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Class VII Physics Chapter 4 Light Energy

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Class VII Physics Chapter 4 Light Energy Class VII Physics Chapter 4 Light Energy When light falls on an object the following may occur : (i) It may get scattered. (ii) It may get reflected. (iii) It may pass through the object. (iv) It may completely get absorbed by the object. Depending upon whether light gets reflected, passes or gets absorbed through the object, we can classify the objects into different types as follows: 1.Translucent : An object which absorbs some light falling on it and reflects the rest of the light is called a translucent object. 2.Transparent: An object which does not reflect any light falling on it and allows all the light to pass through it is called a transparent object. 3.Opaque: An object which absorbs all the light falling on it and allows no light to pass through it is called an opaque object. 4.Smooth and polished: A smooth and polished object absorbs negligible amount of light and reflects almost all the light falling on it. REFLECTION: Reflection takes place from a smooth and and polished surface through which light cannot pass. Definition of reflection: The bouncing back of light rays from surface is called reflection. Examples: Seeing your face in a mirror. Formation of a bright patch of light on a wall when you hold a mirror in sunlight. ACTIVITY: Aim: To show that reflection takes place from smooth and polished surface. Material Required: Small plane mirror ( looking glass),a highly polished brass plate and a book. Procedure: Let the sun rays fall on the mirror. Now turn the mirror through various angles, such that the light of the sun striking the mirror falls on the wall.You will find that as the angle of mirror is changed the position of light on the wall also changes.Thus we observe that the plane mirror reflects light. When we repeat the process with the brass plate, it also shows the same observation. If the process is repeated by holding the book in sunlight, we see that no light patch is formed on the wall.This is due to the reason that the book is an opaque object and that the surface of the book is rough so light rays bouncing from it move in various directions and no light patch is formed . Conclusion : A smooth and polished surface like a plane mirror or a brass plate reflects light. Terms Related to Reflection : Consider the figure given below in which a light ray is shown to fall on a mirror such that it may reflect. (i)Reflecting Surface: The light ray striking a surface get reflected.This surface is known as reflecting surface. AB is the reflecting surface. (ii)lncident Ray: The ray of light falling on the surface AB is known as incident ray.Here PN is the incident ray. (iii)Reflected Ray: The incident ray bouncing back in the same medium after striking the reflecting surface is called the reflected ray. NQ is the reflected ray. (iv)Point of lncidence: The point on the mirror surface, where incident ray strikes or the reflected ray bounces off is known as the point of incidence. Here N is the point of incidence. (v)Normal: It is a line drawn perpendicular to the reflecting surface at the point of incidence. Here MN is the normal. (vi)Angle of lncidence: The angle formed between the incident ray and the normal is called the angle of incidence.<PNM is the angle of incidence. It is denoted by ' i '. (vii)Angle of Reflection: The angle formed between the normal and the reflected ray is called the angle of reflection. <MNQ is the angle of reflection. It is denoted by ' r '. Types of Reflection : There are two types of surfaces (i) Regular (ii) lrregular or diffused surface. Similarly depending upon the surface, we have two types of reflection (i) Regular reflection (ii) lrregular reflection (i)Regular Reflection: If the parallel incident rays are reflected in such a way that all the reflected rays are parallel to each other then it is is known as regular reflection . Regular reflection occurs only if the rays are falling on a highly smooth ,polished surface like a plane mirror . Mirrors ,water, oil ,highly polished metals etc. cause regular reflection. (ii)Irregular Reflection: If parallel incident rays fall on an irregular surface, then the reflected rays scatter in all the direction. Such a reflection is known as irregular reflection. This type of reflection enables us to see the objects around us. Concrete floor, stones, books, walls, cardboards etc. cause irregular reflection. Laws of Reflection: Reflection of light from a reflecting surface takes place according to definite laws.These are known as the laws of reflection.There are two laws of reflection. 1.The incident ray and the normal at the point of incidence and the reflected ray all lie in the same plane. 2.When a ray of light falls on a reflecting surface its angle of incidence <i is always equal to its angle of reflection <r. ACTIVITY: Aim:To verify the Laws of Reflection and to prove that the image in the plane mirror is formed as far behind as the object is in front of it. Material Required:A wooden board, a white sheet of paper, board pins, common pins, ruler, pencil and protractor. Procedure: Fix a white sheet of paper on the wooden board with the help of board pins. Somewhere near the centre of the paper draw straight line AB with a pencil. Place the long face of the plane mirror strip along the line AB and hold it in vertical position with plasticine. Fix two common pins P and Q in front of the mirror strip in an upright position. The distance between the pins should be 4 to 5 cm. Looking from the side B of the plane mirror, fix two more pins R and S, such that these pins and the images of pins P and Q are in the same straight line. Now remove the mirror strip, remove the pins P, Q, R and S one by one and draw small circles around the pinholes. Join PQ and RS and produce them to meet the mirror line AB at O. This is the point of incidence. At O draw ON perpendicular to mirror surface. Measure the angle of incidence <PON and angle of reflection <SON. It is found that both the angles are equal. Further the incident ray OP, the reflected ray OS and the normal ON lie in the plane of the paper, all the three are also in the same plane. From point P, draw a perpendicular PL and produce it behind AB. Also produce SO behind AB. The lines produced meet at the point P'. Thus P' gives the position of image of the pin P. Now measure the distance PL and P'L. It is found that PL=P'L. This proves that images in the plane mirror are formed as far behind as the object are in front of it. PLANE MIRROR: A straight highly polished smooth and reflecting surface is known as a plane mirror. A mirror is made from a thin piece of glass whose one surface is made opaque by silvering it.The silvering is protected by giving a thin coat of lead oxide. TYPES OF MIRROR : (i) Plane mirror. (ii) Spherical mirror. Features of an ideal mirror : (i) It should give maximum reflection. (ii)It should not absorb light. (iii)It should not transmit more light. Following features are achieved in a mirror by: (i) Red paint- protects the silver coat. (ii) Glass sheet- provides smooth surface for maximum reflection. (iii) Silver polish -shining surface reduces transmission. Uses of Plane Mirror : (i) As a looking Glass. (ii) ln optician's room to double the length of room by keeping two mirrors on opposite walls of the room. (iii) ln barber's shop for seeing the rear view at the back, two mirrors are fixed on opposite walls facing each other. (iv) In Periscope, Kaleidoscope, Solar cooker etc. (v) ln many gadgets and electrical appliances. (iv) To get multiple images of an object by keeping them at proper angles to one another. Formation of lmage by a Plane Mirror: Let us take a plane mirror MM' and place a point object O in front of it. Rays of light travel from the object O in all directions.We can consider two rays OA and OB which fall on the mirror and get reflected as AC and BD respectively. To find out the position of the image we produce the reflected rays AC and BD backwards.They meet at a point l behind the mirror.Thus I is the virtual image of object O. On joining the points O and I, we find that OP =IP. Thus the perpendicular distance of the image from mirror is equal to the perpendicular distance of the object from it. Lateral lnversion : The phenomenon due to which the left side of an object appears to be the right side of the object in its image in a mirror(reflecting medium) and vice versa is called lateral inversion. Example: AMIT written on a piece of paper and held in front of a mirror will appear TIMA and likewise TIMA will appear AMIT. Characteristics of image formed in a plane mirror: (i) lmage is of same size as that of the object. (ii) The image formed is laterally inverted. (iii) The image is virtual i.e it cannot be obtained on the screen. (iv) The images is erect. (v) The image is formed as far behind the mirror as the object is in front of the mirror.
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