Title CHAPTER 10 REFLECTION AND REFRACTION.doc ✅

2 CBSE X: PHYSICS : CHAPTER 1 REFLECTION AND REFRACTION OF LIGHT  Keep a bright object, say a burning candle, at a position far beyond C. Place a paper screen and move it in front of the mirror till you obtain a sharp bright image of the candle flame on it.  Observe the image carefully. Note down its nature, position and relative size with respect to the object size.  Repeat the activity by placing the candle (a) just beyond C, (b) at C, (e) between F and C, (d) at F, and (e) between P and F.  In one of the cases, you may not get the image on the screen. Identify the position of the object in such a case. Then, look for its virtual image in the mirror itself.  Note down and tabulate your observations. EXPLANATION Hold the concave mirror in your hand and point its reflecting surface towards a distant object, say a tree. Hold the cardboard screen in the other hand in front of the concave mirror. Move the screen back and forth till a sharp image of the distant object/tree is seen on the screen. Measure the distance of screen from the mirror. This distance is the approximate focal length of concave mirror. Mark a line on a table with a chalk. Place the concave mirror on a stand at this point. This position of the mirror is its pole P. With a chalk, draw two more lines parallel to the previous line such that the distance between any two successive lines is approximately the focal length of the mirror. The line closer to P represents principal focus F and that away from P, the centre of curvature C of the mirror. When we keep a bright object such as a burning candle at a position far beyond C, and move back and forth a paper screen in front of the minor, we get a sharp bright image of the candle flame on the screen. As the candle is moved from a point just beyond C towards the pole of the concave mirror, the image moves from the focus towards the centre of curvature , increasing in size from a point size at the focus to the same size as the object, at the centre of curvature. As the object moves further towards the focus, the image moves towards infinity, increasing in size and always remaining real and inverted. When the object is at the focus, the image is formed at infinity. When the object is moved closer the image becomes virtual and is formed behind the mirror. Therefore no image is seen when the candle lies between the pole of the mirror and its focus. The ray diagrams of these positions are depicted in figure 1.16 through 1.21. These results are as shown in table 1.1. ACTIVITY 4:  Draw neat ray diagrams for each position of the object shown in Table 1.1  You may take any two of the rays mentioned in the previous section for locating the image.  Compare your diagram with those given in Fig. 1.16 to 1.21.  Describe the nature, position and relative size of the image formed in each case.  Tabulate the results in a convenient format. EXPLANATION The diagrams are as shown in figures 1.16 to 1.21. This is discussed in article on concave mirror. ACTIVITY 5:  Take a convex mirror. Hold it in one hand.  Hold a pencil in the upright position in the other band.  Observe the image of the pencil in the mirror. Is the image erect or inverted? Is it diminished or enlarged?  Move the pencil away from the mirror slowly. Does the image become smaller or larger?  Repeat this activity carefully. State whether the image will move closer to or farther away from the focus as the object is moved away from the mirror? EXPLANATION A convex mirror produces an erect, diminished and virtual image of the object for all positions of the object in front of the mirror. When we hold a pencil in the upright position in front of a convex mirror, we observe a diminished, erect and virtual image. As the pencil is moved away from the mirror, the image diminishes further. On repeating the activity, we find that as the object is moved away from the mirror, the image moves closer to focus of the mirror. ACTIVITY 6:  Observe the image of a distant object; say a distant tree, in a plane mirror.
3 CBSE X: PHYSICS : CHAPTER 1 REFLECTION AND REFRACTION OF LIGHT  Could you see a full-length image?  Try with plane mirrors of different sizes? Did you see the entire object in the image?  Repeat this activity with a concave mirror. Did the mirror show full length image of the object?  Now try using a convex mirror. Did you succeed? Explain your observations with reason. EXPLANATION A plane mirror forms an image which is equal to the size of the object. To see a complete image of an object in a plane mirror its size should be at least half the size of the object. A concave mirror forms an image which is equal in size, smaller in size or bigger in size than the object. For certain locations of the object in front of the concave mirror, the entire image of the distant object can be seen. In a convex mirror the image is always virtual, erect and smaller than the object for all positions of the object. Thus for all positions of the object only a convex mirror forms the full, diminished image of the object. ACTIVITY 7:  Place a coin at the bottom of a bucket filled with water.  With your eye to a side above water, try to pick up the coin in one go. Did you succeed in picking up the coin?  Repeat the activity. Why did you not succeed in doing it in one go?  Ask your friends to do this. Compare your experience with theirs. EXPLANATION: This is due to the phenomenon of refraction. When viewed obliquely the depth of a container filled with a liquid seems to be less than its actual depth. When we view the coin with our eye to a side above water, we observe the image of the coin above the coin. When we try to pick up this coin in one go, we do not succeed in picking up the coin because we are trying to catch the coin which is at I and not at O. When our friends try the same way, they also fail to pick up the coin. However, if someone understands the phenomenon of refraction of light he/she can pick up the coin. ACTIVITY 8:  Place a large shallow bowl on a table and put a coin in it.  Move away slowly from the bowl. Stop when the coin just disappears from your sight.  Ask a friend to pour water gently into the howl without disturbing the coin.  Keep looking for the coin from your position. Does the coin become visible again from your position? How could this happen? EXPLANATION: This is again related to the phenomenon of refraction of light. When viewed obliquely the depth of a container filled with a liquid seems to be less than its actual depth. Thus the bottom of the container seems to be raised. When a friend pours water gently into the bowl without disturbing the coin, the coin becomes visible again from the same position of our eye. This happens because on adding water, real depth of the coin decreases i.e., the coin seems to be raised. The apparent position of the coin rises slightly above the real position and it becomes visible from the same position of our eye. ACTIVITY 9:  Draw a thick straight line in ink, over a sheet of white paper placed on a table  Place a glass slab over the line in such a way that one of its edges makes an angle with the line.  Look at the portion of the line under the slab from the sides. What do you observe? Does the line under the glass slab appear to be bent at the edges?  Next, place the glass slab such that it is normal to the line. What do you observe now? Does the part of the line under the glass slab appear bent?  Look at the line from the top of the glass slab. Does the part of the line, beneath the slab, appear to be raised? Why does this happen? EXPLANATION:
4 CBSE X: PHYSICS : CHAPTER 1 REFLECTION AND REFRACTION OF LIGHT Let the glass slab be put on the line at an angle. When we look at the portion of the line under the slab, from the sides, we find that the line is bent at the edges. This is because of refraction of light. When a ray of light enters the glass slab, it is moving from an optically rarer to an optically denser medium. As a result it bends towards the normal. When it comes out of the glass it is bent away from the normal. When the glass slab is so placed that one of its edges is normal to the line then the line under the slab does not appear to be bent. This is because for the incident ray the angle of incidence i = 0. Therefore, angle of refraction r is also zero. Therefore, no refraction occurs at the edge, and there is no bending. Similarly, no refraction occurs at the other edge when the ray of light comes out of the glass slab. When we look at the portion of the line from the top of the glass slab, it appears to be raised. This is also due to the phenomenon of refraction of light. Rays of light starting from the line pass from glass into air. When viewed from the top of slab, the rays of light seem to come from a point above the actual line (apparent depth). Thus the line seems to be raised. ACTIVITY 10:  Fix a sheet of white paper on a drawing board using drawing pins.  Place a rectangular glass slab over the sheet in the middle.  Draw the outline of the slab with a pencil. Let us name the outline as ABCD.  Take four identical pins.  Fix two pins, say E and F, vertically such that the line joining the pins is inclined to the edge AD.  Look for the images of the pins E and F through the opposite edge, CD.  Fix two other pins, say G and H, such that these pins and the images of E and F lie on a straight line.  Remove the pins and the slab.  Join the positions of tip of the pins E and F and produce the line up to AB. Let EF meet AB at O. Similarly, join the positions of tip of the pins G and H and produce it up to the edge CD. Let HG meet CD at O / .  Join O and O / . Also produce EF upto P as shown by a dotted line in the Fig. EXPLANATION: At point O, draw a normal NN / . At O / draw a normal MM / At O, a light ray from the pins enters glass from air. Since the ray moves from an optically rarer to an optically denser medium, therefore it bends towards the normal NN / and travels towards point O / . At O / this light ray enters air (optically rarer) from glass (optically denser) therefore it bends away from normal MM / . This is second refraction, When a ray light is passing from air to glass, that is, from a rarer medium to a denser medium, the refracted ray bends towards the normal drawn at the point of incidence. In this case ir . But when the ray of light is passing from glass to air, that is, from a denser medium to a rarer medium the refracted ray bends away from the normal. In this ri . The emergent ray, O / E which is nothing but the refracted ray emerging out of the glass slab is parallel to the incident ray. This means that the refracted ray (emergent ray) has been displaced from its original path by a distance XY. This displacement is referred to as lateral displacement. ACTIVITY 11: CAUTION Do not look at the Sun directly or through a lens while doing this activity or otherwise. You may damage your eyes if you do so.  Hold a convex lens in your hand. Direct it towards the Sun.