LIGHT CLASS X STUDY MATERIAL & QUESTION BANK: 1. REFLECTION OF LIGHT: The phenomenon of light coming back into the same medium after it strikes a smooth surface is called reflection. 2. LAWS OF REFLECTION: A. The angle of incidence and the angle of reflection are equal. B. The incident ray, the reflected ray and the normal to the surface at the point of incidence are in the same plane. 3. PROPERTIES OF IMAGE FORMED BY PLANE MIRRORS: A. A plane mirror forms a virtual image B. The image is erect C. The image is of the same size as the object D. The image is laterally inverted. 4. SPHERICAL MIRROR: It is a type of mirror in which the reflecting surface is curved. 5. CENTRE OF CURVATURE: It is the centre of that sphere of which mirror is a part. 6. TYPES OF SPHERICAL MIRRORS: A. Concave: It is a type of spherical mirror in which reflection takes place at the surface on the same side as the centre of curvature. B. Convex: It is a type of spherical mirror in which reflection takes place at the bulged surface. Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 1
7. IMPORTANT TERMS ASSOCIATED WITH SPHERICAL MIRRORS: A. Pole (P): The centre of the reflecting surface of the mirror is called pole. B. Principal axis: The line joining the pole and centre of curvature is called principal axis. C. Principal focus (F): concave mirror: It is the point on the principal axis at which the rays which come parallel to principal axis after reflection converge. convex mirror: It is the point on the principal axis at which the rays which come parallel to principal axis after reflection appear to diverge. D. Radius of curvature(r): The distance between pole and centre of curvature is called radius of curvature E. Focal length: (f): The distance between pole and principal focus is called focal length F. Aperture: The width or diameter of the reflecting surface is called its aperture. G. Relation between radius of curvature and focal length: Radius of curvature is twice of its focal length. R = 2f or f = R/2 8. RULES FOR THE FORMATION OF IMAGES: A. A ray of light which passes parallel to principal axis after reflection from a concave mirror, passes through principal focus. A ray of light which passes parallel to principal axis after reflection from a convex mirror, appear to diverge from principal focus Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 2
B. A light ray passing through principal focus, after reflection, passes parallel to principal axis. A light ray projected towards the principal focus of a convex mirror, after reflection passes parallel to principal axis. C. A ray of light which strikes at the pole of a concave or a convex mirror, reflects at the same angle on the other side of principal axis. D. A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path. Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 3
9. FORMATION OF IMAGES BY CONCAVE MIRROR FOR DIFFERENT POSITIONS OF OBJECT IN FRONT OF IT: CONCAVE MIRROR FORMS REAL IMAGES IN ALL THE CASES EXCEPT WHEN THE OBJECT IS PLACED BETWEEN POLE AND PRINCIPAL FOCUS. THE VIRTUAL IMAGE FORMED BY CONCAVE MIRROR WILL BE BIGGER IN SIZE THAN THE OBJECT. Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 4
10. FORMATION OF IMAGES BY CONVEX MIRROR FOR DIFFERENT POSITIONS OF OBJECT IN FRONT OF IT: CONVEX MIRROR ALWAYS FORMS VIRTUAL IMAGES THE SIZE OF VIRTUAL IMAGE FORMED BY A CONVEX MIRROR WILL BE SMALLER THAN THE OBJECT. 11. USES OF CONCAVE MIRROR: A. It is used as a shaving mirror B. It is used in search lights and head lights of automobiles C. It is used by dentists. 12. USES OF A CONVEX MIRROR: A. It is used as a rear view mirror in automobiles B. It is used in apartments to bring the view of the corners which are not directly visible. 13. MIRROR FORMULA ( used both for convex and concave mirrors) 1/f = 1/u + 1/v Here u= object distance v = image distance f = focal length 14. MAGNIFICATION (m): It is defined as the ratio of size of the image to the size of the object m = -v/u = h /h Magnification is negative for real and inverted images Magnification is positive for virtual images m= 1 for plane mirror. It means the size of the object and the image are same m >1, when image is larger than the object m < 1 when image is smaller than the object Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 5
15. SIGN CONVENTION FOR SPHERICAL MIRRORS: All the measurements must be done from the pole Measurements done in the direction of light are taken as positive Measurements done in the opposite direction of light are taken as negative Concave mirror Convex mirror Object distance negative negative Image distance Negative ( real images) Positive Positive ( virtual images) Focal length Negative positive Size of the object Positive positive Size of the image Negative (real images) Positive ( virtual images) Positive 16. REFRACTION OF LIGHT: The process of bending of light at the surface of separating two media when it travels from one medium into the other is called refraction of light. 17. CAUSE OF REFRACTION: Light has different speeds in different media. Therefore, it changes its direction when it travels from one medium into the other. It bends towards the normal when it travels from rarer to denser medium. It bends away from the normal when it travels from denser to rarer medium. 18. LAWS OF REFRACTION: 1. The incident ray, refracted ray and the normal lie in the same plane. 2. The ratio of sine of angle of incidence to the sine of angle of refraction for the same pair of media remains constant. This is called Snell s law of refraction. 19. SNELL S LAW OF REFRACTION: The ratio of sine of angle of incidence to the sine of angle of refraction for the same pair of media remains constant. This is called Snell s law of refraction. The constant is called the refractive index for the pair of the media. μ1 2 = μ 2 Sin i = μ 1 Sin r 20. REFRACTIVE INDEX: The ratio of sine of angle of incidence to the sine of angle of refraction is called refractive index for the given pair of media. μ1 2 = μ 2 Sin i = μ 1 Sin r (OR) The ratio of speed of light in the first medium to the speed of light in the second medium is called refractive index of the second medium with respect to the first medium. μ1 2 = μ 2 Sin i = = v 1 μ 1 Sin r v 2 Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 6
21. ABSOLUTE REFRACTIVE INDEX: The ratio of the speed of light in air (vacuum) to the speed of light in the given medium is called the absolute refractive index of the given medium. μ medium = c v medium ; here c is the speed of light in air (vacuum) 22. OPTICAL DENSITY & MASS DENSITY: The ability of the medium to refract the light when passes through it is called optical density. Mass per unit volume of a substance is known as its mass density. Kerosene has higher optical density than water but its mass density is less than that of water. 23. REFRACTIVE INDEX OF WATER IS 1.33. WHAT DOES IT MEAN? The ratio of speed of light in air to speed of light in water is 1.33 24. REFRACTION OF LIGHT THROUGH A GLASS SLAB: EO = Incident ray i 1 = angle of incidence OO 1 = refracted ray r 1 = angle of refraction O 1 H = emergent ray r 2 = angle of emergence The extent of bending at the opposite faces of the glass slab is equal and opposite. That is the reason; the emergent ray is parallel to incident ray. The perpendicular distance between the emergent ray and the original un deviated incident ray (in the absence of glass slab) is called lateral shift. Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 7
25. REFRACTION BY SPHERICAL LENSES: Lens: A transparent material bounded by two surfaces of which at least one is curved is a called a lens. Convex lens: A lens that is thicker at the middle than at the edges is called a convex lens Concave lens: A lens that is thicker at the edges than at the middle is called a concave lens 26. IMPORTANT TERMS RELATED TO SPHERICAL LENSES: A. Optic centre: The central point of the lens is called Optic centre. A ray of light incident at the optic centre passes almost without any deviation. B. Principal focus: A set of rays parallel to principal axis, after refraction from one of the surfaces, converge or appear to diverge from a point on the principal axis. This point is called principal focus. C. Focal length: The distance of the principal focus from the optic centre is called focal length. 27. RULES FOR THE FORMATION OF IMAGES IN CASE OF SPHERICAL LENSES: A. A ray of light passing parallel to principal axis, after refraction, passes through principal focus on the other side of the lens. (FOR CONVEX LENS) A ray of light passing parallel to principal axis, after refraction, appear to diverge from the principal focus on the same side of the object. (FOR CONCAVE LENS) Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 8
B. A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis. C. A ray of light passing through the optical centre of a lens will emerge without any deviation. 28. FORMATION OF IMAGES BY SPHERICAL LENSES: A. Object at infinity Image is real, point sized Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 9
B. Object beyond 2F image is real, inverted & smaller than object C. Object at 2F image is real, inverted & same size as the object D. Object between F & 2F: Image is real, inverted & bigger in size as the object E. Object at F Image real, inverted & is infinitely large Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 10
F. Object between F & O: Image is virtual, erect & large A CONVEX LENS FORMS REAL IMAGES IN ALL THE POSITIONS OF THE OBJECT INFRONT OF IT EXCEPT WHEN IT IS PLACED BETWEEN PRINCIPAL FOCUS AND OPTIC CENTRE. 29. FORMATION OF IMAGES BY A CONCAVE LENS: A CONCAVE LENS ALWAYS FORMS VIRTUAL, ERECT & SMALLER IMAGES. 28. LENS FORMULA: 1 f = 1 v + 1 u Here: u= object distance; v= image distance; f = focal length 30. MAGNIFICATION: It is defined as the ratio of size of the image to size of the object m = h h 1 = v u FOR CONCAVE LENS m IS ALWAYS POSITIVE FOR CONVEX LENS m IS NEGATIVE FOR REAL IMAGES AND POSITIVE FOR VITUAL IMAGES. Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 11
31. SIGN CONVENTION: Convex Lens Concave Lens Object distance negative negative Image distance Positive (real images) negative Negative (virtual images) Focal length positive negative Size of the object positive positive Size of the image Negative (real images) Positive (virtual images) positive 32. POWER OF A LENS: The ability of a lens to converge or diverge the rays of light is called its power. It is defined as the reciprocal of its focal length when expressed in meters. P = 1 f S.I unit: Dioptre Def of 1Dioptre: The power of a lens is said to be 1 dioptre when its focal length is 1 metre. THE POWER OF A CONVERGING ( CONVEX) LENS IS POSITIVE THE POWER OF A DIVERGING (CONCAVE) LENS IS NEGATIVE 33. COMBINATION OF LENSES: When two or more lenses are in contact: The resultant power is ; P = P 1 + P 2 +.. The resultant focal length; 1 F = 1 f 1 + 1 f 2 + QUESTION BANK: (Know the answers from your teacher) 1. A concave mirror of focal length f can form a magnified, erect as well as inverted image of an object placed in front of it. Justify this statement stating the position of the object with respect to the mirror in each case for obtaining these images. 2. Distinguish between a real and virtual image of an object. What type of image is formed i) by a plane mirror ii) on a cinema screen 3. What is the minimum number of rays required for locating the image formed by a concave mirror for an object? Draw a ray diagram to show the formation of a virtual image by a concave mirror. 4. Explain with the help of a diagram, why a pencil partly immersed in water appears to be bent at the water surface. 5. With the help of a ray diagram explain why a convex mirror is preferred for rear view mirrors in motor cars 6. One half of a convex lens of focal length 20 cm is covered with a black paper. I) will the lens produce a complete image of the object? Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 12
Ii) Show the formation of image of an object placed at 2F of such covered lens with the help of a ray diagram Iii) How will the intensity of the image formed by half-covered lens compare with non-covered lens? 7. Explain why we see the sign in front of some vehicles? 8. A ray of light is incident on the plane mirror normally. What are the angle of incidence and the angle of reflection? 9. With the help of a ray diagram show that angle of incidence is equal to the angle of reflection when a ray is incident on the concave mirror. 10. Discuss the position and nature of the image formed by a concave mirror when the object is moved from infinity towards the pole of the mirror? 11. Why does a ray of light passing through the centre of curvature of a concave mirror after reflection is reflected back along the same path? 12. A pencil partly immersed in water in a glass tumbler appears to be displaced at the interface of air and water. Name the phenomenon of light responsible for it? 13. Name two spherical mirrors. Define them. Write any three points of difference between them. 14. The magnification produced by a concave mirror is m= + 4. Write the information about the image given by this statement. 15. With the help of a ray diagram explain the use of concave mirror as solar concentrators 16. A thin converging lens forms a: 1) real magnified image 2) virtual magnified image of an object placed in front of it. Write the positions of the objects in each case. Draw labelled ray diagrams to show the image formation in each case. 17. How will the following be affected on cutting the convex lens into two halves along the principal axis? 1) Focal length ii) intensity of the image formed by half lens. 18. For what position of the object does a concave mirror form a real, inverted and diminished image of the object? Draw the ray diagram. 19. What lens can be used as a magnifying glass? For which position of the object does a convex lens form: i) a virtual and erect image? Ii) a real and inverted image of same size as that of the object? Draw labelled ray diagrams to show the formation of the required image in each of the above two cases. 20. How will you identify whether a given lens is concave or convex without touching it? Also mention one use of each of the lens. NUEMRICALS: Practice NCERT text book examples and exercise numericals. Prepared by E.V.Ramana, PGT (Phy), KV CRPF, Hyderabad. Page 13