Class 10 Science: Light Reflection and Refraction Revision Notes PDF

Reflection Of Light by Plane Mirror Let us do a quick activity before we move ahead with the lesson. Close your eyes for 10 seconds and count to 10; now when you open your eyes again, what do you observe? Do you observe all the things around you? What are the phenomena that allow you to see things around you? Does the phenomena Reflection of Light have anything to do with this? Let us know more about it. What is Reflection of Light? When a ray of light falls on any object (polished, smooth, shiny object), light from the object bounces back those rays of light to our eyes and this phenomenon is known as “Reflection” or “Reflection of Light”. This phenomenon is what enables us to look at the world around us and is based on the property that light travels in a straight line. For example, twinkling of stars or light reflected by a mirror. (Source: Wikipedia) Laws of Reflection In the diagram given above, the ray of light that approaches the mirror is known as “Incident Ray”. The ray that leaves the mirror is known as “Reflected Ray”. At the point of incidence where the incident ray strikes the mirror, a perpendicular line is drawn known as the “Normal”. This normal is what divides the incident ray and the reflected ray equally and gives us the “Angle of Incidence” (Qi) and “Angle of Reflection” (Qr). Hence the above information gives us the “Laws of Reflection of Light” which state that : a. The angle of incidence is equal to the angle of reflection. b. The incident ray, the normal and the reflected ray, all lie in the same plane. Learn more about Reflection of Light by Spherical Mirrors. Types of Reflection : There are majorly two types of reflection : a. Specular/ Regular reflection b. Diffused/ Irregular reflection a. Specular/Regular reflection : Specular/Regular reflection is a mirror-like reflection of rays of light. Here the rays of light which are reflected from a smooth and shiny object such as a mirror, are reflected at a definitive angle and each incident ray which is reflected along with the reflected ray has the same angle to the normal as the incident ray. Thus, this type of phenomena causes the formation of an image. (Source: Wikipedia) Learn how is Image formed in Spherical Mirror here. b. Diffused/Irregular reflection: Diffused/Irregular reflection is a non-mirror-like reflection of light. In this type of reflection rays of light that hit an irregular object with a rough surface, are reflected back in all directions. Here, the incident ray which is reflected along with reflected ray doesn’t have the same angle to the normal as the incident ray. Thus, this type of reflection doesn’t form an image. (Source: Wikipedia) Image Formation by a Plane Mirror Let us take a mirror MM’ as shown in the diagram given above. Let us suppose an object AB of size ‘h’ on the left-hand side of the mirror at a distance ‘u’. An incident ray of light AP from point A of the object AB falls on the mirror MM’ at point P. This incident ray AP is reflected back in the same path PA. Another ray OC falls on the mirror MM’ at a point O and is reflected along the path OC. Now, since reflected rays PA and OC are diverging and therefore cannot meet each other in front of the mirror, hence we extend these rays PA and OC behind the mirror by dotted lines. On extending these rays behind the mirror, we see that these rays meet at point A’ at a distance V’. Therefore A’ is the virtual image of point A of the object AB. Similarly, a virtual image of point B will be formed behind the mirror as B’ from the incident rays BO and BE. Now, to get a complete image of the object AB, we join the point A and B to point A’ and B’ by a dotted line. In doing so, we find that the image A’B’ being formed is virtual, erect and of same shape and size as the object AB; thereby giving us the characteristics of images formed by the plane mirror. Learn more about the Laws of Reflection of Light in detail. Characteristics of Images formed by Plane Mirror a. Images formed by a plane mirror are “Always Virtual”. b. Images formed by a plane mirror are “Erect/Upright”. c. Images formed by a plane mirror are of “same shape and size” as that of an object. Solved Example for You Q. A ray of light strikes a reflective plane surface at an angle of 42o with the surface. a. Find the angle of incidence. b. Find the angle of reflection. c. Find the angle made by the reflected ray and the surface. d. Find the angle made by the incident and reflected rays. Sol: We’ll use a diagram given below to answer the question: a. Angle of Incidence (Qi)= 900-420= 480 b. Angle of Reflection (Qr)= (Qi)= 480 c. x = 900-Qr = 900-480 = 420 d. Qi + Qr = 480 + 480 = 960 Terms Related to Spherical Mirrors “Mirror, mirror on the wall, who’s the fairest of them all?” We’ve all heard this dialogue from the story of Snow white and the seven dwarfs. But what kind of mirror was it? What were the properties that enabled these spherical mirrors to be so special? Well, let us find answers to it in this section. There’re few basic and important terms one needs to know while studying spherical mirrors inside the chapter “Reflection and Refraction”: a. Center of Curvature (c) b. Radius of Curvature (r) c. Pole (p) d. Principal axis e. Aperture f. Principal Focus or Focal Length g. Focus a. Center of Curvature (c) : (Source: Wikipedia) The Center of Curvature of a spherical mirror is the point in the centre of the mirror which passes through the curve of the mirror and has the same tangent and curvature at that point. It is denoted by the letter ‘c’. b. Radius of Curvature (r): It’s the linear distance between Pole and the Center of curvature. c. Pole (p) : It’s the midpoint of the spherical mirror. d. Principal axis: It’s an imaginary line passing through the optical centre and the centre of curvature of any lens or a spherical mirror. e. Aperture: An aperture of a mirror or lens is a point from which the reflection of light actually happens. It also gives the size of the mirror. f. Principal Focus : Principal Focus can also be called as Focal Point. It’s on the axis of a mirror or lens wherein rays of light parallel to the axis converge or appear to converge after reflection or refraction. Principal Focus is also what determines the Focal Length of the mirror. g. Focus: It’s any given point, where light rays parallel to the principal axis, will converge after getting reflected from the mirror. What is an Image? When an object is placed in front of a mirror, we see an image of the object placed. This image appears to be behind the mirror, and is called “Image”. The object is the source from which incident ray occurs and the image that is formed is because of the reflected rays. The image formed maybe “Real” or “Virtual”. Wherein the “real” image is formed when the light rays from the object actually intersect each other after reflection. Real images are formed inverted and can be projected on to a screen. On the other hand, a “virtual” image is formed when the light rays from the object don’t actually intersect each other after reflection. Although they “appear” to do so when they’re produced behind the mirror. Virtual images are “always” erect and cannot be projected on a screen. Concave and Convex Mirrors. (Source: Wikipedia) See how Image is formed by Spherical Mirrors here Types of Spherical Mirrors Spherical mirrors are of two types: a. Concave Mirror b. Convex Mirror Learn more about Reflection of Light by Spherical Mirrors in more detail here. a. Concave Mirror A concave mirror is curved inward. Since it’s curved inward, when one looks at a concave mirror, it looks like the person is looking into a cave. (Source: National Geographic) A concave mirror is also known as a “Converging Mirror” since in these type of mirrors light rays converge at a point after they strike and are reflected back from the reflecting surface of the concave mirror. In the majority of the cases, a concave mirror produces real and inverted images except when the object is placed very near to the mirror i.e. pole (p) and the focus (f) where the image produced is virtual and erect. Many examples of concave mirrors can be seen in our daily life; few such examples are torch used to reflect light, shaving mirrors, concave mirrors are also used in telescopes and more. Can you find out more such examples? Example of a Concave mirror (Source: Martinhurxford.com) b. Convex Mirror A concave mirror is curved outward. Since it’s curved outward, it looks like one is looking at the bump of a car. (Source: TradeIndia) A convex mirror is also known as a “Diverging Mirror” since here light rays diverge after it strikes the reflecting surface of the convex mirror. Convex mirrors “always” form virtual, erect and diminished regardless of the distance between the object and mirror. Few examples of convex mirrors can also be seen in our daily life such as the rearview mirror in a car, street light reflectors and more. Can you find more such examples for convex mirrors too? Example of a Convex mirror (Source: Telegraph.co.uk) Learn different Laws of Reflection here. Solved Examples for You Question: Fill in the blanks: An image formed by _________ mirror is always of the same size as that of the object. A. Concave B. Convex C. Plane D. Small Answer: Option C Plane A plane mirror forms the same size images as that of the object regardless of the position of the object. Question: When the object is focused on the Concave mirror, where is the Image formed at? Learn Reflection of light by Spherical mirror to know the answer. Image Formation by Spherical Mirrors Have you ever gone camping in the night or have walked inside a cave? Whenever its dark, you must have used a torch to show you the right path. Why not a candle instead? Well, torches have special spherical mirrors which make the light focused in the direction you want to go. Let’s find out the mechanism involved in the image formation by spherical mirrors… Ray Diagrams Ray diagrams are used to depict the image formation by tracing the path of light rays i.e. incident rays and reflected rays. They are drawn in order for anyone to view a point on the image of an object. These ray diagrams depend on the position of the object. General rules for image formation using ray diagrams: Any ray of light that passes through the mirror, is always parallel to the principal axis. Any ray of light that passes through the mirror always passes through the principal focus (f) of the mirror after reflection. A ray of light passing through the center of curvature of any mirror is reflected back along the same path. Any incident ray which isn’t parallel to the principal axis is also reflected diagonally and the incident ray and the reflected ray always follow the laws of reflection i.e. the angles formed by these rays are equal to each other. Ray Diagrams for a Concave Mirror For a concave mirror, there are six possible positions where the object can be positioned and an image is formed: a. Object is positioned at infinity When the object is placed at infinity, rays PQ and RS parallel to the axis are reflected from points Q and S respectively. Rays PQ and RS intersect each other and get converged at the principal focus (f). And since when the object is placed at infinity, the properties of the images formed are highly diminished, point sized and real and inverted. b. Object is positioned between infinity and center of curvature(c) Here the object MN is placed between infinity and center of curvature (c) of a concave mirror, then a ray MP parallel to the principal axis and another ray MQ that pass through the center of curvature(c) intersect each other at M’ after reflection between focus (f) and center of curvature (c). Therefore the properties of the images formed here are that the image formation is between principal focus (f) and center of curvature (c), the image formed is diminished and real and inverted. c. Object is positioned at Center of Curvature (c) When the object MN is placed the at the center of curvature (c), then a ray MP parallel to the principal axis and another ray MQ that passes through the principal focus (f) after reflection, intersect each other at point M’ right below where the object MN is positioned. Hence the properties of the images formed in this case are that image is formed at the center of curvature, the image is the same size as the object and images are real and inverted. d. Object is positioned between the center of curvature (c) and principal focus (f) Object MN is placed between the center of curvature (c) and principal focus (f), then the ray MP parallel to the principal axis and another ray MQ passing through principal focus (f) intersect each other beyond the center of curvature (c) at point M’. Hence the properties of the images formed here are that the image is formed beyond the center of curvature (c), and the image is real and inverted. e. Object is positioned at principal focus (f) Object MN is positioned at the principal focus (f), then ray MP parallel to the principal axis passes through principal focus (f) giving the reflected ray PS. Second ray MQ that passes through the center of curvature is reflected along the same path giving the reflected ray QR. Here, since the rays, PS and QR become parallel to each other and therefore the image formation is at infinity. Here the properties of the images formed are highly enlarged images and real and inverted images. f. Object is positioned between principal focus (f) and pole (p) Object MN is positioned between principal focus (f) and pole (p), then the ray MP parallel to principal axis passes through principal focus (f) giving the reflected ray PS and the second ray MQ that passes through the center of curvature is reflected along the same path giving the reflected ray QR. Now, since the reflected rays PS and QR are diverging away hence cannot intersect each other, hence reflected rays PS and QR are extended behind the mirror by dotted lines. In doing so, rays PS and QR appear to intersect each other at point M’ backwards. Therefore, the properties of the images formed here are formed behind the mirror, images are highly enlarged, images are virtual and erect. Ray Diagrams for a Convex Mirror In case, of a convex mirror, there are only two possible positions where the object can be positioned and an image can be formed. a. Object is positioned at Infinity When the object is at infinity, the rays MN and PX that are parallel to the principal axis (f) are divergent in the direction NZ and XY respectively; after getting reflected from the convex mirror. The diverged rays NZ and XY are extended behind the mirror, where they intersect each other at the principal focus (f). Hence, in this case, the properties of the images formed are formed at the principal focus (f) behind the mirror and are highly diminished, the images are virtual and erect. b. Object is positioned between the pole (p) and the principal focus (f) When the object MN is placed between pole (p) and infinity, a ray MC that starts from point M of the object MN that’s running parallel to the principal axis is reflected along CY. On extending behind the mirror, CY appears to come from principal focus (f) and another ray MD from point M of the object MN that goes towards the center of curvature is reflected along DM. The two rays, CY and DM are diverging rays and when extended behind the mirror, they appear to intersect each other at point M’. Therefore, the properties of the images formed here are formed behind the mirror, between the pole and principal focus (f), the images are diminished and are virtual and erect. Solved Example for You Q: The value of the focal length of the lens is equal to the value of the image distance when the rays are: a. Passing through the optic center b. Parallel to the principal axis c. Passing through the focus d. In all the above cases Sol: b. Parallel to the Principal axis According to the rule of ray optics, all the rays parallel to the principal axis must pass through the focus of the lens after getting refracted from that lens as shown in the figure. Thus, in this case, the image formation is at the focus of the lens and hence the value of image distance is equal to the focal length of the lens. Mirror Formula and Magnification Let us do a quick activity. Stand in front of a mirror and mark your position with a colored tape and label it as point A. Now, from point A, walk a little away from the mirror and mark it again with a colored tape and label it as point B. Can you calculate the distance from point A to point B? Need help in solving this problem? This distance can easily be calculated using the mirror formula. Let’s scroll ahead to find more. Sign Conventions The sign convention for spherical mirrors follows a set of rules known as the “New Cartesian Sign Convention”, as mentioned below: a. The pole (p) of the mirror is taken as the origin. b. The principal axis is taken as the x-axis of our coordinate system. c. The object is always placed on the left side of the mirror which implies that light falling from the object on the mirror is on the left-hand side. d. All the distances parallel to the principal axis are measured from the pole (p) of the mirror. e. All the distances measured from the pole (p) on the right-hand side of the mirror are taken as positive and those on the left-hand side of the mirror are taken as negative. f. Distances measured perpendicular to and above the principal axis are taken as positive. g. All the distances below the principal axis are taken as negative. Learn Reflection of Light by Plane Mirror here Mirror Formula Now, that all these conventions are clear; let us know move on the mirror formula. Mirror Formula helps us to find: a. Image distance which is represented as ‘v’. b. Object distance which is represented as ‘u’. c. Focal length which is represented as ‘f’. And is written as : 1 v + 1 u = 1 f This formula is valid for all kinds of spherical mirrors, for all positions of the object. Although one needs to be careful about the values, one puts for u,v and f with appropriate sign according to the sign convention given above. Learn about Terminology of Spherical Mirrors and its types here Magnification Physically, we all understand what is magnification. It can be defined as the extent to which the image appears bigger or smaller in comparison to the object size. It is represented as the ratio of the height of the image to the ratio of the height of the object. Magnification is denoted as the letter ‘m’. Where, Magnification (m) = h/h’ And h’ is the image height and h is the object height. Magnification can also be related to the image distance and object distance; therefore it can also be written as: m = -v/u Where v is the image distance and u is the object distance. Hence, the expression for magnification (m) becomes: m = h’/h = -v/u Learn more about Reflection of Light here Solved Example for You Q. What will be the distance of the object, when a concave mirror produces an image of magnification m? The focal length of the mirror is f. a. 1 m(m+1) b. (m-1)f c. f m(m−1) d. (m+1)f Sol: (c.) f m(m−1) Given, m = -v/u => v = -mu By mirror formula, 1/f = 1/v + 1/u = 1/-(mu) + 1/u => 1/f = 1/u ( -1/m + 1 ) OR u= f m(m−1) Refraction and Refractive Index Have you ever observed that whenever you put a straw in your cold-drink, it looks bent? Or while travelling on a road on a hot summer day, distantly, water appears in the middle of the road out of no-where! Do you know how this happens? Does the straw really bend when immersed in water? Or is it just our eyes? Scroll ahead to know more about refraction and refractive index. Refraction The change in direction or bending of a light wave passing from one transparent medium to another; caused by the change in wave’s speed is known as “Refraction”. An example to understand this better is that of placing a straw/stick in a glass of water wherein it to be bent when viewed from any other angle than 900 to the surface. This happens because of bending of light rays as they move from air to glass. This bending of light depends on the speed of light in air and glass and the speed is dependent on the wavelength. Refractive Index The extent of bending of light rays entering from one medium to another is known as “Refractive Index”. It is denoted by the letter ‘n’. And can be represented as : n = c/v Where c = velocity/speed of light of a certain wavelength in the air and v = velocity of light in any medium. The refractive index depends on the following factors: a. nature of the medium b. physical conditions c. the color of the wavelength of light The nature of a medium is defined as: a. A medium is said to be “optically rarer” medium if the light in it travels faster. b. A medium is said to be “optically denser” medium if the light in it travels slower. If the value of the refractive index is high then, the bending effect light too will be higher. When passing from air into any medium. Any medium with a greater value of the refractive index (n) is an optically denser medium. When we imagine a ray of light passing from air to any medium, say water; we draw a perpendicular to its surface known as the ‘normal’. Therefore when this ray of light passes from : a. Optically denser medium to optically rarer medium it bends away from the normal. b. Optically rarer medium to optically denser medium it bends towards the normal. Source: Medium.com Source: Medium.com When air is taken as a medium and the velocity of light is taken in it, then the refractive index with respect to air is termed as “Absolute refractive index”. Refractive index can also be defined as the constant obtained from the ratio of the sine of the angle of incidence to the sine of the angle of refraction i.e. n= sini sinr = c v This gives rise to the second law of refraction also known as the “Snell’s Law”. Snell’s Law It gives the amount of bending of light rays. It determines the relationship between the angle of incidence, the angle of refraction and relative indices of given pair of media. It is defined as the : “Ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant for the light of given color and for the given pair of media”. Source: Britannica.com Laws of Refraction The angle of incidence is the angle between the incident ray and the normal; denoted as ‘i’. The angle of refraction is the angle between the refracted ray and the normal; denoted as ‘r’. Laws of refraction state that: i. The incident ray, reflected ray and the normal, to the interface of any two given mediums; all lie in the same plane. ii. The ratio of the sine of the angle of incidence and sine of the angle of refraction is constant. Solved Examples For You Q1. For the same angle of incidence in the mediums A, X and Y, the angles of refraction are 550, 350, 150 respectively. In which of the medium will the velocity of light be minimum? Sol: Snell’s law says that, n= sini sinr = c v For the given angle of incidence (i), V will be minimum, when angle of refraction

Class 10 Science: Light Reflection and Refraction Revision Notes PDF

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