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1 1 Class X: Physics Chapter 10: Light- Reflection and Refraction Points to remember Key learnings: 1. When light falls on a body, it may be absorbed, may be transmitted or light may come back to the same medium. 2. Reflection of a light means light waes are neither transmitted nor absorbed but are deflected from the surface of the medium back into the same medium. 3. Laws of Reflection: First law of reflection: The incident ray, the normal to the surface at the point of incidence and the reflected ray, all lie in the same plane. Second law of reflection: The angle of incidence is equal to the angle of reflection. 4. Real image is obtained when the rays of light after reflection, actually conerge at a point. It can be obtained on the screen and can be seen with the eye. 5. Virtual image forms when rays of light do not actually meet, but appear to meet when produced backwards. It cannot be obtained on the screen. 6. Characteristics of the image formed by the plane mirror: i. The image formed by a plane mirror is always irtual and erect. ii. Size of image = Size of object and the image is laterally inerted.

2 2 iii. The image formed by the plane mirror is as far behind the mirror as the object is in front. 7. Lateral Inersion: The phenomenon due to which the right side of the object appears as left and the left side of the object appears as right. That is the image is inerted sideways. 8. A spherical mirror whose reflecting surface is cured outwards and polished on the inner spherical surface is conex mirror. 9. A spherical mirror whose reflecting surface is cured inwards and polished on the outer spherical surface is concae mirror. 10. Concae mirror mostly forms real images, which can be receied on the screen. 11. Conex mirror forms irtual images, which cannot be receied on the screen. 12. Differentiating between a plane mirror, a concae mirror and a conex mirror, without touching them: i) If the image formed is erect and is of same size as in reality then it is a plane mirror. ii) If the image formed is still erect but smaller in size then it is a conex mirror. iii) If the image is erect but magnified when mirror is close to the object then it is a concae mirror. 13. Solar concentrators use huge concae mirrors to focus large amounts of solar energy thereby producing high temperature conditions in a solar power plant.

3 3 14. Concae mirrors are used as shaing and make up mirrors. 15. The centre of the reflecting surface of a spherical mirror is a point called the pole of the mirror and usually represented by P. 16. The horizontal line passing through the centre of curature and pole of the spherical mirror is known as Principal axis. 17. The centre of curature of a spherical mirror is the centre of the hollow sphere of glass, of which the spherical mirror is a part and usually represented by C. 18. The radius of curature of a spherical mirror is the radius of the hollow sphere of glass, of which the spherical mirror is a part and usually represented by R. 19. The diameter of the reflecting surface that is twice the radius is called its aperture. 20. Radius of curature (R) = 2 focal length (f) 21. Rules for ray diagram- i. The path of the reflected light ray depends upon how the incident ray is oriented with respect to the principal axis. ii. A ray of light parallel to the principal axis, after reflection pass through the principal focus. iii. A ray of light passing through the principal focus of a mirror becomes parallel to the principal axis of the mirror, on reflection

4 4 i. A ray of light incident obliquely towards the pole of mirror is reflected obliquely as per the laws of reflection. A ray of light passing through centre of curature of a mirror is reflected back along the same path 22. Image formation by a concae mirror for different positions of the object Position of Position of Size of image Nature of image object image At infinity At focus F Highly diminished, Real and inerted point sized Beyond C Between F and C Diminished Real and inerted At C At C Same size Real and inerted Between C and F Beyond C Enlarge Real and inerted At F At infinity Highly enlarged Real and inerted Between P and F Behind mirror Enlarged Virtual and erect 23. Nature, position and relatie size of the image formed by a conex mirror: Position of object Position of image Size of image Nature of image At infinity At focus F behind Highly Virtual and the mirror diminished, point erect

5 5 sized Between infinity Between P and F Diminished Virtual and and pole of the mirror behind the mirror erect 24. The ratio of the height of the image to the height of the object is known as the magnification. 25. Magnification is positie for irtual image and negatie for real image. 26. The phenomenon of change in the path of light from one medium to another is called refraction of light. 27. The angle formed between the incident ray and the normal is called angle of incidence and the angle formed between the refracted ray and the normal is called angle of refraction. 28. The cause of refraction is the change in the speed of light as it goes from one medium to another medium. 29. Larger the difference in speeds of light, the greater will be the angle of bending and ice-ersa. 30. When a ray of light passes from a rarer medium to a denser medium, it bends towards the normal. Also, the angle of incidence is greater than the angle of refraction. 31. When a ray of light passes from a denser medium to a rarer medium, it bends away from the normal. Also, the angle of incidence is less than the angle of refraction

6 6 32. Laws of refraction: The first law of refraction: The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane. Second law of refraction: The ratio of the sine of angle of incidence in the first medium to the sine of angle of refraction in the second medium is a constant for a gien pair of medium and for a gien waelength of light. sini n sinr where n is a constant known as refractie index of the second medium with respect to the first medium. This law is also called as Snell s Law 33. The phenomenon of change in path of light in going from one medium to another is called refraction of light. 34. The ratio of speed of light in acuum to the speed of light in a medium is called the refractie index of the medium. It has no unit. 35. When a ray of light traeling from a rarer to a denser medium slows down and bends towards the normal. 36. When it traels from a denser medium to a rarer medium, it speeds up and bends away from the normal. 37. The shifting of emergent ray sideways from the direction of original incident ray is called lateral displacement or lateral shift 38. The extend of the lateral shift depends on - the angle of incidence, - the thickness of slab, and - Refractie index of the glass slab

7 7 39. Lateral displacement is produced during refraction through a glass slab and not through a glass prism. 40. The angle through which a ray of light deiates on passing through a prism is called the angle of deiation. 41. Conex lens is thicker at the middle than at the edges. 42. Concae lens is thicker at the edges than at the middle. 43. Conex lens conerge the rays of light while concae lens dierges. 44. As the object moes towards the optical centre of conex lens, the image moes away from the optical centre except when the object is placed between focus and optical centre of the lens. 45. As the object moes towards the optical centre of the conex lens, the size of image increases and it s highly enlarged when object is at focus. 46. Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is Dioptre. Top Formulae Mirror Formula: u f Magnification produced by mirror: Height of the image (h') m Height of the object (h) Snell s law of refraction:

8 8 If I is the angle of incidence and r is the angle of refraction, then, sin i cons tant sin r The refractie index of medium 2 with respect to medium 1 n 21 Speed of light in medium 1 Speed of light in medium The refractie index of medium 1 with respect to medium 2 n 12 Speed of light in medium 2 Speed of light in medium Absolute refractie index: n m Speed of light in air Speed of light in the medium Lens Formula: u f Magnification produced by lens: Height of the Image h' m Height of the Image h c Magnification (m) = h / h = / u The power p of a lens of focal length f: P = 1 f