LIGHT-REFLECTION AND REFRACTION. It is a form of energy which is needed to see things around us. It travels in a straight line.

Transcription

1 LIGHT-REFLECTION AND REFRACTION Class: 10 (Boys) Sub: PHYSICS NOTES-Reflection Light: It is a form of energy which is needed to see things around us. It travels in a straight line. Nature of Light: Light has a dual nature. Wave theory of light light consists of electromagnetic waves which do not require a medium. Particle theory of light- Light is composed of particles which travel in a straight line at very high speed. Some phenomenon of light such as Reflection, refraction, shadows follows the particle theory of light. And some phenomenon of light such as Diffraction, interference, polarization follows the Wave theory of light. The modern theory of light called Quantum Theory of Light combines both the wave and particle models of light as light exhibits the properties of both waves and particles. Reflection of Light: The process of sending back the light rays which fall on the surface of an object, is called reflection of light. The ray of light which falls on the mirror surface is called incident ray. The ray of light which is sent back by the mirror is called reflected ray. The normal is a line at right angles to the mirror surface at the point of incidence. The angle of incidence is the angle made by the incident ray with the normal at the point of incidence. The angle of reflection is the angle made by the reflected ray with the normal at the point of incidence.

2 Laws of Reflection: 1 st law -The incident ray, the reflected ray and the normal all lie in the same plane. 2 nd law The angle of incidence is equal to angle of reflection. Types of Reflection: Regular reflection In this, the parallel beam of light is reflected as parallel beam in one direction. It occurs from smooth surfaces such as plane mirrors, still water etc. Diffuse reflection In this, the parallel beam of light is reflected in different directions. It occurs in rough surfaces such as paper, book etc. Types of Images: Real images The image which can be obtained on a screen. It is formed when light rays coming from an object actually meet at a point after reflection. For ex: image at eye s retina, image formed by a projector etc Virtual images The image which cannot be obtained on a screen. It is formed when light rays coming from an object only appear to meet at a point when produced backwards after reflection. For ex: image on a TV screen, image in a mirror, image formed by convex mirror etc. Difference between Real and Virtual Images: Real Virtual 1. Can be obtained on a screen or wall Cannot be obtained on a screen or wall 2. Formed in front of the mirror Formed behind the mirror 3. The images are always are inverted. The images are always erect. 4. Can be touched Cannot be touched 5. Formed by concave mirrors only. Formed by all types of mirrors. ie. Plane, convex and concave Characteristics of the Images formed in a Plane mirror: The image formed is a virtual image. The image is erect. The size of the image is equal to that of the object.

3 The image formed is as far behind the mirror as the object is in front of it. The image is laterally inverted. Spherical mirrors: A spherical mirror is that mirror whose reflecting surface is the part of a hollow sphere of glass. The spherical mirrors are of two types: Concave Mirror It is that spherical mirror in which the reflection of light takes place at the concave surface(cave-in) Convex Mirror It is that spherical mirror in which the reflection of light takes place at the convex surface(bulging-out) Terms related to Spherical mirrors: Aperture - It is the portion of a mirror from which the reflection of light actually takes place. The aperture of a spherical mirror is represented by the diameter of its reflecting surface. Pole It is the centre of the reflecting surface of a spherical mirror. It lies on the surface of the mirror and represented by the letter P.

4 Centre of curvature It is the centre of the hollow sphere of glass of which the mirror is a part which is represented by the letter C. The centre of curvature of a concave mirror is in front of it but the centre of curvature of a convex mirror is behind it. Principal axis It is the straight line passing through the centre of curvature and pole of a spherical mirror Radius of Curvature It is the radius of the hollow sphere of glass of which the mirror is a part and represented by R. Principal Focus - The Principal Focus of concave mirror is the point on its principal axis to which all the light rays which are parallel and close to the axis, converge after reflection from the concave mirror. The concave mirror has a real focus as all the parallel rays of light converge at it focus after reflection and formed in front of the concave mirror. The Principal Focus of a convex mirror is a point on its principal axis from which a beam of light rays, initially parallel to the axis, appears to diverge after being reflected from the convex mirror. The convex mirror has a virtual focus as all the reflected rays diverge and appear to meet at its focus and formed behind the convex mirror.

5 Focal length It is the distance between its pole and principal focus. Relation between Radius of Curvature and Focal Length of a Spherical mirror: The focal length of a spherical mirror is equal to half of its radius of curvature. If, f is the focal length and R is the radius of curvature, then: f = R/2 or R = 2f Rules for obtaining images formed by spherical mirrors: 1 st Rule -A ray parallel to the principal axis, after reflection, will pass through the principal focus for concave mirror or appear to diverge from the principal focus for convex mirror. 2 nd Rule - A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis. 3 rd Rule - 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.

6 4 th Rule - A ray incident obliquely to the principal axis, towards a point P on the concave mirror or a convex mirror is reflected obliquely. Formation of different types of images by a concave mirror: Object at infinity: Image formed at the focus Highly diminished, point-sized Real and inverted Object beyond C Image between F and C Size is diminished Real and inverted Object at C Image formed at C Same size Real and inverted

7 Object between C and F Image formed beyond C Size is enlarged Real and inverted Object at F Image is formed at infinity Size is highly enlarged Real and inverted Object between P and F Image is formed behind the mirror Size is enlarged Virtual and erect Uses of Concave Mirrors : Used in torches, search-light and vehicles headlights to get powerful beams of light. Used as shaving mirrors to see a larger image of the face. Dentists use it to see large images of the teeth of the patients. Concentrate sunlight to produce heat in solar furnaces.

8 Formation of different types of images by a convex mirror: At infinity: Image is formed at the focus Behind the mirror Highly diminished, point sized Virtual and erect Between infinity and pole: Image is formed between P and F, behind the mirror Diminished Virtual and Erect Uses of Convex Mirrors : Used as rear-view mirrors in vehicles: i. As it produces an erect image of the objects. ii. As the image formed is highly diminished or much smaller than the object, due to which it gives a wide field of view Used as shop security mirrors in order to help the shop owner to keep an eye on the customers. Sign convention for spherical mirrors: According to the New Cartesian Sign convention: The object is always placed to the left of the mirror. All the distances are measured from pole of the mirror as origin. Distances measured in the same direction as that of incident light are taken as positive. Distances measured against the direction of incident light are taken as negative. Distances measured upward and perpendicular to the principal axis are taken as positive. Distances measured downward and perpendicular to the principal axis are taken as negative.

9 Conclusions : The object distance(u) is always negative. For a concave mirror, if the image is formed behind, the image distance(v) is positive but if the image is formed in front of the mirror then the image distance will be negative. In convex mirror, the image is always formed on the right hand side, so v is always positive. The focus of concave mirror is in the front on the left side, so its negative but the focus of the convex mirror is behind the mirror, so its positive. The object is always placed above the principal axis in the upward direction, so the height is positive but the image can be formed above the principal axis or below, so if the image is formed above the principal axis, its height is taken as positive and if the image is formed below the principal axis, then its height is taken as negative. All the virtual images are erect and formed above principal axis, so the height of all virtual and erect images is positive but all real images are inverted and formed below the principal axis, so the height of all the real and inverted images is taken as negative. Mirror Formula : It s the relationship between object distance, image distance and focal length = 1 v u f Where u is the object distance v is the image distance f is the focal length

10 [ Magnification : Magnification produced by a spherical mirror gives the relative extent to which the image of an object is magnified with respect to the object size. Linear magnification is the ratio of the height of the image to the height of the object. It is represented by letter m. If h is the height of the object and h is the height of the image, then the magnification m produced by a spherical mirror is given by m= Height of the image(h ) Height of the object(h) Or m = h h Conclusions: The object is always placed above the principal axis, so the height of the object is always positive. The virtual image is always formed above the principal axis, so the height will be positive. The real image is formed below the principal axis, so the height of a real image will be negative. For virtual image, h is positive and h is also positive, so the magnification of a virtual image is always positive. So, if the magnification has a plus sign, then the image is virtual and erect. For a real image, h is negative and h is positive, so the magnification for a real image is always negative. So, if the magnification has a minus sign, then the image is real and inverted. If the magnification, m has a value greater than 1 then the image is enlarged. If the magnification, m is 1, then the image is of the same size as the object. If the magnification, m is less than 1, then the image is smaller than the object. Magnification in relation to object and image distance : The linear magnification produced by a mirror is equal to the ratio of the image distance to the object distance with a minus sign. That is, Magnification = - Image distance Object distance Or m = - v u Combining both the formulas, we get, h = - v h u ***********