LIGHT - REFLECTION AND REFRACTION.pdf

LIGHT – REFLECTION AND REFRACTION Important terms related with light Light It is form of energy that enables us to see. An object reflects the light rays that fall on it. These reflected light rays, when received by our eyes, make the objec...

LIGHT – REFLECTION AND REFRACTION Important terms related with light Light It is form of energy that enables us to see. An object reflects the light rays that fall on it. These reflected light rays, when received by our eyes, make the object visible to us. The speed of light in vacuum or in air is 3×108 m/s. Light propagates in straight line. There are 2 theories about the nature of light: wave theory and particle theory According to wave theory - Light consists of electromagnetic waves which do not require any medium to propagate. According to particle theory – Light is composed of particles which travels in a straight line at very high speed. These particles are known as “photons”. ❖ Physics experiments over the past hundred years or so have demonstrated that light has a dual nature (double nature) : light exhibits the properties of both waves and particles (depending on the situation it is in.). the modern theory of light called ‘Quantum Theory of Light’ combines both the wave and particle models of light. 1. Source- A source of light is an object, from which light is given out. Some sources of light are natural and many others are manmade sources. The sources of light are of two kinds: Luminous objects: the objects which emit their own light are called luminous objects. For example, the sun, electric lamps. Candle etc. are luminous or self-luminous objects. Non-luminous- the objects which does not emit light of its own but reflect or scatter the light which falls on them are known as non-luminous object. For example, the moon, a book, a table, a chair etc. are non-luminous objects. 2. Medium- A medium is a substance through which light propagates or tries to do so. There are three types of media of light. Transparent- it is a medium through which light propagates easily. Ex- air, water, glass etc. are transparent media Translucent- it is a medium through which light propagates partially. Ex= paper, ground, glass etc are translucent Opaque- it is a medium through which light cannot propagate. Ex- metals, wood etc. 3. Ray A ray of light is the straight-line path along which light travels. It is represented by an arrow head on straight line. The arrow head represents the direction of propagation of light The number of rays combined together form a beam of light. A narrow of light is called a pencil; of light A pencil of light may be of 3 types (i) Convergence (ii) Divergence (iii) Parallel pencil I. Convergence pencil a. It is that which rays of light go to meet at a particular point O. b. The width of the pencil goes on increasing as the rays proceed forward. II. Divergence pencil c. It is that in which rays of light come out a point source S. d. The width of the pencil goes on increasing as the rays proceed forward. III. Parallel pencil e. It is that which all the rays move parallel to one another. f. The width of the parallel pencil remains constant throughout.  LIGHT – REFLECTION AND REFRACTION REFLECTION OF LIGHT ▪ The process of bouncing back of light rays which fall on an opaque surface is known as reflection of light. ▪ The polished, shining surfaces reflect more light. ▪ During reflection of light medium does not change only the direction of light changes. (Silver is one of the best reflectors of light. Thus, a plane mirror has a shining coating of silver metal on one side.) Laws of reflection MM’ is the mirror. AO is the incident ray. OB is the reflected ray.O is the point of incidence ON is the normal. Incident ray – the ray of light which falls on the surface is called the incident ray. Reflected ray – the ray of light which sent back by the surface is called the reflected ray. Point of incidence – the point on which the incident ray falls on the mirror is called the point of incidence. Normal – it is a line right angle to the mirror at the point of incidence. (if a light falls through the NORMAL it reflects through NORMAL) Angle if incidence ∠𝒊 – the angle between the incident ray and the normal at the point of incidence is known as angle of incidence. Angle of reflection ∠𝒓 – the angle between the reflected ray and the normal at the point of incidence is known as angle of reflection. There are two laws of reflection 1. The incidence ray, the reflected ray and the normal at the point of incidence, all lie in the same plane. 2. Angle of incidence is always equal to the angle of reflection, i.e. ∠𝑖 = ∠𝑟. Regular Reflection If parallel beam of incident rays is reflected as parallel beam in one direction is known as regular reflection. Regular reflection occurs from smooth surfaces like plane mirror or highly polished metal. Diffused reflection If parallel beam of incident rays is reflected in different directions is known as diffused reflection. Diffused reflection occurs from rough surfaces like paper, cardboard, unpolished metal objects. ❖ Diffused reflection of light is not due to the failure of the laws of reflection. It is caused by the roughness in the surface. Image If light coming from an object meet or appeared to be meet at a point after reflection then image of that object is formed. Image of 2 types- Real image and Virtual image Real image If the light rays coming from an object actually meet after reflection, then the image formed is called a real image. Real image formed on the screen/mirror. Real image is inverted.  LIGHT – REFLECTION AND REFRACTION Virtual image- If the light rays coming from a point, after reflection does not meet actually, but appears to be meet at another point, then the image formed is called a virtual image. (virtual image is also known as unreal image or an illusion) Virtual image is formed behind the screen or mirror. Virtual image is erect. MIRROR Mirror is a polished surface, which reflects almost all the light incident on it. Mirrors are of two types : 1. Plane Mirror 2. Spherical Mirror 1. Plane Mirror If the reflecting surface of a mirror is plane, then the mirror is called a plane mirror. Properties/characteristics of image formed by a plane mirror The image formed in a plane mirror is virtual and erect. The size of image is equal to the size of the object. The distance between the image and the mirror is equal to the distance between the object and the mirror. The image is laterally inverted. (left seems right and vice-versa) ❖ The focal length of a plane mirror is infinite. Uses of plane mirrors Plane mirrors are commonly used as looking glass, in making periscopes. Kaleidoscope etc. 1. Wood is a ___ source of light. b. Size a. Non-luminous source c. Thickness b. Self-luminous d. Polishing c. Man made 6. Image formed of an object formed in a plane mirror d. Natural is 2. Rays arising from sun is an example of ____pencil. a. Laterally inverted a. Parallel pencil b. Virtual b. Divergent pencil c. Erect c. Convergent pencil d. All of the above d. All of the above 7. A ray of light strikes a silvered surface inclined to 3. When the parallel incident rays falling on the rough another one at an angle 90 then, the reflected ray reflecting surface, reflected in different direction. It will turn through____ is called ____ a. 0 a. Regular reflection b. 45 b. Diffuse reflection c. 90 c. Irregular reflection d. 180 d. Both band c 8. The laws of reflection hold good for ___ mirror. 4. What happens to the medium during reflection of a. Plane mirror only light? b. Concave mirror only a. Changes c. Convex mirror only b. Does not change d. All mirrors irrespective of their shape 5. Difference between a mirror and glass door is of 9. What happens to the image if you wave left hand in ____ front of a plane mirror? a. Shape a. Right hand will wave  LIGHT – REFLECTION AND REFRACTION b. Left hand will wave a. It retraces its path c. Both the hands will wave b. It will make 90 angle with normal d. None of the above c. It will make 60 angle with normal 10. What happens to the path of the reflected say if a d. None of the above light is incident normally to the reflecting surface? 2. Spherical Mirror If the reflecting surface of the mirror is curved inwards or outwards, then the mirror is called a spherical mirror. When a parallel beam of light rays falls on a spherical mirror, it is reflected as convergence or divergence beam. Spherical mirror are of 2 types : Concave mirrors and Convex mirrors. Concave Mirror – A spherical mirror with inwards curved reflecting surface is called concave mirror. A beam of light generally converges after reflection from such surfaces, hence it is also known as convergent mirror. Ex- the inner curved surface of a shinning spoon can be considered as a concave mirror. Convex Mirror – A spherical mirror with outward curved reflecting surface is called convex mirror. A beam of light generally diverges after reflection from this surface, hence it is also known as divergent mirror. Ex- the outer curved surface of a shining spoon can be treated or considered as convex mirror. Some definitions related to spherical mirrors Centre of Curvature – the centre of the imaginary sphere of which, the mirror is a part is known as the centre of curvature a spherical mirror. In case of concave mirror, the centre of curvature lies in front of it, while in case of convex mirror, the centre of curvature lies behind it. Radius of curvature – the radius of imaginary sphere of which, mirror is a part is known as radius of curvature of a spherical mirror. Pole – the centre or the mid-point of the spherical surface is known as pole of the mirror. Principal axis – the line joining the pole and the centre of curvature of a spherical mirror is known as the principal axis. Aperture – the diameter of the reflecting surface of a spherical mirror is called its aperture. It is equal to the straight line distance between two ends of the mirror. In other words, it is the part of the reflecting surface of a mirror which exposed to the incident light. Focus or Principal Focus – principal focus or focus of a concave mirror is a point on the principal axis of the mirror at which the light rays coming parallel to principal axis, converges after reflection actually meet. For a concave mirror, the focus is in front of the mirror. The focus of a concave mirror is real. Principal focus of a convex mirror is a point on the principal axis of the mirror at which the light rays parallel to principal axis, appears to diverge after being reflected from the mirror. For a convex mirror, the focus is behind the mirror. The focus of a convex mirror is virtual. Focal Length – the distance between pole and principal focus of a spherical mirror is called its focal length. It is represented by “f”. Relation between Radius a curvature and focal length of a spherical Mirror – If the aperture of the mirror is small, then the principal focus of a spherical mirror lies midway between the pole and the centre of curvature. So, the focal length of a spherical mirror is equal to half of its radius of curvature.  LIGHT – REFLECTION AND REFRACTION 𝑅 𝑓= 2 Representation of images formed by spherical mirrors using Ray diagrams. I. The rays incident parallel to the principal axis, pass through the focus after reflection in concave mirror or appear to come from focus in convex mirror. II. The rays passing through the focus of a concave mirror or passing towards focus of a convex mirror, because parallel to principal axis after reflection from the mirror. III. A ray passing through centre of curvature of a concave mirror or towards the direction of centre of curvature of a convex mirror, reflects back along the same path on striking the mirror surface. IV. A ray incident obliquely to principal axis, towards a pole P of the concave or convex mirror is reflected obliquely, following the laws of reflection, i.e., ∠𝑖 = ∠𝑟. IMAGE FORMATION BY SPHERICAL MIRRORS Image Formation by a Concave Mirror S.No. Position of Object Ray Diagram Position of Nature of Size of the image the image image 1. At infinity At focus (f) Real and Highly inverted diminished 2. Beyond the centre of Between F Real and Diminished curvature (C) but at and C inverted finite distance from mirror 3. At the centre of At Centre of Real and Equal to the curvature curvature (C) inverted object 4. Between focus and Beyond Real and Enlarged or centre of curvature centre of inverted Magnified curvature (c) 5. At the focus At infinity Real and Highly inverted enlarged or magnified  LIGHT – REFLECTION AND REFRACTION 6. Between the pole and Behind the Virtual and Enlarged or focus mirror erect magnified USES OF CONCAVE MIRRORS 1. Concave mirrors are used as shaving mirrors to see larger image of the face. 2. Dentists use concave mirrors to see large image of the teeth of patients. 3. Concave mirrors are commonly used in torches, search-lights and headlights of vehicles, to get powerful parallel beams of light. 4. Large concave mirrors are used to converge sunrays on a point to produce large amount of heat in a solar furnace. 5. Concave dishes are used in TV dish antennas to receive TV by signals from distant communications satellites. IMAGE FORMATION FOR CONVEX MIRROR S.No. Position of object Ray diagram Position of Nature of Size of image image image 1. At infinity At the Virtual and Highly principal erect diminished focus, behind the mirror 2. Between infinity and Between the Virtual and Diminished the pole principal focus erect and the pole, behind the mirror USES OF CONVEX MIRRORS Convex mirrors are commonly used as rear view mirrors in vehicles. (because they always give an erect image and have wider field of view as they are curved outward.) Big convex mirrors are used as shop security mirrors. Sign Convention For Spherical Mirrors → While dealing with the reflection of light by spherical mirrors, we shall follow a set of sign convention called the cartesian sign convention. → Cartesian sign convention is based on cartesian coordinates. → In this convention, the pole (p) of the mirror is taken as the origin. → The principal axis of the mirror is taken as X-axis of the coordinate system.  LIGHT – REFLECTION AND REFRACTION The conventions are as follow- (i) The object is always placed to the left of the mirror. (ii) All distances parallel to principal axis (X-axis) are measured from the pole of the mirror. (iii) Distances to the left of pole (-ve X-axis) are negative. Distances to the right of pole (+ve X-axis) are positive. (iv) Distances measured perpendicularly above the principal axis (along + Y-axis) are taken as positive. (v) Distances measured perpendicularly below the principal axis (along – Y-axis) is taken as negative. ❖ Focal length of convex mirror is taken as positive. Focal length of concave mirror is taken as negative. MIRROR FORMULA The relation between object distance (u), image distance (v) and focal length (f) of a spherical mirror is known as the mirror formula. 1 1 1 The mirror formula can be written as : 𝑣 +𝑢 =𝑓 Object distance (u) – The distance of the object from its pole is called the object distance (u). Image distance (v) – The distance of the image from the pole is called the image distance (v). Focal length (f) – The distance of the focus from the pole is called focal length (f). MAGNIFICATION OF SPHERICAL MIRROR The ratio of the height of image to the height of the object is known as linear magnification. It is represented by m. 𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑖𝑚𝑎𝑔𝑒 (ℎ𝑖 ) Magnification, m = 𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑜𝑏𝑗𝑒𝑐𝑡 (ℎ𝑜 ) Or, magnification is also related to the object distance (u) and image distance (v). it can be expressed as 𝐼𝑚𝑎𝑔𝑒 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑣) −𝑣 Magnification, m = − = 𝑂𝑏𝑗𝑒𝑐𝑡 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 (𝑢) 𝑢 Magnitude of magnification of a mirror or a lens gives information about the size of the image relative to the object. Magnification Size of image (hi) m=1 hi = ho m2 hi >ho Sign of magnification by mirrors or lens gives information about nature of the image produced. Sign of Magnification Image -ve Real and Inverted +ve Virtual and Erect Identification of mirrors How to distinguish between a plane mirror, a concave mirror and a convex mirror without touching them? ANS- By observing the images produced by a mirror for different position of objects, its nature can be identified. If the image formed by the mirror is of the same size as that of object for different positions of object, then the mirror is a plane mirror. If the image formed by the mirror is diminished for all positions of object, then the mirror is convex mirror. If the image formed behind the mirror is longer/bigger/magnified than the object, then the mirror is concave mirror. ❖ Focal length of a spherical mirror is independent of the medium in which it is placed. Q) No matter how far you stand from a mirror, your image appears erect. The mirror may be : I) plane II) concave III) convex IV) either plane or convex LIGHT – REFLECTION AND REFRACTION