Refraction of Light PDF

## KNOWING CONCEPTS ### Refraction: - **Definition:** The change in direction of the path of light when it passes from one optically transparent medium to another is called refraction of light. - **Curved mirrors:** - **Convex:** Examples of refraction - **Concave:** Reflecting surface(co...

## KNOWING CONCEPTS ### Refraction: - **Definition:** The change in direction of the path of light when it passes from one optically transparent medium to another is called refraction of light. - **Curved mirrors:** - **Convex:** Examples of refraction - **Concave:** Reflecting surface(convex and concave). Uses of curved mirrors. Terms related to curved mirrors: Focus, principal axis, centre of curvature, radius of curvature. Rules for making ray diagrams of spherical mirrors. Real and virtual images. Ray diagrams with curved mirrors where real images are formed. - **Dispersion of white light into constituent colors:** ## SPEED OF LIGHT IN DIFFERENT MEDIA - In class VII, we have read that light travels faster in air than in water or glass. - speed of light in air: $3 × 10^8\ m/s$ - speed of light in water: $2.25 × 10^8\ m/s$ - speed of light in glass: $2 × 10^8\ m/s$. - We say that glass is optically denser than water and water is optically denser than air (or air is optically rarer than both water and glass). - A medium is said to be denser if the speed of light in it decreases, while it is said to be rarer if the speed of light in it increases. - In no medium the speed of light can be more than $3 × 10^8\ m/s$. ## REFRACTION OF LIGHT - Light travels in a straight line path in a medium. - When a ray of light travelling in one transparent medium falls obliquely on the surface of another transparent medium, it travels in the other medium in a direction different from its initial path. ### It has been experimentally observed that: - **(1)** When a ray of light travels from a rarer to a denser medium (say from air to water or from air to glass), it bends towards the normal as shown in **Fig 5.1**. - **(2)** When a ray of light travels from a denser to a rarer medium (say from water to air or from glass to air), it bends away from the normal as shown in **Fig 5.2**. - **(3)** When a ray of light falls normally on the surface separating the two media, it passes undeviated (i.e., along the same path) as shown in **Fig 5.3**. ### SOME TERMS RELATED TO REFRACTION OF LIGHT: 1. **Incident ray:** The ray of light falling on the surface separating the two media, is called the incident ray. 2. **Refracted ray:** The ray of light traveling in the other medium in the changed direction, is called the refracted ray. 3. **Normal:** The perpendicular drawn on the surface separating the two media, at the point where the incident ray strikes it, i.e. at the point of incidence, is called the normal. 4. **Angle of incidence:** The angle between the incident ray and the normal is called the angle of incidence 'i'. 5. **Angle of refraction:** The angle between the refracted ray and the normal is called the angle of refraction 'r'. **Note:** When a ray of light passes form one transparent medium to another transparent medium having the same refractive index, it also remains undeviated. ### LAWS OF REFRACTION (SNELL'S LAW) - Refraction of light obeys the following two laws also known as Snell's laws of refraction: - The incident ray, the normal at the point of incidence and the refracted ray, all lie in the same plane. - For a given pair of media and given colour of light, the ratio of the sine of angle of incidence i to the sine of angle of refraction r is a constant i.e., $\frac{sin\ i}{sin\ r} = constant$. This constant is denoted by the symbol μ (read as mew) and is known as the refractive index of the second medium with respect to the first medium. It is given as: $\mu = \frac{Speed\ of\ light\ in\ first\ medium}{Speed\ of\ light\ in\ second\ medium}$ For example, if a ray of light travels from air to water, then the constant $\mu = \frac{sin\ i}{sin\ r}$ is the refractive index of water with respect to air. It is given as: $\mu = \frac{sin\ i}{sin\ r} = \frac{3×10^8\ m/s}{2.25×10^8\ m/s} = \frac{4}{3}$ (or 1.33) Similarly: if a ray of light travels from air to glass, then $\mu = \frac{sin\ i}{sin\ r} = \frac{3×10^8\ m/s}{2×10^8\ m/s} = 1.5$ **Note:** The refractive index of air is 1. No medium can have refractive index less than 1. ## EFFECTS OF REFRACTION - **The depth of water in a vessel when seen from air appears to be less** - Consider a vessel containing water as shown in **Fig 5.6**. Its real depth is AO. But when seen obliquely (i.e., at an angle above O from air), its depth appears to be AI which is less than AO. - **Reason:** A ray of light OA from the point O at the bottom of the vessel is incident normally on the water-air surface XY. It travels straight along AD in air. Another ray OB incident from water on the surface XY, when passes to air, bends away from the normal BN, and goes along the path BC. The two refracted rays AD and BC when produced back, meet at I. Thus I is the image of O. - **Thus to the observer in air, the depth of the vessel appears to be AI instead of AO, due to refraction of light from water to air. The apparent depth AI is less than the real depth AO.** - **Do You Know?** Real depth/ Apparent depth = Refractive index. Since, refractive index of water is 4/3, so the apparent depth is 2/3 the real depth. - **The change in depth due to refraction can be demonstrated by the following activities:** - **(1)** Take a coin and an empty glass vessel. Place the coin in the vessel. Put the vessel on a table and step back till the coin is just out of your view. It is hidden from your eye by the edge of the vessel as shown in **Fig 5.7(a)**. - **(2)** Keep your eye in this position and ask your friend to pour water gradually in the vessel. You will find that when there is sufficient water in the vessel, the coin becomes visible because then it appears to be slightly raised from position A to position B as shown in **Fig 5.7(b)**. - **Explanation:** In **Fig 5.7(a)**, when there is no water in the vessel, the coin is not visible because the ray of light from the coin traveling in a straight line does not reach the eye. In **Fig 5.7(b)**, when water is poured in the vessel, the coin becomes visible because the ray of light from the point A of the coin, travelling in a straight line changes its direction (i.e., it bends) at the surface of water and reaches the eye. Thus, the light ray bends as it leaves water and enters air. The ray now appears to come from a point B instead of A. In other words, the coin appears to be raised from position A to position B. - **The pencil in water appears to be bent** - **Explanation:** The ray of light coming from the tip of the pencil bends at the surface of water as it enters in air and it appears to be coming from the point D. In other words, it is due to refraction of light from water to air that the pencil ABC appears as ABD. - **From the above, we conclude that when a light ray passes from one transparent medium to another, it bends. The direction which light ray bends, depends upon whether light travels from a rarer medium to a denser medium or from a denser medium to a rarer medium.** - **Early sunrise and late sunset** Before sunrise and after sunset, the upper atmospheric layers are warmer than the layers near the earth’s surface. So the atmospheric layers near the earth’s surface are denser than those above. When the sun is just below the horizon, the light from sun, while coming towards the earth, suffers refraction from a rarer to a denser layer and so it bends towards the normal at each refraction. Due to continuous bending of light rays at different successive layers, the sun can be seen even when its actual position is just below the horizon as shown in **Fig 5.9**. As a result, sun is seen in advance, a few minutes before it rises in the morning, in the morning. Similarly, in the evening, sun is seen delayed by 3 to 4 minutes longer above the horizon after the sun set. - **Mirage in a desert** Sometimes, in deserts, an inverted image of a tree is seen which gives a false impression of water under the tree. This is called a mirage. The cause of mirage is the refraction of light. In a desert, the sand becomes very hot during day-time and it rapidly heats the layers of air in contact with it. Therefore, the layers of air near the ground are warmer (and hence rarer) than the upper layers. In other words, the successive upper layers are denser than those below them. - When a ray of light from sun after reflection from the top of a tree travels from a denser to a rarer layer, it bends away from the normal. As a result, in refraction at the surface of separation of successive layers, each time the angle of refraction increases and the angle of incidence of ray going from denser medium to rarer medium also increase, till a stage is reached when the angle of refraction becomes 90°. On further increase in angle of incidence, the ray of light travelling from a denser to a rarer layer, is not refracted, but it suffers reflection. This reflected ray now travels from the rarer to the denser layer, so it bends towards the normal, at each refraction. On reaching the eye of the observer, an inverted image of the tree is seen. Thus it gives a false impression of a pool of water in front of the tree (**Fig 5.10**). ## REFRACTION OF LIGHT THROUGH A RECTANGULAR GLASS BLOCK - **Fig 5.11** shows a rectangular glass block PQRS. A light ray AB falls on the surface PQ. NBM is the normal at the point of incidence B to the surface PQ. At the surface PQ, the ray AB enters from air to glass, so it bends towards the normal NBM and travels along BC. At the surface RS, another refraction occurs. N₁CM₁ is the normal at the point of incidence C to the surface RS. - The ray BC now enters from glass to air, so it bends away from the normal N₁CM₁ and travels along CD. The ray AB is called the incident ray, BC the refracted ray, and CD the emergent ray. - The emergent ray CD is parallel to the incident ray AB. Thus, both the incident and emergent rays are in the same direction, but the emergent ray is laterally displaced from the incident ray. In **Fig 5.1**. The lateral displacement is shown by CE (x) which is the perpendicular distance between the incident ray and the emergent ray. ## PRISM - A prism is a transparent medium bounded by five plane surfaces with a triangular cross section. Two opposite surfaces of a prism are identical and parallel triangles, while the other three surfaces are rectangular and inclined on each other as shown in **Fig 5.12**. - In symbol form, it is represented by the triangle ABC. ## REFRACTION OF LIGHT THROUGH A PRISM - **Fig 5.13** shows a prism ABC. A ray of light PQ of single colour falls obliquely on the face AB of the prism. This ray enters from air to glass (i.e., from a rarer medium to a denser medium), so it bends towards the normal NQM to the face AB and travels along QR. At the face AC of the prism, another refraction occurs. - The emergent ray through a prism is not in the direction of the incident ray, but it bends towards the base of the prism because in a prism, refraction occurs at inclined surfaces. On the other hand, in a rectangular glass block, refraction of light occurs at two parallel surfaces, so the emergent ray is in the direction of incident ray, but laterally displaced. ## DISPERSION OF WHITE LIGHT - Newton allowed white light from the sun to enter a dark room through a small aperture in a window and placed a glass prism in the path of light rays. The light coming out of the prism was received on a white screen. On the screen, a coloured patch like a rainbow was found as shown in **Fig 5.14**. This patch was termed as spectrum. - **Since, refractive index = (speed of light in air)/(speed of light in medium)**, the refractive index of a medium is maximum for the violet light and minimum for the red light. Therefore, when white light enters a prism, it splits into its constituent colours while refraction at the first surface of the prism. These colours get further separated from each other on refraction at the second surface of prism. - **Do You Know?** In rainy season, sometimes after the rains, you see a rainbow in the sky, just opposite to the sun. It is due to dispersion of white light of sun by the rain drops which behave like small prisms. - **The dispersion of white light can be demonstrated by the following activities:** - **(1)** Take a thick cardboard sheet. Make a small hole in it. Allow the sunlight to pass through it in a dark room. Place a prism in the path of sunlight coming through the hole and then a white screen behind the prism as shown in **Fig 5.15**. You will see that a band of colours is obtained on the screen with colours violet, indigo, blue, green, yellow, orange and red in order from the base of the prism upwards as shown in **Fig 5.15**. - **(2)** Take a circular disc of cardboard and divide it into seven sectors. Then paint the sectors with the seven colours (violet, indigo, blue, green, yellow, orange and red) in order, as shown in **Fig 5.16**. Rotate the disc rapidly. You will notice that the disc appears white. This shows that seven colours violet, indigo, blue, green, yellow, orange and red being the constituent colours of white light, when combined produce the white colour effect. ## SPHERICAL MIRRORS - Spherical mirrors are made by silvering the part AB of the hollow glass sphere as shown in **Fig 5.17**. ### Kinds of spherical mirrors: - There are two kinds of spherical mirrors: - **(i) Concave mirror:** A concave mirror is made by silvering on the outer surface of a hollow sphere such that the reflection takes place from the inside hollow (or concave) surface as shown in **Fig 5.18(a)**. - **(ii) Convex mirror:** A convex mirror is made by silvering on the inner surface such that the reflection takes place from the outer convexed (or bulged) surface as shown in **Fig 5.18(b)**. ### SOME TERMS RELATED TO A SPHERICAL MIRROR: 1. **Pole:** The geometric centre of the spherical surface of the mirror is called the pole of the mirror. It is the mid point of the aperture AB of the mirror. It is represented by the symbol P in **Fig 5.19**. 2. **Centre of curvature:** The centre of curvature of a mirror is the centre of the sphere of which the mirror is a part. It is represented by the symbol C in **Fig 5.19**. 3. **Radius of curvature:** The radius of curvature of a mirror is the radius of the sphere of which the mirror is a part. Thus, it is the distance of the centre of curvature C from any point of the surface of mirror. In **Fig 5.19**, this is represented by the symbol R. 4. **Principal axis:** It is a straight line joining the pole of the mirror to its centre of curvature. In **Fig 5.19**, the line PC represents the principal axis. It may extend on either side of the pole. ## FOCUS AND FOCAL LENGTH - In class VII, you have learnt about reflection of light at a plane mirror. When a ray of light is reflected from a plane mirror, it obeys the following two laws of reflection: - The angle of incidence i is equal to the angle of reflection r. - The incident ray, the reflected ray and the normal, all lie in the same plane. - The above laws of reflection of light hold true for the spherical mirrors as well. ### Focus: - **Fig 5.20** shows the rays of light falling on a spherical mirror parallel to its principal axis. These rays are reflected by the mirror obeying the laws of reflection (i.e., angle of incidence i = angle of reflection r). The normal at the point of incidence is obtained by joining this point to the centre of curvature C. The reflected rays are not parallel to each other, but they are converging towards a point in a concave mirror, while diverging from a point in a convex mirror. - **In case of a concave mirror**, the reflected rays meet at point F on the principal axis [**Fig 5.20(a)**]. This point is called the focus of the concave mirror. - **In case of a convex mirror** [**Fig 5.20(b)**], the reflected rays do not meet at any point, but they appear to come from a point F on the principal axis, behind the mirror. This point is called the focus of the convex mirror. ### Focal Length: - The distance of the focus from the pole of the mirror is called the focal length of the mirror. In **Fig 5.20**, the focal length of mirror is marked by the distance PF. Thus, Focal length f = PF. - It can be proved (by simple geometry) that Focal length = $1\over{Radius of curvature}$ or Radius of curvature = 2 x Focal length. - The approximate focal length of a concave mirror can be determined by the following simple activity. - **To find the approximate focal length of a concave mirror** - Take the concave mirror and hold it such that it faces the sun. Now place a piece of paper in front of it and adjust its distance from the mirror such that at one position, a very small image of sun is seen on the paper. You will notice that the paper chars at this point (**Fig 5.21**). This point is the focus of the concave mirror. - Measure the distance f of this point from the mirror with a metre ruler. This distance f gives the approximate focal length of the concave mirror. - **Rules for making ray diagram in a spherical mirror** - The object is kept in front of the reflecting surface on its left side. - The object is always kept on the principal axis such that it is perpendicular to the principal axis and its foot touches the principal axis. - For constructing a ray diagram, take at least two rays of convenience whose paths can be traced after reflection. - **To construct the image of an object due to reflection by a spherical mirror, any two of the following three rays can be drawn according to our convenience** - **Convenient rays:** - **(i)** A ray passing through the centre of curvature is reflected along its own path: A line joining the centre of curvature to any point on the surface of a mirror is always normal to it. Thus, a ray passing through the centre of curvature is incident normally on the spherical mirror. Its angle of incidence is zero, therefore, the angle of reflection is also zero. It means that the ray gets reflected back along its own path (**Fig 5.22**). - **(ii)** A ray parallel to the principal axis: A ray of light incident parallel to the principal axis, after reflection passes through the focus in case of a concave mirror or appears to come from the focus in case of a convex mirror (**Fig 5.23**). - **(iii)** A ray passing through the focus: A ray passing through the focus in case of a concave mirror or appearing to pass through the focus in case of a convex mirror, gets reflected parallel to the principal axis (**Fig 5.24**). - **To construct the image formed by a mirror, we take at least two rays incident on the mirror from a given point of the object. The point where the rays after reflection from the mirror, meet or appear to meet, gives the image of that point of the object.** ### REAL AND VIRTUAL IMAGE - **If the reflected rays actually meet at a point, the image is real, but if the reflected rays appear to meet at a point when produced backwards, the image is virtual. A real image can be obtained on a screen, but a virtual image cannot be taken on a screen. A real image is inverted, but a virtual image is erect**. - **Distinction between real and virtual images** - **Real Image:** - A real image is formed when the reflected rays actually meet at a point. - It is inverted. - It can be obtained on a screen. - **Virtual Image:** - A virtual image is formed when the reflected rays meet on producing them backwards. - It is erect or upright. - It cannot be obtained on a screen. ## IMAGES FORMED BY A CONCAVE MIRROR 1. **When an object is at infinity:** When an object is at infinity, the image is formed at the focus. It is a real, inverted and highly diminished image (**Fig 5.25**). 2. ** When an object is beyond the centre of curvature: ** An object AB is placed beyond the centre of curvature C of the concave mirror (**Fig 5.26**). A ray AD is incident on the mirror parallel to the principal axis. This ray after reflection passes through the focus F along DA'. The other ray AE passing through the centre of curvature C after reflection retraces its path EA (i.e., it gets reflected along EA). The two reflected rays DA' and EA intersect at A'. Thus, A' is the real image of the point A. When we take rays incident from other points of the object, we will find that A'B' is the image of AB which is between C and F. The image formed is real, inverted and smaller in size than the object. 3. **When an object is at the centre of curvature:** An object AB is placed at the centre of curvature C of the concave mirror (**Fig 5.27**). A ray AD incident on the mirror parallel to its principal axis after reflection passes through the focus F along DA'. The other ray AE passing through the focus F after reflection becomes parallel to the principal axis along EA'. The two reflected rays DA' and EA' intersect at point A'. Hence, A' is the real image of the point A. In a similar way, taking rays incident from other points of the object, A'B' is the image of AB formed at C. The image formed is real, inverted and of the same size that of the object. 4. **When an object is between the centre of curvature and focus:** An object AB is placed between focus F and the centre of curvature C of the concave mirror (**Fig 5.28**). A ray AD incident on the mirror parallel to the principal axis after reflection passes through the point F along DA'. The other ray AE passing through the focus F after reflection becomes parallel to the principal axis along EA'. The two reflected rays DA' and EA' intersect at A'. Thus, A' is the real image of A. In a similar way, taking rays incident from other points of the object, A'B' is the image of AB formed beyond C. The image thus formed is real, inverted and of bigger size than the object. 5. **When an object is at the focus:** When an object is at the focus, the image formed is at infinity. It is real, inverted and highly magnified (**Fig 5.29**). 6. **When an object is between the focus and pole:** An object AB is placed between the pole P and focus F of a concave mirror (**Fig 5.30**). A ray AD incident on the mirror parallel to the principal axis after reflection passes through the focus F along DF. The other ray AE passing through the centre of curvature C of the mirror after reflection retraces its path (i.e., it gets reflected as EC). The two reflected rays DF and EC do not actually intersect, but they simply appear to diverge from a point A' behind the mirror. This is shown by the dotted lines. Thus, A' is the virtual image of A. In a similar way, taking rays incident from other points of the object, A'B' is the image of AB formed behind the mirror. The image formed is virtual, erect and of size bigger than the object. - **Thus a concave mirror forms real as well as virtual images. The image is virtual if the object is very close to the mirror before its focus. For the object at focus or beyond it, the image is real. The virtual image is always magnified for each position of object between the pole and focus of mirror. The real image is magnified if the object is at focus or between focus and centre of curvature. It is of same size when object is at centre of curvature. But it is diminished when object is beyond centre of curvature.** - **Do You Know?** The image formed by a mirror (both plane mirror and spherical mirror) shows lateral inversion (i.e., the right side of object appears at left side of image or vice-versa). - **Images formed by a concave mirror for different positions of the object** | No. | Position of the object | Position of the image | Nature of the image | |---|---|---|---| | 1 | At infinity | At focus (F) | Real, inverted and diminished | | 2 | Beyond the centre of curvature (C) | Between focus (F) and the centre of curvature (C) | Real, inverted and smaller than the object | | 3 | At the centre of curvature (C) | At the centre of curvature (C) | Real, inverted and of same size | | 4 | Between the centre of curvature (C) and focus (F) | Beyond the centre of curvature (C) | Real, inverted and bigger than the object | | 5 | At the focus (F) | Infinity | Real, inverted and highly magnified | | 6 | Between the focus (F) and pole (P) | Behind the mirror | Virtual, erect and enlarged | ## Images Formed by A Convex Mirror - An object AB is placed in front of a convex mirror. A ray AD incident on the mirror parallel to the principal axis after reflection appears to diverge from the focus F along DA₁. The other ray AE passing towards the centre of curvature C, after reflection retraces its path EA as shown in **Fig 5.31**. (i.e., it gets reflected back along EA). The two reflected rays DA, and EA do not actually meet, but they appear to meet at A' behind the mirror when produced backwards as shown by the dotted lines. Thus, A' is the virtual image of the point A. In a similar way, taking rays incident from the other points of the object, A'B' is the image of AB. - **As the object is brought closer to the convex mirror, the image moves towards the pole of mirror. Its size increases (but always remains smaller than the size of the object). It is virtual, erect and diminished, and is always formed between the pole and focus.** - **Position, size and nature of image formed by a convex mirror** | No. | Position of the object | Position of the image | Size of the image | Nature of the image | |---|---|---|---|---| | 1 | At infinity | At focus | Diminished to a point | Virtual and upright | | 2 | Any other point | Between focus and pole | Diminished | Virtual and upright | - **Conclusion:** - **(1) In a concave mirror, the image formed can be real or virtual, inverted or erect, diminished or of same size as the object, depending upon the position of the object. As the object is brought from infinity towards the mirror, the image is real, inverted and diminished till the object is beyond centre of curvature. For the object at centre of curvature, the image is real, inverted and of same size. When object comes closer up to focus, the image is real, enlarged and inverted But if object comes still closer, the image becomes virtual, erect and enlarged.** - **(2) In a convex mirror, the image formed is always virtual, erect, diminished and it is situated between the pole and focus, for each position of the object in front of the mirror. As the object moves closer to the mirror, the image shifts towards the pole and it increases in size. We can verify it by drawing ray diagrams for different positions of the object. The formation of virtual images by a concave and a convex mirror can be demonstrated by the following activity.** - **ACTIVITY 6** Take a polished steel spoon. The inside surface of the spoon is curved inwards and has a concave shape while the outside surface of the spoon is curved outwards and has a convex shape. - **(1)** Hold the spoon in such a way that the inside surface of the spoon (concave side) is closer to you. See image of your face (**Fig 5.32**). It is erect and magnified. Now move the spoon away from you, you will notice that the image becomes inverted. - **(2)** Now hold the spoon with its outside surface towards you. You will observe that the image is erect but diminished as shown in **Fig 5.33**. If you move the spoon away from you, the image remains always erect and diminished. - **Do You Know?**A real image formed by a mirror is always formed in front of the mirror, while a virtual image is formed behind the mirror. ## USES OF A CONCAVE MIRROR - A concave mirror is put to the following uses: - **(i) As a shaving mirror,** - **(ii) As a reflector,** - **(iii) As a doctor's head mirror,** - **(iv) To converge solar radiations in a solar cooker, and** - **(v) In flood lights as a reflector**. - **(i) Use of concave mirror as a shaving mirror:** A concave mirror forms an erect and magnified image of an object placed close to it. This fact enables us to use it as a shaving mirror. - **(ii) Use of concave mirror as a reflector:** If a source of light is placed at the focus of a concave mirror, we get a parallel beam of reflected light (**Fig 5.34**). This fact enables us to use it as a reflector in torch, searchlight and headlights of a car and other vehicles. The source of light (bulb) is placed at the focus of the concave reflector. - **(iii) Use of concave mirror as a doctor's head mirror:** If a parallel beam of light is incident on a concave mirror, it converges the beam to a point called focus (**Fig 5.35**). This fact enables us to use it as a doctor's head mirror to concentrate light on a small area to be examined, like nose, throat, ear, teeth, etc - **(iv) Use of concave mirror in a solar cooker to converge the sun-rays:** In a solar cooker a concave mirror is used to reflect the sun rays so as to converge them on the cooking material placed at the focus of the concave mirror. - **(v) Use of concave mirror in flood lights as a reflector:** In flood lights, the source of light (i.e. bulb) is placed between the pole and focus of a concave mirror so to obtain a diverging beam of light. ## USES OF A CONVEX MIRROR - A convex mirror is put to the following uses: - **(i) As a rear view mirror,** - **(ii) As a reflector in street lamps, and** - **(iii) As a vigilance or anti-theft mirror.** - **(i) Use of convex mirror as a rear mirror:** A convex mirror diverges incident light rays and always forms a small and erect image between its pole and focus. This fact enables us to use it as a rear view mirror by a driver to see all the traffic behind him approaching the mirror. **Fig 5.36** shows that a convex mirror has a wider field of view than a plane mirror. - **(ii) Use of convex mirror as a reflector in street lamps:** The fact that a convex mirror diverges the light rays incident on it enables us to use it as a reflector in street lamps. The light from a bulb placed in front of a convex mirror diverges over a large area in the street as shown in **Fig 5.37**. - **(iii) Use of convex mirror as a vigilance mirror:** In big showrooms and departmental stores, convex mirrors are used to have a view on the customers entering in as well as going out. The mirrors so used are called vigilance or anti-theft mirrors. ## Recapitulation - The change in direction of path of light when it passes from one transparent medium to another, is called the refraction of light. - The speed of light in air is $3 × 10^8\ m/s$. In any other transparent medium (such as water, glass, etc.), the speed of light is less than that in air. The air is, therefore, optically rarer than any other transparent medium. - When a ray of light travels from a rarer to a denser medium, it bends towards the normal. - When a ray of light travels from a denser to a rarer medium, it bends away from the normal. - When a ray of light falls normally on the surface separating the two media, the angle of incidence is zero, so it passes undeviated. - Refraction takes place at the two parallel surfaces when light passes through a rectangular glass block. The emergent ray and the incident ray are in the same direction, but they are laterally displaced. - When a light ray of single colour passes through a prism, refraction takes place at the inclined surfaces of the prism and the light ray bends towards the third surface when it passes through it. - When white light passes through a prism, it splits into several colours namely violet, indigo, blue, green, yellow, orange and red (VIBGYOR) with violet colour towards the base and red is towards the other end. This band is called the spectrum of white light.