CHAPTER 5 LIGHT . 5.1 Understanding reflection of light. Answers: 1.The characteristics of image when light from an object is reflected by a plane mirror includes laterally inverted , same size, same distance from the mirror at perpendicular line and virtual. 2. The Law of reflection of light states that i) the angle of incidence equals the angle of reflection ii) the incident ray, the reflected ray and normal line are all lie at the same plane 3. a) Complete the image of L in the diagram of reflection below. Mirror lo li i r (b) On the diagram above, i. show the light ray direction and the normal line of the mirror, ii iii show the angle of incidence and label as i, the angle of reflection and label as r show the distance of object, lo and distance of image li perpendicular to the mirror 4. When the parallel rays are directed to concave mirror, the reflected rays will meet at a point before the mirror. It is called the focal point of the concave mirror. 5. When the parallel rays are directed to convex mirror, the reflected rays will rays are extrapolated, it will meet at a point behind the mirror, it’s the diverge. When the focal point . 6. Diagram below shows three parallel rays are directed to a concave and a convex lens, draw the reflected rays and indicate the focal point, F in each case. Show the direction of reflected rays. Relate the relation between centre of curvature, C, with focal point, F, of a mirror. F F 7. Convex mirror is often installed at the cornering of a hidden road and in the convenience store. This mirror allows us to see wide angle of objects but the size of the image is smaller / diminished. Diagram below shows a ray directed on a plane mirror. i) Uses a red / blue ink ball pen, draw the normal line and the reflected ray for the given incident ray (use protractor and ruler to assist you). ii) Now, uses a pencil draw to show the mirror turned 10o clockwise. Thus, draw the new normal line and new reflected ray. Observe the changes of angle for the reflected ray. 8. This angle reduce 10o 60 o 50 o Just now angle between 2 rays is 120 o Now angle between 2 rays is 100 o 9. When a mirror turns xo, the angle between the two rays (incident and reflected) will have a change of 2xo. 10. Mirror is used in measuring instruments like ammeter, voltmeter and galvanometer. It is to help the user to read accurately and avoid parallax error. 5.2 Understanding refraction of light. Answers: 1. Light travels in straight line, but when travels from one medium to another medium with different density, it changes direction. The change in direction when light ray travels from one medium to another medium is called refraction . normal line of the boundary between two medium 2. The light ray that travels along the does not bend. 3. When a light ray travels from an optically denser medium to a less dense medium, it refracts far from the normal. 4. When a light ray travels from an optically less dense medium to a denser medium, the angle of incident ray (at less dense medium) is larger then the angle of refracted ray (at denser medium). 5. In the diagram below, a. draw two normal lines of the ray to and from the glass block b. show the refracted light of the glass block c. complete the direction of light d. indicate the angle of incident, i , and the angle of refraction, r . 6. Angle of incident or angle of refraction is an angle made between the ray and the normal line. 7. Snell’s Law refractive index says that the ratio of sin i is a constant. It is known as the sin r of a medium (water / glass / Perspex), n, where i > r . 8. The speed of light decreases as it travel into a medium, therefore the refractive index of speed of light in air / vaccum a medium n = , speed of light in medium c simplified n = . v 9. Due to refraction of light, a straight straw / rod appears bent at the boundary of liquid. b the bottom of a pool liquid appears shallower / nearer to the surface that it actually is. 10. The refractive index of liquid can be calculated using, n = simplified n = real depth , apparent depth D . d 11. In the diagram below use the following steps to construct the refraction of light of a coin in a beaker of water. a draw the two normal lines at the boundary of liquid b draw and show the two rays refracted at the air (further from normal line) c draw an eye at the correct position d from the eye, followed the refracted rays, extrapolated dotted lines backward and meet above the original coin. Draw a dotted oval shape to represents the image of the coin. Eye 12. The phenomenon of refraction of light include mirage, at night and wriggling of distant objects on hot day. sunrise, sunset, twinkling stars 5.3 Understanding total internal reflection of light Answers: 1. Diagram below shows a light ray travels through a semicircle glass block. If the angle, of incident is small, most of the light is refracted but some is reflected. Label in the diagram i) the refracted light and reflected light ii) the angle of incident, i and angle of refraction, r. Refracted light r Reflected light 2. The critical angle, c is the incident angle, i when the maximum refraction occurs at r = 90 o. On the diagram below, label the critical angle, c and refraction angle, r. i r = 90 o C 3. The total internal reflection occurs when, (a) i) light travels from a denser medium to a less dense medium and ii) when the incident angle, i is greater than the critical angle, c (b) name the phenomena occurs in the diagram below. Total internal reflection 4. Another formula of refractive index, n = sin 90 o 1 = sin c sin c 5. The phenomenon of total refraction of lights includes a) mirage in the desert b) mirage on hot afternoon on highway c) rainbow 6. The phenomenon of mirage occurs is due to the different layers of hot air, hot air on the ground is less enough straight dense compare with cold air at higher level on hot day. The water droplets from the cloud are further and further from the normal line. When the angle of incident ray is large , it created a total internal reflection on the ground level. as though there are water drops on the ground. Our eye sight sees refracted 7. Refers to the diagram given below: i) In each of the box fill in two of the following characteristics of air: hot air, colder air, denser, less dense, ray (closer to / further) from normal line ii) Complete the ray diagram of mirage starts from the object. iii) Show how we can see the pool of water (mirage) on the highway. Object Colder air Denser Ray closer to normal line Eye Hot air Less dense Ray further from normal line Image Pool of water appears at the highway on a very hot day. 5.4 Understanding lenses. Answers: 1. When light passes through a convex lens, the light converges point. Convex lens is also known as a converging lens. 2. Concave lens is also known as a diverging lens a point, the focal point, F. to a point and we call it focal because light passes through it diverge from 3. For the ray diagram below fill in the following labels: Principle axis, P, Optical centre, O, Focal point, F, focal length, f P O F f 4. Complete the following ray diagrams to find out the images of each one. Given the characteristics of images includes real or virtual, upright or inverted, diminished / smaller, same or magnified or larger. State the characteristics for each. a) ● ● ● ● Characteristics of image: Real Inverted Diminished b) ● ● ● ● Characteristics of image: Real Inverted Same size c) ● ● ● ● Characteristics of image: Real Inverted Magnified 5. a) When the object is too near the convex lens, the image is virtual, magnified and upright. b) Concave lens produce only one type of image that is virtual, diminished and upright. Base on the two statements above, draw the images for the following diagrams. i) ii) ● ● ● ● 6. Complete the following ray diagram and name the characteristics for each. ● ● ● ● Characteristics of image: Characteristics of image: 7. The symbol for image distance is v and the symbol for object distance is u . 8. The formula for magnification m = size of image, D image dis tan ce, v or m = object dis tan ce, u size of object , d 9. Given the formula 1 1 1 = + can be used to solve optical problems. Explain what does it means f u v when f is negative value, f is positive value, v is negative value and v is positive value. f value is positive when it’s a convex lens, f value is negative when it’s a concave lens, v value is positive shows the image is real, v value is negative shows the image is virtual.