OPTICAL INSTRUMENTSLECTURE BY PROF.ARDAMAN SIDHUHKS INSTITUTE OF SCIENCESIN COLLABORATION WITH WIZIQ.COM : OPTICAL INSTRUMENTSLECTURE BY PROF.ARDAMAN SIDHUHKS INSTITUTE OF SCIENCESIN COLLABORATION WITH WIZIQ.COM hksidhuinstitute@gmail.com 09814123832
OPTICAL INSTRUMENTSL 11/AS 1st,AUG,09 7.00 P.M. : OPTICAL INSTRUMENTSL 11/AS 1st,AUG,09 7.00 P.M. THE EYE.
THE
MICROSCOPE.
TELESCOPE.
OPTICAL INSTRUMENTS : OPTICAL INSTRUMENTS The optical instruments are the devices which help human eye
in observing highly magnified images of tiny objects, for detailed examination,
in observing very far off objects terrestrial or astronomical.
THE EYE : THE EYE THE ESSENTIAL PARTS :
Crystalline lens: The image of an external object is formed by the crystalline lens
Ciliary muscles
IRIS : IS a contractable diaphragm In front the lens. The function of iris is to adjust and allow suitable quantity of light into the eye.
PUPIL It is a circular aperture, near the centre of Iris .
Slide 5 : CORNEA:
The front transparent part of the hard outer surface of the eye ball is called cornea.
RETINA:
The retina is a film of nerve fibers covering the curved back surface of the eye. crystalline lens forms image on the retina
EYE – ESSENTIAL PARTS : EYE – ESSENTIAL PARTS The retina contains RODS AND CONES, which sense light intensity and colour respectively.
They transmit electrical signals via the OPTIC NERVE to the brain, which finally processes this information.
The least sensitive spot on the retina is called the blind spot.
Slide 7 : The average refractive index ? of crystalline lens is 1.43 and average refractive index of aqueous humour and vitreous humour is 1.337.
Slide 8 : The ability of the eye to observe distinctly the objects situated at widely different distances from the eye. This property of the eye is called accommodating power or accommodation of the eye.
Slide 9 : It become possible by changing the curvature of crystalline lens and hence its focal length by a change in tension in the cilliary muscles holding the lens. For example, when the muscle is relaxed, focal length of the lens is about 2.5 cm and objects at infinity are in sharp focus on the retina.
accommodation of the eye : accommodation of the eye The most distant point which an eye can observe clearly is called Far Point (F) of the eye. For a normal eye, F lies at infinity.
The closest distance for which lens can focus light on the retina is called the least distance of distinct vision or the distance of Near point (N) of the eye.
ACCOMMODATION OF THE EYE : ACCOMMODATION OF THE EYE For a normal eye, value of least distance of distinct vision, d = 25 cm. This distance increases with age, because of decreasing effectiveness of the cilliary muscles and loss of flexibility of the eye lens.
The distance between near point and far point of the eye is called Range of vision.
DEFECTS OF VISION : DEFECTS OF VISION The common optical defects of the eye are:-
Myopia or short sightedness.
Hypermetropia or long sightedness.
Presbyopia.
Astigmatism.
Slide 13 : Myopia or short sightedness is that defect of human eye by virtue of which, the eye can see clearly the objects lying near it, but the far off objects cannot be seen distinctly
Slide 14 : Parallel rays from infinity are focused on the retina by the normal eye. However, the defective eye focuses rays from a near point F on the retina. And the image of far of object is formed before the retina
CAUSES FOR MYOPIA : CAUSES FOR MYOPIA The two possible causes of this defect are:
(i) Increase in size of the eye ball i.e. distance of retina from the eye lens increases.
(ii) Decrease in focal length of the eye lens, when the eye is fully relaxed.
CURE OF MYOPIA : CURE OF MYOPIA To correct a myopic eye of this defect, the person has to use spectacles with a concave lens of suitable focal length.
HYPERMETROPIA OR LONG SIGHTEDNESS : HYPERMETROPIA OR LONG SIGHTEDNESS Hypermetropia or long sightedness is that defect of human EYE, by virtue of which the eye can see clearly the far off objects, but the nearby objects cannot be seen clearly.
In other words, for a hypermetropic eye, the near point shifts away from the eye. It is no longer at 25 cm, the least distance of distinct vision.
CAUSE AND CORRECTNESS OF HYPERMETROPIA : CAUSE AND CORRECTNESS OF HYPERMETROPIA CAUSE
(i) Increase in the focal length of the eye lens, when the eye is fully relaxed.
To correct
a hypermetropic eye of this defect, the person has to use spectacles with a convex lens of suitable focal length.
PRESBYOPIA (OLD SIGHT) : PRESBYOPIA (OLD SIGHT) With increasing age, the cilliary muscles holding the eye lens weaken and the lens loses some of its elasticity. Therefore, power of accommodation of the eye decreases with age. This defect is called presbyopia.
CORRECTION OF PRESBYOPIA : CORRECTION OF PRESBYOPIA To remove this defect, converging spectacle lenses are employed(as in the case of hypermetropia) Bifocal lenses are used.
ASTIGMATISM : ASTIGMATISM To a normal; eye, all the lines in fig.
Look equally black. But an astigmatic eye will find variation in the intensity of different lines.
CAUSES OF ASTIGMATISM : CAUSES OF ASTIGMATISM This defect arises when cornea has different curvature in different directions.
Light focuses in more than one point on the retina causing blurry and distorted vision at all distances
ASTIGMATISM CORRECTED : ASTIGMATISM CORRECTED Astigmatism can be corrected by using a cylindrical lens of suitable radius of curvature, and suitable axis.
SIMPLE MICROSCOPE OR MAGNIFYING GLASS : SIMPLE MICROSCOPE OR MAGNIFYING GLASS A simple microscope is used for observing magnified images of tiny objects. It consists of a converging lens of small focal length. A virtual, erect and magnified image of the object is formed at the least distance of distance of distinct vision from the eye held close to the lens
SIMPLE MICROSCOPE : SIMPLE MICROSCOPE An object AB is held between optical centre C and principal focus F of the lens perpendicular to the principal axis.
A virtual, erect and magnified image A’B’ is formed as traced in Figure.
The eye is held close to the lens and CB’ = d, least distance of distinct vision for the normal eye.
MAGNIFYING POWER OF A SIMPLE MICROSCOPE : MAGNIFYING POWER OF A SIMPLE MICROSCOPE Magnifying power of a simple microscope is defined as the ratio of the angles subtended by the image and the object on the eye, when both are at the least distance of distinct vision from the eye.
Slide 27 : Let
Imagine the object AB to be displaced to A1 B’ at distance d. Let
MAGNIFICATION OF SIMPLE MICROSCOPE : MAGNIFICATION OF SIMPLE MICROSCOPE
USES OF MAGNIFYING GLASS : USES OF MAGNIFYING GLASS A simple microscope or magnifying glass is used:
(i) by watch makers and jewelers for having magnified view of tiny parts of watch and fine jeweler work.
(ii) by students in Science laboratories for reading vernier scales etc.
COMPOUND MICROSCOPE : COMPOUND MICROSCOPE A compound microscope is an optical instrument used for observing highly magnified images of tiny objects.
Construction. : Construction. A compound microscope consists of two converging lenses (or lens systems); an objective lens O of very small focal length and short aperture and an eye piece E of moderate focal length and large aperture.
The two lenses are held co-axially at the free ends of a tube, at a suitable fixed distance from each other.
The distance of the objective lens from the object can be adjusted by rack and pinion arrangement.
MAGNIFYING POWER OF A COMPOUND MICROSCOPE : MAGNIFYING POWER OF A COMPOUND MICROSCOPE Magnifying power of a compound microscope is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye by the object, when both the final image and the object are situated at the least distance of distinct vision from the eye.
MAGNIFYING POWER OF A COMPOUND MICROSCOPE : MAGNIFYING POWER OF A COMPOUND MICROSCOPE Magnifying power, …(5)
For small angles expressed in radians,
Tan
MAGNIFICATION : MAGNIFICATION where
and
Now, me = …(8)
As the object AB lies very close to F0, the focus of lens O, therefore,
Discussion. (i) As magnifying power (m) is negative, the image seen in a microscope is always inverted i.e. upside down and left turned right.
ASTRONOMICAL TELESCOPE : ASTRONOMICAL TELESCOPE As astronomical telescope is an optical instrument which is used for observing distinct images of heavenly bodies like stars, planets etc.
ASTRONOMICAL TELESCOPE : ASTRONOMICAL TELESCOPE It consists of two lenses ( or lens systems), the objective lens O, which is of large focal length and large aperture and the eye piece E, which has a small focal length and small aperture. The two lenses are mounted co-axially at the free ends of the two tubes. The distance between these lenses can be adjusted using a rack and pinion arrangement.
In normal adjustment of telescope, the final image is formed at infinity. The course of rays in normal adjustment of telescope is shown in Fig.
Magnifying Power : Magnifying Power Magnifying Power of an astronomical telescope in normal adjustment is defined as the ratio of the angle subtended at the eye by the final image to the angle subtended at the eye, by the object directly, when the final image and the object both lie at infinite distance from the eye.
Magnifying power,
Magnifying Power : Magnifying Power As angle ? and ? are small, therefore,
Negative sign of m indicates that final image is inverted.
Slide 39 : Thus to increase magnifying power of an astronomical telescope in normal adjustment focal length of objective lens should be large and focal length of eye lens should be small.
For Real Adjustment:- : For Real Adjustment:- Angular magnification produced by the telescope =
=
Discussion: : Discussion: As magnifying power is negative, the final image in an astronomical telescope is inverted i.e. upside down and left turned right.
(iii) In normal setting of telescope, final image is at infinity, Magnifying power is minimum. When final image is at least distance of distinct vision, magnifying power is maximum. Thus
To have large magnifying power, must be as large as possible and must be as small as possible.
In a telescope, aperture of objective lens is made large to increase magnifying power and resolving power of the telescope.
REFLECTING TYPE TELESCOPE (CASSEGRAIN TELESCOPE) : REFLECTING TYPE TELESCOPE (CASSEGRAIN TELESCOPE) It is an improvement over the refracting type astronomical telescope described above. In such a telescope, objective lens is replaced by a concave parabolic mirror of large aperture, which is free from chromatic and spherical aberrations. The image formed is much brighter and the reflecting type telescope has much higher resolving power compared to the refracting type telescope.
Slide 43 : Casasegrainian type telescope is shown.C is a parabolic concave reflector of about 200 inch aperture with a narrow hole at the centre.
MAGNIFYING POWER : MAGNIFYING POWER In normal adjustment, magnifying power of a reflecting type telescope is given by
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Newtonian reflecting type telescope. : Newtonian reflecting type telescope. Concave reflector C on to a plane mirror M. This mirror is inclined at an angle of 45° to the axis of C.
Advantages: : Advantages: Advantages of a reflecting type telescope
high resolution astronomy.
There is no chromatic aberration as the objective is a mirror.
Spherical aberration is reduced using mirror objective in the form of a paraboloid.
Image is brighter compared to that in a refracting type telescope.
Mirror requires grinding and polishing of only one side.
High resolution is achieved by using a mirror of large aperture. : High resolution is achieved by using a mirror of large aperture.
Slide 48 : A mirror weighs much less than a lens of equivalent optical quality. Therefore, mechanical support of mirror is much less of a problem compared to the support required for lens. Further, mirror can be supported over its entire back surface unlike the lens which is supported over the rim only.
The largest reflecting telescopes in the world are the pair of Keck telescopes in Hawaii, U.S.A. Reflecting mirror has a diameter of 10 meter. : The largest reflecting telescopes in the world are the pair of Keck telescopes in Hawaii, U.S.A. Reflecting mirror has a diameter of 10 meter.
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Slide 51 :