Humor. These liquids serve mainly to keep the spherical shape
of the eye.
Main Defects of Eye
1. Short Sightedness (Myopia)
If a person can see object placed near, but cannot see distant object,
this defect is known as short sightedness.
Causes:
This defect appears due to increase in thickness of eyeball. The focal
length decreases making the image to form before retina.
(Diagram)
Removal of Defect:
It is removed by using a concave lens of suitable focal length.
(Diagram)
2. Long Sightedness (Hypermetropia)
If a person can see distant objects, but not near objects, this defect
is called longsightedness.
Causes:
This defect appears due to decrease in thickness of ball. The focal
length increases so that the image is formed beyond the retina.
(Diagram)
Removal of Defect:
It is removed by sing a convex lens of suitable focal length.
(Diagram)
3. Astigmatism
It is the defect in which the clear image of an object does not form
on the retina.
Causes:
This defect appears due to non-sphericity of the cornea.
Removal:
This defect can be removed by using lenses of different focal length.
4. Presbyopia
The accommodation power of eye loses by which a person suffers a long
sightedness. This defect is called Presbyopia or Lack of
Accommodation.
Causes:
This defect appears due to loss of accommodation power of the lens of the eye.
Removal:
This defect can be removed by using convex lens.
2. CAMERA
A camera is an optical device for obtaining still photographs or for
exposing cinematic films.
Construction:
It consists of a light proof box with a lensat one end and a
photographic plate or film at other end and a shutter to control the
light rays.
Working:
To make an exposure, the shutter is opened and an image is formed by
lens on the photographic plate or film, small in size. Photographic
plate or film saves this image. In this way an image is obtained.
3. COMPOUND MICROSCOPE
Construction:
It consist of two convex lenses at the endof two tubes. One tube can
slide into other so that the distance between them can be change. The
lens near the object is the small convex lens of short focal length is
called objective. The lens near the eye is the larger convex of longer
focal length is called eyepiece.
(Diagram)
Working:
The object is placed between F and @F and its real, inverted and
magnified image A'B' is formed. The eyepiece is brought close to it so
that it comes within its focal length. The first image A'B' acts as an
object and a virtual, erect and magnified final image A"B" is formed.
The magnification of a microscope can be varied by using different
objectives.
4. ASTRONOMICAL TELESCOPE
It is used to see heavenly bodies.
Construction:
It consists of two convex lenses at the end of the two metallic tubes.
One tube can slide into other so that the distance between can be
changed. The lens near the object is a convex lens of longer
focallength called the objective, while the lensnear the eye is a
small convex lens of shorter focal length called the eyepiece.
(Diagram)
Working:
The rays from distant object entering theobjective and form a real,
inverted and diminished image A'B' near the principal focus. The
eyepiece is adjusted so that the image formed by the objective
comeswithin its focal length. Thus the eyepieceacts as a magnifying
glass and a virtual, erect and magnified image A"B" is formed by the
first image.
Difference between Real Image and Virtual Image
*.Real Image:
1. Real image is formed when rays after reflection actually meet at a point.
2. Real image is inverted and can be seenon a screen.
3. It has a physical existence.
*.Virtual Image:
1. Virtual image is formed when rays do not actually meet but appear
to diverge from a point.
2. Virtual image is erect and cannot be seen on a screen.
3. It does not have a physical existence.
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