CHAPTER-10 LIGHT REFLECTION AND REFRACTION Light rays; are; electromagnetic in nature, and do not need material medium for Propagation Speed of light in vacuum in 3*10 8 m/s When a light ray falls on a polished surface like a mirror it gets reflected in accordance with law of reflection. Laws of reflection are applicable to planar and spherical surfaces LAWS Angle of incidence is equal to the angle of reflection. Incident ray, the normal to the mirror at the point of incidence and the reflected ray all lie in the same plane. RULES To represent image formation by spherical mirror through ray diagram A ray parallel to the principal axis after reflection will pass through the principal focus in concave mirror or appear to diverge from principal focus in case of convex mirror. A ray of light passing through principal focus of a concave mirror or a ray which is directed towards the principal focus of convex mirror after reflection emerges parallel to the principal axis. A ray passing through center of curvature of concave mirror or directed in the direction of center of curvature of a convex mirror after reflection i.e. it is reflected back along the same path. A ray incident obliquely to the principal axis, toward the pole of the mirror on the concave mirror or the convex mirror is reflected obliquely. Position Of the Object Position Of the Image Size Of the Image Nature Of the Image At Infinity At the Focus Highly Diminished, Point Real and Inverted Sized Beyond C Between F and C Diminished Real and Inverted At C At C Same size Real and Inverted Between C and F Beyond C Enlarged Real and Inverted At F At Infinity Highly Enlarged Real and Inverted Between P and F Behind the mirror Enlarged Virtual and Erect a. Plane mirror b.concave mirror c.convex mirror a) The nature of image formed by a plane mirror is virtual laterally inverted and of the same size as the object b) Concave mirror is a converging mirror as it focuses a set of parallel rays at a point called its principle focus. (Activity 10.2 NCERT) c) Convex mirror is a diverging mirror as it diverges a set of parallel rays falling on it. d) Real images can be projected on a screen while virtual images can not be projected on a screen. e) The size and the nature of the image formed by a concave mirror depends on the position of the object in front of the mirror. (Activity 10.2 NCERT)
f) The nature of the image formed by a convex mirror is always virtual. g) The size and the nature of the image formed by a convex lens depends on the position of the object. ( Linear Magnification) h) The nature of the image formed by the concave lens is always virtual and diminished. i) Linear magnification produced by a mirror or lens is the relative the image(h ) as compared to the object(h) i.e; m=h /h. j) A refraction is the process by which the speed and the path of the light rays changes when it enters form one medium to another.(activity 10.7, 10.8 & 10.9) k) Refraction occurs through a glass slab, prism, spherical surfaces(convex and concave lens). l) Refraction occurs on a accordance with the laws of Refraction. m) The incident ray and the emergent ray in a glass slab is parallel as the extent of Refraction at the two parallel faces are equal and opposite. n) Refraction index is denoted as n21=sin i/sin r, where n21means refractive index of the 2 nd medium with respect to1 st medium. o) Refractive index is also the ratio of the velocity of light in medium1 to that in medium 2. n21=v1/v2. p) When the first medium is vacuum it is said to e absolute refractive index.n=c/v, where c=3*10 8 m/s(speed of light of vacuum) v=velocity of light in medium2. q) An optical denser medium refracts a light ray more than a optically rarer medium. While comparing two media the one with the larger refractive index is optically denser than the other medium of lower refractive index in optically rarer. The speed of light is higher in a rarer medium the denser medium. (Table 10.4 NCERT Pg 178 Table 10.5 Pg 179) r) Power of lens in the degree of convergence or divergence of light ray falling on it. s) Power of the lens is the reciprocal of its focal length P=1/f t) SI unit of power is Dioptre (D), (1D=1/m) Important formulae 1. R=2f 2. 1/v+1/u=1f (mirror formulae) 3. 1/v-1/u=1/f (lens formulae) 4. m=-v/u (linear magnification of mirror) 5. m=v/u (linear magnification of lens) 6. p=-1/f 7. n21=sin i/sin r 8. n=c/v Sign convention at the glance u f v
Concave mirror - - +(Only when object is between O and f) Concave lens - - - Convex mirror - + + Convex lens - + +(Only when between u and f) Question bank 1. What is a magnification of a plane mirror? Ans: m=+1. 2. What is the radius of curvature of a plane mirror? ans: infinity. 3. Density of turpentine is less than water yet has a higher refractive index why? It is optically denser that water. 4. Power of a convex lens is +3D what is its focal length? F=1/p=1/3=0.33 Meter 5. Which lens bends a light ray more lens with a shorter or with longer focal length? the lens with the shorter focal length bends the light more. 6. Which type of mirror is used in a) Headlight of a car b) Rear view mirror. c) Dentist s mirror. Ans a) Concave mirror b) Convex mirror c) Concave mirror 7. A lens has a power +3D. is it a convex lens or concave lens? Convex lens. 8. What is the relation between radius of curvature and focal length of a spherical mirror? R=2f. 9. The magnification of a spherical mirror is -4, what does it signify the image is real,inverted and enlarged 10. if a convex lens is used to focus sunlight on a paper, where should the paper be placed so that it catches fire?
at the principal focus. 11. the radius of a curvature of a spherical mirror is 40cm. what is the focal length? The focal length is 20cm. 12. what happens if a light ray falls on a glass slab 90 o to its surface? it undergoes normal refraction i.e. there is no deviation in the light ray. 13. Write an expression for snells law? n= Sin i Sin r 14. If alight ray travels obliquely from a rarer medium to a denser medium how will it bend? it would bend towards the normal. 15. Where should be an object placed in front of a convex lens so as to use it as a magnifier? Between the pole and the focal length 2 mark questions 1. State the laws of reflection. a. Angle of incidence (i) = Angle of reflection (r) b. The incident ray,the normal to the mirror at the point of the incidence and the reflected ray lie in the same plane. 2. State the laws of refraction. Page 174 NCERT text 3. light enters from air to glass having refractive index of 1.50 what is the speed of light in glass. Formula n= c v n= 1.50 c= 300000000 v=? n = 300000000 = 200000000m/s or 2 X 10 8 m/s. 1.50 4. Draw a ray diagram to show image formed when object is placed between infinity and optical center of a concave lens Page 181 NCERT fig 10.17(b) 5. Apply Cartesian sign convention for the diagram on page 181 NCERT fig 10.16 for real image. (i) for u, v, f, h & h` u=v=+ f=+ h=+ h`=-
An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm.find position and nature of the image? Given f=+15cm U=-10cm V=? 1/v+1/u=1/f 1/v+1/-10=1/15 1/v-1/10=1/15 1/v=1/15+1/10 1/v=2+3/30 1/v=5/30 v=30/5=6cm. The nature of the image is virtual, erect and diminished. How far should an object be placed from the pole of a concave mirror of focal length 20 cm to form a real image whose size is 1/5 th of the size of the object? given f=-20 cm m=-1/5 m=-v/u v=1/5 x u 1/v+1/u=1/f 1/u/5+1/u=-1/20 5/u+1/u=-1/20 6/u=-1/20 u=-120cm An object of size 4 cm is placed 25 cm in front of a concave mirror of focal length 15 cm find position nature and size of image given h=+4cm u=-25 cm f=-15 cm h`=? v=? 1/v+1/u=1/f 1/v+1/-25=1/-15 1/v-1/25=-1/15 1/v=-1/15+1/25 1/v=-5+3/75 1/v=-2/75 v=-75/2 v=-37.5 cm m=h`/h=-v/u h`/4=-(-75/2/-25) h`/4=-1.5
h`=-2cm The real, inverted and diminished images form at 37.5cm in front of the concave mirror. A concave lens of focal length 15 cm forms a image 10 cm from the lens how far is the object placed from the lens given f=-15 cm v=-10 cm u =? 1/v-1/u=1/f (1/-10)-(1/u)=1/-15-1/10-1/u=-1/15 1/10+1/u=1/15 1/u=1/15-1/10 1/u=2-3/30 1/u=-1/30 u=-30 cm