1. Most of the things we see around us do not emit their own light. They are visible because of reflection.

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Moris Shelton
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1 Chapter 12 Light Learning Outcomes After completing this chapter, students should be able to: 1. recall and use the terms for reflection, including normal, angle of incidence and angle of reflection 2. state that, for reflection, the angle of incidence is equal to the angle of reflection and use this principle in constructions, measurements and calculations 3. recall and use the terms for refraction, including normal, angle of incidence and angle of refraction 4. recall and apply the relationship sin i 7 sin r = constant to new situations or to solve related problems 5. define refractive index of a medium in terms of the ratio of speed of light in vacuum and in the medium 6. explain the terms critical angle and total internal reflection 7. describe the action of a thin lens (both converging and diverging) on a beam of light 8. define the term focal length for a converging lens 9. draw ray diagrams to illustrate the formation of real and virtual images of an object by a thin converging lens 12.1 Reflection of Light page Most of the things we see around us do not emit their own light. They are visible because of reflection. 2. The topic on reflection is best covered experimentally. Mirrors and ray-boxes, torches or optical pins may be used. 3. The laws of reflection may be modelled in 3D using rulers to represent the rays and the normal and the desk top to act as a mirror. 4. Observe that there are many rays for which the angle of reflection equals the angle of incidence but the correct one lies in the same plane as the normal and the incident ray. Emphasise that all angles are measured between the ray and the normal. s. Emphasise that an image formed by a mirror is the location from which the light seems to come. It has not actually travelled behind the mirror. An eye or a photographic plate placed at the image position would not detect anything. 6. Consider more complicated examples such as two mirrors at right angles or a kaleidoscope. I 3 I ~~.., 2 I "".... ",',,,,, " I ',...,1, ~, rnirror2 o mirror I Answer to Think Time question page 188 The letters which have the property of symmetry are B, C, D, E, H, I, 0 and X. Find out words which can be formed using these letters.

2 ,,,,, Answers to Section Review questions page We can see the Moon because it reflects light from the Sun. The reflected light enters our eyes. 2. It is difficult to read because the words are laterally inverted. 3. This is because the rough surface of paper reflects light rays in many different directions and causes diffused reflection. 4. The angle of incidence is 60, so the angle of reflection is When the man stands 2.5 m from the mirror, his distance from the image is 5 m. So he must walk a distance of (10-2.5) m = 7.5 m towards the mirror. 6. The observer is able to see the reflections of the corners B and C. To check for this, draw the straight line from the image of B to the eye, if the line cuts through the mirror, it means that reflection by the mirror reaches the eye. C B ~ / ~, /, /, ~~/ \... --:.-::..,..:;. ""'... ' '\... ""... \ /,, ' " ~ C' 7. The girl and her image are closer by 3.0 m after 1 second. The speed of her image towards her is 3.0 m S-I Refraction of Light page Refraction occurs because light travels with different speed in different medium. 2. Give some simple examples of refraction: the bent stick and apparent reduction in depth. 3. Carry out experiments with a glass block and a ray-box, torch or optical pin. 4. Emphasise that as the light leaves the second face of a rectangular block, it reverses the procedure which occurred at the first and emerges parallel to the incident ray. Answer to Think Time question page 194 The displacement of the beam in water will decrease. Total Internal Reflection page Explain that when a ray emerges from glass into air, the emergent angle is larger than the angle at which it strikes the surface. In due course, it emerges at 90 to the lower face. If the angle at which it strikes the surface increases, then the emergent angle exceeds 90 and it passes back into the glass. This is reflection not refraction. Refraction out of the glass is now impossible.,... - _.. L "'_ : _ n4._ 1...1

3 2. Conduct an experiment by passing a ray into the curved face of a semi-circular glass block, into the side face of a glass block or a tank of water. Move the ray. There comes a point at which no light at all emerges through the surface. 3. Emphasise the fact that two conditions must be met before total internal reflection can occur: (a) The light must pass from the slow to the fast medium. (b) The angle at the surface must exceed the critical angle. 4. Optical fibres rely on total internal reflection. Digital pulses (light/no light), transmit the information. Advantages include: more information can be sent per second (many phone calls on one line), less prone to interference, less easy to tap into, fewer repeaters. Optical fibres need to be sheathed to prevent light passing from one fibre into the next. Unlike electricity, light is not affected by temperature and fluctuations in surrounding magnetic fields, and so the signal is clearer. 5. There are transoceanic optical fibre cables and it is possible to have an optical link straight into a personal computer or telephone. Answer to Think Time question page 199 Periscope that uses prisms does not have multiple images. Answers to Section Review questions page sin i n=-- sinr 1.33 = sin 30 sin r r = = sin 60 sin r n = sin 65 = 1.54 sin n = velocity of light in vacuum velocity of light in medium 3.0xIQ = velocity of light in medium Velocity of light in medium = 2.1 x 10 8 m S-1 n = _f_e_al_d_e-,-pt_h_ apparent depth ± _ real depth 3-1.2m Real depth = 1.6 m 3. (a) Angle of incidence = 60 (b) 1.50 = sin 60 smr r = 35 Angle of refraction = Total internal reflection will take place if the angle of incidence is more than p Q p Q Physics in Daily Life: Laser Technology. page 203 Answers to Q -- I. A very narrow beam of intense collimated light

4 2. The laser beam can focus large amount of energy onto a very small region, thus enabling it to cut through the material. 3. Surgeons will have to depend on knives in their operations. There will not be CDs, DVDs and telecommunications will be limited to fixed telephone lines Thin Lenses page Emphasise to student that a convex lens is a converging lens but a converging lens is not necessarily convex. 2. Draw ray diagrams. Emphasise three types of ray: (a) Rays parallel to the principal axis refract through the focal point. (b) Rays striking the optical centre emerge undeviated. (c) Rays which pass through or seem to have passed through the focal point emerge parallel to the principal axis. 3. Emphasise to students that the three special rays mentioned above are not the only ones to originate from the object point, but all the others also meet at or seem to have come from the same image point. 4. It is usual to use the top point of an object as the object point but every other point on the object also produces an image. 5. Emphasise to students that the total amount of light passes through a lens depends on the area of the lens. The bigger the lens, the more light passes through it and the image will be brighter. 6. Describe the use of a single lens as a magnifying glass and in a camera, projector and photographic enlarger and draw ray diagrams to show how each forms an image. Answer to Think Time question page 206 If the speed of light is the same in glass and in air, glass lenses will not change the direction of the light rays. Answer to Think Time question page 208 It is true. Real image formed by a converging lens is always inverted. Answers to Section Review questions page A real image can be formed on a screen but a virtual image cannot. 2. The image is real, inverted and magnified. It is formed at a distance beyond the distance 2/ 3. (a) The object is placed between the lens and the focal point. (b) The image is upright. (c) The image is magnified. Answers to Misconception Analysis page True 2. False. 3. False. 4. True 5. True 6. True Angle of incidence is the angle between the incident ray and the normal The image is upright but laterally inverted 7. False. The critical angle is in the optically denser medium 8. False. The angle of incidence must be greater than the critical angle 9. False. The focal point of a lens is a point where parallel rays after passing through the lens, converges. 10. False. There are many rays. The three rays are meant for drawing ray diagrams....:;:.. r""\ r..j... :... C...,.+h J\.C"' i~ O ta I trl

5 Answers to Multiple Choice Questions page 211, D The new angle of incidence is C Initially the distance between object and image = 100 cm Finally the distance between object and image = 60 cm So distance between original position and final position of the image = (100-60) cm = 40 cm 3. D 4. C 5. C 6. B refractive index = speed of light in air = _ c_ = 1.25 speed of light in medium 0.8e 7. D 8. B 9. B When object distance = image distance, object distance = 2f Answers to Structured Questions page 212, (a) Refer page 225, Fig (b) Distance of chart from the mirror = 3.0 m Distance between the chart and the image of the chart = 6.0 m Since the chart is 0.5 m behind the man, so he is 5.5 m away from the image of the chart. 2. (a) & (b) jar filled with water glass (c) The glass can pass through the glass (transparent) as well as reflect the light. 3. x x y y (a) (2x + 2y) = 160 cm (x + y ) = 80 cm (b) The image will be the same. Pearson Education South Asia Pte Ltd Science in Focus: Physics '0' Level

6 4. (a) (b) n = _ r_e_al_d_e..:.. p_th_ apparent depth 1.25 = 45_cm apparent depth apparent depth = 36 cm. 1 1 smc = -=-- n (a) Because the speed of light in glass and in air are different (b) n = sini = sin40 = 1.29 sin r sin (c) smc = -=-- n (a) Total internal reflection (b) (i) The light ray travels from an optically denser medium towards an optically less dense medium. (ii) The angle of incidence is greater than critical angle. 7. (a) Focal length is the distance between the focal point and the optical centre of the lens. (b) lcm:2cm F Scm 10 em F The image is virtual, magnified and upright. The image is positioned 10 cm from the lens and is on the same side as the object. Image height is 8 cm. Answers to Critical Thinking Questions page The reflected beam will be moving away. This is because the incident angle is increased and so does the reflected angle. The laws of reflection are still valid in this situation. 2. Reflection can occur in the optically less dense medium or optically denser medium. When reflection occurs, there is partial refraction. Total internal reflection can only occur in optically denser medium. When total internal reflection occurs, there is no refraction. Pearson Education South Asia Pte Lid Science in Focus: Physics '0 ' Level

7 3. The thickest lens will have the image formed nearest to the lens. The thinnest lens has the longest focal length. 4. This is because different colour light refracted differently. Blue colour light, which has the lowest speed in glass, refracts more and the blue image is nearer to the lens than the red image. Extension page A one-way mirror has a reflective coating applied in a very thin, sparse layer - so thin that it is called a half-silvered surface. The name half-silvered comes from the fact that the reflective molecules coat the glass so sparsely that only about half the molecules needed to make the glass an opaque mirror are applied. The half-silvered surface will reflect about half the light that strikes its surface, while letting the other half go straight through. So why is it that the people in one room see the glass as a mirror whereas the people in the other 'observation room' see the glass as a window? The answer lies in the lighting of the two rooms. The room in which the glass looks like a mirror is kept very brightly lit, so that there is plenty of light to reflect back from the mirror's surface. The other room, in which the glass looks like a window, is kept dark, so there is very little light to transmit through the glass. If the lights in the room with the mirror are suddenly turned out, or the lights in the observation room suddenly turned on, then the one-way mirror becomes a window, with people in each room able to see those in the other. Refer website: 2. Optical fibre relies on total internal reflection to transmit images or data from one end to the other using digital pulses (lightlno light). Although expensive to produce and install, optical fibres can carry more data than traditional cables, and are less susceptible to interference. Standard optical fibre transmitters can send up to 10 billion bits of information per second by switching a laser beam on and off. Optical fibres are increasingly being used to replace metal communications cables, the messages being encoded as digital pulses of light rather than as fluctuating electric current. B undies of optical fibres are also used in endoscopes to inspect otherwise inaccessible parts of machines or of the living body. Refer to websites: Pearson Education South Asia Pte Ltd SciAnr.p. in J="n("IIC::- Dh\lc-i,...C'." Ir'\' I... 1

Converging Lenses. Parallel rays are brought to a focus by a converging lens (one that is thicker in the center than it is at the edge).

Converging Lenses. Parallel rays are brought to a focus by a converging lens (one that is thicker in the center than it is at the edge). Chapter 30: Lenses Types of Lenses Piece of glass or transparent material that bends parallel rays of light so they cross and form an image Two types: Converging Diverging Converging Lenses Parallel rays