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. (vii)Angle of Reflection: The angle formed between the normal and the reflected ray is called the angle of reflection. 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 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 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. Real and Virtual image: The image which can be obtained on a screen is called a real image. It is formed when light rays after reflection actually meet or intersect. Whereas the image which cannot be obtained on a screen is called a virtual image. lt is formed when light rays after reflection do not actually meet but are made to intersect by producing backwards. A plane mirror forms a virtual image. Difference between Real and Virtual Image: Difference between Shadow and Image: Speed of Light: Light travels in straight lines. The speed of light in air or vacuum is 3×108m/s. It is the maximum speed of light. In transparent medium like water and glass the speed of light is less. In water the speed of light is 2.25×108m/s and in glass it is 2×108m/s. Constituent Colours of White Light from the Sun: White light from the sun consists of seven colours which are violet, indigo, blue, green, yellow, orange and red and are remembered as VIBGYOR. Primary Colours: The colours of light which on mixing give white light are called Primary Colours. These colours are Red , Green and Blue. We cannot get Red, Blue or Green colours by mixing light of any two colours whereas lights of all other colours can be obtained by mixing the light of any two of the three primary colours .Hence Red, Green, and Blue are called the primary colours. Secondary Colours: Secondary colours are those colours of light which are obtained by mixing two primary colours out of three. They are Yellow, Cyan and and Magenta . Red + Green =Yellow Blue +Green= Cyan Red + Blue = Magenta Complementary Colours : When two colours are added together to produce white, they are called complementary colours. Example: A combination of Blue and Yellow is used to produce the effect of White light in lighting during stage performances. Blue+ yellow =Blue+(Red +Green) = White Other complementary colours of light are Red and Cyan, Green and Magenta . Red +Cyan= Red+(Blue+Green) = White Green+Magenta=Green+( Red + Blue) =White. Appearance of colour of an object: Most of the objects around us reflect light. Those which are sources of light like lamps and lasers emit light. When light falls on an object some part of it is absorbed and some part is reflected. The light that is reflected appears to be the colour of the object in that light. All other colours are absorbed. Example: A Blue object absorbs all the colours of the spectrum except Blue as it reflects blue light. When we pass sunlight through a prism the light splits into its seven component colours. If you keep Red rose in Red part of the spectrum it appears Red but in other parts of the spectrum it appears Brown or Black as there is no Red light striking on it to be reflected to our eyes. When a Red rose and Green leaves are kept in White light the rose petals appear Red and the leaves Green because petals reflect Red part of the White light and leaves reflect Green part of the White light. Hence reflection and absorption of light cause the appearance of colour of an object. In White light some objects appear Black because they absorb all colours and reflect none. Colour Subtraction :Chemicals that are capable of selectively absorbing one or more frequency of white light are known as pigments. Material permeated by specific pigments absorb only one colour to produce a desired appearance. Example: Consider a shirt made of a material that is capable of absorbing Green light.Such a material will absorb Green light and reflect other colours. So the shirt will appear Magenta White - Green=( Red + Green + Blue) - Green = Red+ Blue = Magenta. Resultant colours of some other substances are given below: White - Red =( Red + Green + Blue) - Red =Blue + Green= Cyan White - Green=(Red + Green + Blue) - Green = Red+Blue = Magenta . White - Blue =( Red + Green + Blue) - Blue = Red + Green =Yellow Cyan - Blue =( Green + Blue) - Blue = Green Magenta - Blue=(Red+Blue) -Blue= Red Exercises Q.1. Define the following terms: (i) Reflection . (ii) Incident ray. (iii) Reflected ray. (iv) Normal. (v) Angle of incidence. (vi) Angle of reflection. (vii) Lateral inversion. Q. 2. Differentiate between the following: (i) Translucent and Transparent object. (ii)Regular and lrregular reflection. (iii)Real and Virtual image. (iv)Plane and Spherical mirror. (v) Shadow and lmage . Q.3.State the laws of reflection. Q.4.What are the features of an ideal mirror. Q.5.Write six uses of plane mirror. Q.6.Draw a ray diagram to show the formation of image of a point object kept in front of a plane mirror. Q.7.Write five characteristics of image formed in a plane mirror. Q.8.Compare the speed of light in (i) vacuum (ii) water and (iii) glass. Q.9. What are primary colours ? Q.10.What are the secondary colours? Q.11.Write the names of any two secondary colours. How can they be formed from primary colours? Q.12.Give reason why red rose appears red in white light.