Light : Reflection And Refraction (Part I Reflection)

Transcription

1 1 Light : Reflectin And Refractin (Part I Reflectin) Light is a frm f energy which enables us t see bjects either frm which it cmes r frm which it is reflected. Luminus bjects are thse bjects which emit their wn light. Example Sun, Candle, Electric Lamp etc. Nn-luminus bjects are thse which d nt emit light themselves but nly reflect (r scatter) the light which falls n them. Example Mn, Chair, Tables, Pt, etc. Light travels in straight lines. The fact can be prved frm, a small surce f light casts a sharp shadw f an paque bject. Nature f Light There are tw theries abut the nature f light (1)Wave Thery and (2)Particle Thery Wave Thery : Light cnsists f electrmagnetic waves which d nt require a material medium (like slid, liquid r gas) fr their prpagatin. Particle Thery : Light is cmpsed f particles which travel in a straight line at a very high speed (being abut 3 x 10 8 metres per secnd in vacuum). The elementary particle that defines light is called phtn. Light has dual nature : light exhibits the prperties f bth waves and particles. Wave nature is prved by phenmena f diffractin, interference and plarizatin. Particle nature is prved by phenmena f reflectin and casting shadw f paque bjects. Reflectin f Light When light falls n the surface f an bject, it may be (1) absrbed, (2) transmitted, r (3) reflected. If an bject absrbs all the light which falls n it, then it will appear perfectly black. If an bject transmits the light, i.e. allws t pass thrugh it, then the bject is said t be transparent. The light may bend smewhat frm its riginal path if it transmits thrugh an bject, then it is called as refractin. The prcess f sending back the light rays which fall n the surface f an bject, is called reflectin f light. Fr example a plished silver mirrr reflects back mre than 95% f light falling n it. The bjects having plished, shining surfaces reflect mre light than bjects having unplished, dull surfaces. Silver metal is ne f the best reflectrs f light. Hence, rdinary mirrrs are made by depsiting a thin layer f silver metal n ne side f a plane glass sheet. The silver layer is then prtected by a cat f red paint. The reflectin f light in a plane mirrr takes place at the silver surface in it. Laws f Reflectin f Light In rder t understand the laws f reflectin f light, we shuld first understand sme terms related t it The ray f light which falls n the mirrr surface is called the incident ray. This dcument is riginally created by ssc2pg.cm and is made available fr persnal use and nncmmercial free distributin. Mre such lessns n : visit cmmunity sectin.

2 2 The pint at which the incident ray falls n the mirrr is called the pint f incidence. The ray f light which is sent back by the mirrr is called the reflected ray. The Nrmal is a line at right angle (perpendicular) t the mirrr surface at the pint f incidence. Angle f Incidence is the angle made by the incident ray with the nrmal at the pint f incidence. Angle f reflectin is the angle made by the reflected ray with the nrmal at the pint f incidence. There are tw laws f reflectin f light as fllws First Law f Reflectin : The incident ray, the reflected ray and the nrmal (at the pint f incidence), all lie in the same plane. Secnd Law f Reflectin : The angle f reflectin is always equal t the angle f incidence. A ray f light which is incident nrmally (r perpendicularly) n a mirrr, is reflected back alng the same path, because the angle f incidence as well as the angle f reflectin fr such a ray f light are zer. The laws f reflectin f light apply t all kinds f mirrrs, plane mirrrs as well as spherical mirrrs (cnvex r cncave). Objects and Images Anything which gives ut light rays, either its wn r reflected by it, is called as bject. Small bjects, usually dented by a pint in the diagrams are called, pint bjects. Large bjects, usually dented by a vertical arrw in the diagrams are called extended bjects. Image is an ptical appearance prduced when light rays cming frm an bject are reflected frm a mirrr r refracted thrugh a lens. An image is frmed when the light rays cming frm an bject meet r appear t meet at a pint, after reflectin frm a mirrr r refractin thrugh a lens. Real Image : An image which can be btained n a screen is called as real image. Fr example image n cinema screen. Virtual Image : The image which cannt be btained n a screen is called as virtual image. Virtual images are als called as unreal images. Fr example yur wn image in a mirrr. A real image is frmed when the light rays actually pass thrugh a real image. Fr example in cinema the light passes thrugh a film cntaining an image. A virtual image cannt be frmed n the screen because light rays d nt actually pass thrugh an image. Images in Plane Mirrr Nature The nature f image frmed in a plane mirrr is virtual and erect. Size The size f image frmed in a plane mirrr is equal t that f the bject. Psitin The image frmed in a plane mirrr is at the same distance behind the mirrr as the bject is in frnt f the mirrr. Lateral Inversin When an bject is placed in frnt f a plane mirrr, then the right side f bject appears t becme the left side f the image; and the left side f bject appears t becme the right side f image. This change f sides f an bject and its mirrr image is This dcument is riginally created by ssc2pg.cm and is made available fr persnal use and nncmmercial free distributin. Mre such lessns n : visit cmmunity sectin.

3 3 called lateral inversin. The image frmed in a plane mirrr is laterally inverted (r sideways reversed) with respect t the bject. When a parallel beam f light rays falls n a plane mirrr, it is reflected as a parallel beam. S a plane mirrr changes nly the directin f incident light rays and it DOES NOT cnverge r diverge the parallel rays f light. Spherical Mirrrs A spherical mirrr is that mirrr whse reflecting surface is the part f a hllw sphere f glass. The spherical mirrrs are f tw types : (1) Cncave Mirrrs, and (2) Cnvex Mirrrs. Cncave Mirrr : A spherical mirrr in which the reflectin f light takes place at the cncave surface (bent in surface) is called Cncave Mirrr. Cnvex Mirrr : A spherical mirrr in which the reflectin f light takes place n the cnvex surface (bulging ut surface) is called Cnvex Mirrr. There are certain terms related t the spherical mirrrs which are required in the study f image frmatin by spherical mirrrs, which are as fllws Centre f Curvature The centre f curvature f a spherical mirrr is the centre f the hllw sphere f glass f which the mirrr is a part. The centre f curvature f a cncave mirrr is in frnt f it but the centre f curvature f a cnvex mirrr is behind it. Radius f Curvature The radius f curvature f a spherical mirrr is the radius f the hllw sphere f glass f which the mirrr is a part. Ple The centre f a spherical mirrr (the middle pint) is called its ple. The ple f a spherical mirrr lies n the surface f the mirrr. Principle Axis It is the straight line passing thrugh the centre f curvature and ple f a spherical mirrr. The principal axis is nrmal (perpendicular) t the mirrr at its ple. Aperture That prtin f mirrr frm which the reflectin f light actually takes place. The aperture f the mirrr represents the size f the mirrr. Cncave Mirrr Principle Fcus It is a pint n its principle axis t which all the light rays which are parallel and clse t the axis, cnverge after reflectin frm the cncave mirrr. Fcal Length The fcal length f a cncave mirrr is the distance between its ple and principle fcus. Cnvex Mirrr Principle Fcus It is a pint n its principle axis frm which a beam f light rays, initially parallel t the axis, appears t diverge after being reflected frm the cnvex mirrr. A cnvex mirrr has virtual fcus. Unlike cncave mirrr, the reflected rays d nt actually pass thrugh the fcus f the cnvex mirrr. The fcus f a cnvex mirrr is situated behind the mirrr. This dcument is riginally created by ssc2pg.cm and is made available fr persnal use and nncmmercial free distributin. Mre such lessns n : visit cmmunity sectin.

4 4 Fcal Length The fcal length f a cnvex mirrr is the distance frm the ple t its principle fcus. Fcal length f a spherical mirrr having small aperture The fcal length f a spherical mirrr, whether cncave r cmplex, is equal t the half f its radius f curvature. S if f is the fcal length f a spherical mirrr and R is its radius f curvature then : Rules fr btaining images frmed by Cncave Mirrrs Rule 1 A ray f light which is parallel t the principal axis f a cncave mirrr, passes thrugh its fcus after reflectin frm the mirrr. Rule 2 A ray f light passing thrugh the centre f curvature f a cncave mirrr is reflected back alng the same path. Rule 3 A ray f light passing thrugh the fcus f a cncave mirrr becmes parallel t the principal axis after reflectin. Rule 4 A ray f light which is incident at the ple f a cncave mirrr is reflected back making the same angle with the principal axis. Different types f images frmed by a Cncave Mirrr The type f image frmed by a cncave mirrr depends n the psitin f bject in frnt f the mirrr. We can place the bject at different distances frm the cncave mirrr t get different types f images. Case 1 : Object is placed between ple and fcus f the mirrr The image frmed is : Virtual and erect Behind the mirrr, and Larger than the bject (magnified) The applicatin f this type f mirrr (cncave) is used by dentists t see the large image f the teeth fr examinatin f decayed tth. Fr this purpse, the dentist hlds a small cncave mirrr in such a way that the tth lies within its fcus. Case 2 : Object is placed at the fcus f the cncave mirrr The image frmed is : Real and inverted At infinity, and Highly enlarged than the bject (highly magnified) The applicatin f this type f mirrr is in trches, car head-lights and search lights. When a lighted bulb is placed at the fcus f a cncave mirrr reflectr, the diverging light rays f the bulb are cllected by the cncave reflectr and then reflected t prduce a strng, parallel sided beam f light. Case 3 : Object is placed between fcus and centre f curvature The image frmed is : Beynd the centre f curvature Real and inverted, and Larger than the bject (magnified) Case 4 : Object is placed at centre f curvature f cncave mirrr The image frmed is : This dcument is riginally created by ssc2pg.cm and is made available fr persnal use and nncmmercial free distributin. Mre such lessns n : visit cmmunity sectin.

5 5 At the centre f curvature Real and inverted, and Same size as the bject. Case 5 : Object beynd the centre f curvature f cncave mirrr The image frmed is : Between the fcus and centre f curvature Real and inverted, and Smaller in size than the bject (diminished) Case 6 : Object is at infinity in frnt f cncave mirrr The image frmed is : At the fcus Real and inverted Much much smaller than the bject (highly diminished) Rules fr btaining images frmed by cnvex mirrrs Rule 1 A ray f light which is parallel t the principal axis f a cnvex mirrr, appears t be cming frm its fcus after reflectin frm the mirrr. Rule 2 A ray f light ging twards the centre f curvature f a cnvex mirrr is reflected back alng the same path. Rule 3 A ray f light ging twards the fcus f the cnvex mirrr becmes parallel t the principal axis after reflectin Rule 4 A ray f light which is incident at the ple f a cnvex mirrr is reflected back making the same angle with the principal axis. Different Types f images frmed by the cnvex mirrr - Whatever be the psitin f bject in frnt f a cnvex mirrr, the image frmed by a cnvex mirrr is always behind the mirrr, it is virtual, erect, and smaller than the bject (diminished). Case 1 : Object is at a finite distance away frm the cnvex mirrr The image frmed is : Behind the mirrr between ple and fcus Virtual and erect, and Diminished (smaller than the bject) If we mve the bject mre and mre away frm the ple f the cnvex mirrr, the image becmes smaller and smaller and mves away twards the fcus, but it remains virtual and erect. Case 2 : Object is lcated at infinity The image frmed is : Behind the mirrr at fcus Virtual and erect Highly diminished (much much smaller than the bject) Sign cnventin fr spherical mirrrs Accrding t the New Cartesian Sign Cnventin All the distances are measured frm ple f the mirrr as rigin. Distances measured in the same directin as that f incident light are taken as psitive. This dcument is riginally created by ssc2pg.cm and is made available fr persnal use and nncmmercial free distributin. Mre such lessns n : visit cmmunity sectin.

6 6 Distances measured against the directin f incident light are taken as negative. Distances measured upward and perpendicular t the principal axis are taken as psitive. Distances measure dwnward and perpendicular t the principal axis are taken as negative. The bject is always assumed t be placed n the left side f the mirrr, s that the directin f incident light is frm left t right. S frm this we cnclude all distances measured frm ple f mirrr t the right side will be cnsidered as psitive. All the distances measured frm ple f mirrr t the left side will be cnsidered negative. The bject distance (u) is always negative. If an image is frmed behind a cncave mirrr (t the right hand side), then the image distance (v) is psitive but if image is frmed in frnt f mirrr (n left side), then the image distance (v) is negative. The image distance (v) fr a cnvex mirrr will always be psitive. The fcal length fr cncave mirrr is always cnsidered negative. The fcal length fr cnvex mirrr is always cnsidered psitive. The Mirrr Frmula A frmula which gives the relatinship between image distance (v), bject distance (u) and fcal length (f) f a spherical mirrr is knwn as the Mirrr Frmula. It is given as Linear Magnificatin prduced by Mirrrs The rati f the height f image (h 2 ) t the height f bject (h 1 ) is knwn as linear magnificatin (m). It is given by If the magnificatin has a plus sign, then the image is virtual and erect. If the magnificatin has a minus sign, then the image is real and inverted. Hwever smetime the height f image may nt be measurable, s the same frmula may be written in terms f bject and image distances The linear magnificatin (m) prduced by mirrr equals t the rati f the image distance (v) t the bject distance (u), with a minus sign This implies that, we can write frm these tw frmulas This dcument is riginally created by ssc2pg.cm and is made available fr persnal use and nncmmercial free distributin. Mre such lessns n : visit cmmunity sectin.