UPSC  >  Plane & Curved Mirror, Refraction, Lenses, Defects of Vision, Optical Instruments

# Plane & Curved Mirror, Refraction, Lenses, Defects of Vision, Optical Instruments - Notes | Study Science & Technology for UPSC CSE - UPSC

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Laws of Reflection: (i) The angle of incidence (with the normal) is equal to the angle of reflection (made with the normal). (ii) The two rays (incident and reflected) and the normal lie in the same plane. In multiple images, formed in a thick plane mirror, the second image will be the brightest, while the other will be less bright. When an object is placed between two inclined mirrors, the number of images formed depend on the following formula:

No. of images =

Applications of Plane Mirror

Kaleidoscope: It is a toy in which multiple images are formed by two strips of plane mirrors placed at an angle of 60° inside a tube. Small, bright-coloured glass pieces are scattered on a ground-glass plate at the bottom of the tube. When viewed from other end of the tube, beautiful symmetrical patterns, formed by the coloured glass pieces and their five images, are seen.
Periscope: It consists of two plane mirrors, fixed facing one another at an angle of 45° to the line joining them. The user is enabled to see over the heads of a crowd or over the top of any obstacle. Periscope used in submarines are made of prisms instead of mirrors and the tube supporting them incorporates a telescope to extend the range of vision.

Pepper’s ghost: It is a method of producing the illusion of a ghost on the theathrical stage.

Curved Mirror

Concave Mirror: When held quite near to the face, it gives an erect, magnified image, and hence it is used in shaving or make up and to diagnose a disease. The first telescope made by Sir Issac Newton was of concave mirrors.

Convex mirrors: A convex mirror always produces a virtual, erect and smaller image of the object. As they diverge the light, they give a wide range of view, hence used in vehicles as rear window or reflector of street light.

Parabolic mirror: This is a concave mirror whose section is the shape of a parabola. It is used as reflectors in search lights, head lights of motor vehicles and reflecting astronomical telescopes.

Refraction

When a ray of light goes from rarer to denser medium it bends towards the normal and if light goes from denser to rarer medium it goes away from the normal.
During refraction, bending of light is due to the change in the speed of light in media of different densities.The speed of light in vacuum, denoted by C, is nearly 3 x 108 m/sec. In air it is slightly (0.03%) less than C, and in glass it is nearly 0.66 C.

The Refractive Index of a medium

Speed of light in vacuum / Speed of light in the medium

Examples of refraction of light:

(i) Stone or any object lying at the bottom of a pond appears to be at higher point than it actually is.

(ii) Ponds appears shallower than it actually is.

(iii) A man standing on the bank of pond appears giant to fishes, conversely, a fish inside water appears shortened to a man.

(iv) Due to refraction through varying densities of atmosphere sun is visible for several minutes after it has set below or it appear elliptical when near the horizon or, twinkling of stars.

Since planets are nearer to us, the light received from them is much greater. Therefore, minor variations in intensity caused by the above effect are not noticeable.
Thus planets do not appear to twinkle.

Total Internal Reflection

The phenomenon occurs when a ray of light passes from an optically denser to a rarer medium, the refracted ray is bent away from the normal, so that the angle of refraction is always greater than the angle of incidence. If the angle of incidence is increased until the angle of refraction is 90° then the angle of incidence is called the critical angle. When the angle of incidence exceeds the critical angle, total internal reflection occurs.

Formation of mirage in hot deserts and looming in cold desert are due to total internal reflection.

An optical fibre of high quality glass, which follows the principle of total internal reflection, finds various uses—

(i) In producing decorative table lamps;

(ii) Used by doctors to see inside of a patient’s stomach;

(iii) In telecommunications.

Due to wave character, light shows diffraction, and due to diffraction, no image formed by even the most ideal mirrors or lens can be perfect.Parallax: While moving in a train or vehicle, the apparent shift between relative position of two objects is due to the phenomenon Parallax.

 Fibre Optics In an optical fibre communication, since there is a model dispersion signals can travel only.In India, the first trial of the fibre optical communication (FOC) system was made in Pune in 1979 by the P & T Deptt.Though there are many applications for the FOC systems, India at present is concentrating on mass communication application.FOC can provide access to a very large number of TV channels and could carry simultaneously multiple channels of hi-fi sound and data.Advantages of Fibre Optic Transmission It has immunity to electro-magnetic interference; equipment is of light weight; components have small diameters; ground loops and cross talks are totally eliminated.They are proof against signal tapping or interception; possess high temperature tolerance and large band width.Disadvantages of fibre-optic Transmission Fibres are made of ultrapure glass and must be protected and strengthened for rough installation and maintenance.It requires more components than the conventional cable system and sophisticated fault detection techniques.The operating life of high power light sources is not high at present.E-Mail is generic name for non-interactive communication of text, data image or voice messages between sender and receiver.Example: Telegraph, telex, facsimile, voice mail and computerbased messaging systems.

Lenses

A transparent medium bounded by two non-parallel curved surface or by one plane surface and one curved surface is called a lens.

Principally of two types—concave and convex.

Power of a lens is defined as the reciprocal of the focal length in meters.

Power of a lens = 1/Focal length of the lens (inmetres)

Power of a lens is expressed in Dioptre (D). The power of a convex lens is positive a concave lens is negative.

Lens formula. Lens formula is the relation between the distance (u) of an object and the distance (v) of its image from the optical centre of the lens and the focal length (f) of the lens after applying the sign convention, i.e.,

Magnification. A linear magnification produced by a lens is equal to the ratio of the image distance to the object distance.

m = v/u

 Nature of LensConvexDiminishedConvexConvexConvexConvexfiedConvexConcaveConcave Position of Real Object AtBetween 2F and InfinityAt 2FBetween F and 2FAt FBetween lens and focusAt infinityBetween infinity and lens Position of Image At focusBetween F and 2FAt 2FBetween 2F and infinity At infinity Farthest from lens than theon the same side At focusBetween focus and lens Nature of ImageReal Inverted infinityReal InvertedReal InvertedReal InvertedMagnified Real InvertedVirtual erect object and objectVirtual erect Virtual erect Size of ImageDiminishedSame size as objectGreatlymagni-Magnified as theDiminishedDiminished

Human eye. The eye consists of a convex lens (called eye-lens) and a screen (called retina). The eye lens forms a real image of the objects on the retina of the eye.

Accommodation. The ability of an eye to focus the distant objects as well as nearby objects on the retina by changing the focal length of its lens is called accommodation. A normal eye has a power of accommodation which enables objects as far as infinity and as close as 25 cm to be focussed on the retina.

Defects of Vision

The common defects of vision are

(i) Long-sightedness (or hyper-metropia)

(ii) Short-sightedness (or myopia)

(iii) Astigmatism

(i) Long-sightedness is that defect of vision due to which a person can not see the nearby objects clearly. Long-sightedness can be due to two reasons: either the focal length of the eye-lens is too large or the length of the eye ball is short. The defect can be carried by using a convex lens of suitable power.

(ii) Short sightedness (or myopia) is that defect of vision due to which a person can not see the distant objects clearly. Short-sightedness can be due to two reasons: either the focal length of the eye lens is too short or the eyeball may be longer than the normal size. Shortsightedness (or myopia) can be corrected by using a concave lens of suitable power.

(iii) Astigmatism: In an eye having this defect the front surface of the eye ball is not curved equally in all directions like a sphere and this produces indistinct images. The images are formed a varying distances from the retina. Using a cylindrical lens—one that is curved in one direction only—remedies this condition.

 Differences Real Image  Always invertedCan be taken on a screen  The rays always meet at the mage point Distinction between convex lens and concave lens.Convex lens1. It is thicker at the centre than at the edges.2. It converges light.3. It has real focus.Interference1. Interference is the result of interaction of light coming fromtwo different wave fronts originating from two coherent sources.2. Interference fringes may or may not be of the same width. Virtual ImageAlways erectCannot be taken on a screenThe rays appear to diverge from the image point.  Concave lens1. It is thinner at the centre than at the edges.2. It diverges light.3. It has virtual focus. Diffraction1. Diffraction pattern is the result of interaction of light coming from different parts of the same wave front.2. Diffraction fringes are not of the same width.

Optical Instruments

In a Magnifying Glass or Simple Microscope, a single converging or convex lens is used and the image formed is erect, virtual and magnified and on the same side of the lens. In Projection Lantern a magnified real image of a slide or film is formed on the screen. In a simple camera a small-real inverted image is formed on the plate.

A Compound Microscope is a combination of two converging or, convex lens system—an objective of very short focal length and an eyepiece of moderately short focal length. The magnification reached by such microscope is as much as 2,500.

Telescope: A telescope provides angular magnification of a distant object, and therefore produces an effect as if the object were either larger or closer to the eye. Magnification attained by a telescope is 1,500-2,000. Even more important to astronomers than magnification is the light-gathering power of a telescope, which determines how faint a star can be and still be seen. This depends on the area of objective, and it is one reason for making telescope of large diameter. In a terrestrial telescope an extra lens is placed between the objective and the eye-lens to produce an erect image of the object.

 Comparison between the human eye and the camera The eye The camera Points of similarity1. In an eye, the image is formed by a convex lens made of a transparent and flexible substance.2. A real and inverted image is formed on the retina.3. The iris of the eye controls the amount of light entering the eye.4. The time of exposure is controlled by the eye lids.Points of difference1. The focal length of the eye lens can be changed by the action of ciliary muscles.2. The focussing in the eye is done by changing the focal length of the eye-lens.3. The retina of the eye-retains the image only up to 1/10 of a second after the object is removed. 1. In a camera, the image is formed by a convex lens made of glass.2. In camera, real and inverted image is formed on the photographic film.3. The diaphragm controls the amount of light in a camera.4. The time of exposure in a camera is controlled by a shutter.1.  The focal length of a camera lens is fixed. It cannot be changed.2. The focussing in a camera is done by changing the distance. between the lens and the film.3. The photographic film of a camera retians the image of the object permanently.
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