PHYSICS. Light FORM 4. Chapter 5. Compiled by Cikgu Desikan

Transcription

1 PHYSICS RM 4 Chapter 5 Light Compiled by Cikgu Desikan

2 PRE SPM PHYSICS 2016 Chapter 5 Light Dear students, The two basic processes of education are knowing and valuing. Learning bjectives : 1. Understanding reflection of light 2. Understanding refraction of light 3. Understanding total internal reflection of light 4. Understanding lenses P2 P3 Analysis of Past Year Questions P A B C A B

3 Chapter 5 Light Dear students, Great dreams of great dreamers are always transcended. (by Dr. Abdul Kalam) Concept Map Light Reflection of Light Refraction of Light Lenses Laws of Reflection Concave Laws of Refraction Convex Lens Concave lens Mirror Convex Plane Refractive index Total internal Reflection Ray diagram Ray Diagram Positions and characteristics of image n sin i sin r D n d v of light invacuum n v of light in medium n 1 sinc 1 f 1 u 1 v Positions and characteristics of image

4 5.1 Understanding Reflection f Light Reflection 1. Mirror works because it reflects light.. 2. The light ray that strikes the surface of the mirror is called incident ray. 3. The light ray that bounces off from the surface of the mirror is called reflected ray. 4. The normal is a line perpendicular to the mirror surface where the reflection occurs. 5. The angle between the incident ray and the normal is called the angle of incidence, i 6. The angle between the reflected ray and the normal is called the angle of reflection, r. A A B i r i N Mirror = Incident ray = Reflected ray = Angle of incident = Angle of reflection r B Laws of Reflection 4

5 Draw ray diagrams to show the positioning and characteristics of the image formed by a plane mirror. bject Characteristics of the image formed by reflection of light. Student Plane Mirror Notes: Real image : Image that can be seen on a screen Virtual image : Image that cannot be seen on a screen. Great dreams of great dreamers are always transcended. APJ Abdul Kalam 5

6 Reflection of light on curved mirror Concave Mirror Common terminology of curved mirrors Centre of curvature, C C P The center of sphere of the mirror Principle axis The connecting line from the centre of curvature to point P Convex Mirror Radius of curvature, CP The distance between the centre of curvature and the surface of the mirror. ocal point, P C The focal point of a concave mirror is the point on the principle axis where all the reflected rays meet and converge. The focal point of convex mirror is the point on the principle axis where all the reflected rays appear to diverge from behind the mirror. 6

7 Common terminology of curved mirrors ocal length, f The distance between the focal point and the surface of the mirror. (P or ½ CP) bject distance, u The distance between the object and the surface of the mirror. Image distance, v The distance between the image and the surface of the mirror. Differences Concave Mirror Rays travelling parallel to the principal axis converge to a point, called the focal point on the principal axis. Convex Mirror Rays travelling parallel to the principal axis appear to diverge from a point behind the mirror, called the focal point on the principal axis. 7

8 Construction Rules for Concave Mirror and Convex Mirror Rule 1 Concave Mirror Convex Mirror bject bject A ray parallel to the principal axis is reflected through. Rule 2 bject A ray parallel to the principal axis is reflected as if it comes from. bject A ray passing through is reflected parallel to the principal axis A ray directed towards is reflected parallel to the principal axis. 8

9 Construction Rules for Concave Mirror and Convex Mirror Concave Mirror Convex Mirror Rule 3 bject bject C C A ray passing through C is reflected back along the same path through C. A ray is directed towards C is reflected back along the same path away from C. If an egg is broken by an outside force. A life ends. If an egg breaks from within.... Life begins. Great things always begin from within. 9

10 Ray Diagrams of concave mirror u > 2f u = 2f or u = c C f < u < 2f or f < u < c u = f C C 10

11 Ray Diagram of concave mirror u < f bject distance u > 2f Characteristics of the image: C u = 2f f < u < 2f u = f u < f Sir, we are surrounded from all sides by enemies! Excellent! We can attack in any direction. 11

12 Ray Diagram of convex mirror f < u < 2f u < f C C C C bject distance u > 2f u = 2f f < u < 2f u = f u < f Characteristics of the image: Diminished, upright, virtual Image formed within 0 < v < f 12

13 Application of Reflection of Light Anti-parallax Mirror in Ammeters or Voltmeter 1. A parallax error occurs when the scale is viewed at an improper angle (the eye sees both the pointer and its image). 2. Some meters provide a mirror within the display, so that a user can easily determine the correct viewing angle by checking the needle's reflection. 3. The proper angle is achieved when the needle's reflection is not visible to the user's eye. pointer mirror strip Periscope ray from a far object mirror 45 pointer s image 1. A periscope can be used to see over the top of high obstacles such as a wall. 2. It is also used inside a submarine to observe the surrounding above water surface. 3. Consist of 2 plane mirror inclined at an angle of The final image appears upright

14 Ambulance Reflector of torchlight Why is the word AMBULANCE purposely inverted laterally on an ambulance car? Images seen through the rear mirror of a car is laterally inverted. Make-up Mirror The light bulb is fixed in position at the focal point of the concave mirror to produce a beam of parallel light rays. The beam of parallel light rays will maintain a uniform intensity for a greater distance. ther applications are the headlight of motor vehicles and the lamp of slide projectors. Concave mirrors with long focal lengths, produce virtual, magnified and upright images bulb parallel light rays 14

15 Widening the field of vision ield of vision a) Plane mirror Wider field of vision b) Convex mirror When a convex mirror is used, the field of vision is larger than a plane mirror Convex mirrors are used as rear view mirrors in motor vehicles to give drivers a wide-angle view of vehicles behind them. It is also used as shop security mirrors. Transmission of radio waves and signals A concave parabolic surface is used to focus the radio wave signals. 15

16 5.2 Understanding Refraction f Light Refraction of light A i N Denser medium r Less Dense medium B A i N Less Dense medium Angle of incidence, i the angle between the incident ray and the normal. Denser medium r B Angle of refraction, r i > r the angle between the refracted ray and the normal the ray bent towards the normal, and the speed of light decreases. A = Incident ray r < i the ray bent away from the normal B = Refracted ray and the speed of light increases. N = Normal line i = Angle of incident r = Angle of refraction 16

17 3 ways in which a ray of light can travel through two medium When a light ray travels from less dense medium to denser medium A i N Less Dense medium When a ray of light travels from denser medium to less dense medium. A i N Denser medium When light ray is incident normally on the boundary between the two medium. Denser medium r B r Less Dense medium B The light ray is refracted towards the normal. The speed of light decreases. The light ray is refracted away from the normal. The speed of light increases. The light ray is does not bend. The Laws of Refraction 17

18 Refractive Index, n Snell s Law Real Depth and Apparent Depth sin i sin r constant n Velocity of light in medium 1 Air Water h H I Normal H = Real depth h = Aparent depth The refraction of light is caused by the change in velocity of light when it passes from a medium to another medium. velocity of light invacuum n velocity of light in medim 2 1. Rays of light coming from the real fish, travels from water (more dense) to air (less dense) 2. The rays are refracted away from the normal as they leave the water. 3. When the light reaches the eye of the person, it appears to come from a virtual fish, I which is above the real fish. The refractive index has no units. It is an indication of the light-bending ability of the medium as the ray of light enters its surface from the air. H n 3 h 18

19 Exercise a) Draw a ray diagram from point P to the eye to show how the legs appear shorter. b) The depth of water is 0.4 m. Calculate the distance of the image of the foot at point P from the surface of the water. [Refractive index of water = 1.33] 19

20 20 2. The light ray travels from air to medium x. ind the: a) incident angle b) refracted angle c) refractive index 45 Air 60 Medium X 3. A light ray is incident normally on a glass prism which has a refractive index of a) complete the ray diagram. b) ind the incident angle and the refractive angle 30 60

21 5.3 Total Internal Reflection f Light i < c 1. When light travels from a denser medium to a less dense, it bends away from normal. 2. A small part of the incident ray is reflected inside the glass. 3. The angle of refraction is larger than the angle of incidence, r > i Air Glass Incident ray Normal i r Refracted ray Weak reflected ray i = c 1. When the angle of incidence, i keeps on increasing, r too increases and the refracted ray moves further away from the normal and thus approaches the glass air boundary. 2. The refracted ray travels along the glass-air boundary. 3. This is the limit of the light ray that can be refracted in air as the angle in air cannot be any larger than The angle of incidence in the denser medium at this limit is called the critical angle, c. Air Glass i = c Incident ray Normal c r = 90 c Refracted ray Weak reflected ray 21

22 i > c 1. If the angle of incidence is increased further so that it is greater than the critical angle, the light is not refracted anymore, but is internally reflected. 2. This phenomenon is called total internal reflection. Air Glass Incident ray Normal i > c c Strong reflected ray Total internal reflection Conditions The two conditions for total internal reflection to occur are: 1. light ray enters from a denser medium towards a less dense medium 2. the angle of incidence in the denser medium is greater than the critical angle of the medium. 22

23 Exercise Calculate the critical angle, c [ Refractive index of water = 1.33 ]. Air Water c 2. igure shows a light ray strikes the surface of a prism. The refractive index of glass is 1.5. ind the critical angle. Complete the path of the light ray that passes into and out of the prism

24 Natural Phenomenon involving Total Internal Reflection Mirage 1. Mirage is caused by refraction and total internal reflection. 2. Mirage normally occur in the daytime when the weather is hot. 3. The air above the road surface consists of many layers. 4. The layers of air nearest the road are hot and the layers get cooler and denser towards the upper layers. 5. The refractive index of air depends on its density. The lower or hotter layers have a lower refractive index than the layers above them. Sunset 1. The Sun is visible above the horizon even though it has set below the horizon. 2. Light entering the atmosphere is refracted by layers of air of different densities producing an apparent shift in the position of the Sun. 24

25 Applications of Total Internal Reflection Prism Periscope object image 45 prism prism The periscope is built using two right-angled prisms. 2. The critical angle of the glass prisms is Total internal reflection occurs when the light rays strike the inside face of a 45 angles with an angle of incidence, I, greater than the critical angle, c,. 4. The image produced is upright and has the same size as the object. Advantage of the prisms periscope compared to a mirror periscope: a) the image is brighter because all the light energy is reflected. b) the image is clearer because there are no multiple images as formed in a mirror periscope. 25

26 ish s Eye View 1. A fish is able to see an object above the water surface because the rays of light from the object are refracted to the eyes of the fish or diver. 2. Due to total internal reflection, part of the water surface acts as a perfect mirror, which allows the fish and diver to see objects in the water and the objects around obstacles. 3. A fish sees the outside world inside a 96 cone. utside the 96 cone, total internal reflection occurs and the fish sees light reflected from the bottom of the pond. The water surface looks like a mirror reflecting light below the surface. refraction ish sees outside world inside 96 cone Total internal reflection 26

27 Prism Binoculars 1. A pair of binoculars uses two prisms which are arranged as shown in figure. 2. Light rays will be totally reflected internally two times in a pair of binoculars. 3. The benefits of using prisms in binoculars: a) an upright image is produced. b) The distance between the objective lens and the eyepiece is reduced. This make the binoculars shorter as compared to a telescope which has the same magnifying power. 45 bjective lens Prism B bject Image Prism A Eyepiece lens 45 27

28 ptical fibers external wall Internal wall 1. iber optics consists of a tubular rod which is made from glass and other transparent material. 2. The external wall of a fiber optic is less dense than the internal wall. 3. When light rays travel from a denser internal wall to a less dense external wall at an angle that exceeds the critical angle, total internal reflection occurs repeatedly. 4. This will continue until the light rays enter the observer s eye. 5. ptical fiber is widely used in telecommunication cables to transmit signal through laser. It can transmit signal faster and through long distance with high fidelity. 6. ptical fiber is also used in an endoscope for medical emerging. Advantage of using optical fibres cables over copper cables: (a) much thinner and lighter (b) a large number of signals can be sent through them at one time. (c) transmit signals with very little loss over great distances. (d) signals are safe and free of electrical interference (e) can carry data for computer and TV programmes. 28

29 5.4 Lenses Lenses are made of transparent material such as glass or clear plastics. They have two faces, of which at least one is curved. Convex Lens Convex converging lenses - thicker at the centre Concave Lens Biconvex Plano-convex Concavo - convex Concave diverging lenses - thinner at the centre Biconcave Plano-concave Concavo - concave 29

30 ocal Point and ocal Length of a Lens Light rays Principal axis C ptical center f ocal point ocal the principal focus, A point on the principle axis to which incident rays of light traveling parallel to the axis converge after refraction through a convex lens. ocal length, f Distance between the focal point, and the optical centre, C Principal axis Light rays ocal point C ocal principal focus, A point on the principal axis to which incident rays of light traveling parallel to the axis appear to diverge after refraction through a concave lens. ocal length, f f Distance between the focal point, and optical centre, C on the lens. 30

31 Rules for Ray Diagrams Convex Lens The ray parallel to the principal axis is refracted through the focus point,. A ray passing through the focus point is refracted parallel to the principal axis. A ray passing through the optical centre travels straight without bending. Concave Lens The ray parallel to the principal axis is refracted as if it appears coming from focus point, which is located at the same side of the incident ray. A ray passing the focus point is refracted parallel to the principle axis. A ray passing through the optical centre travels straight on without bending. 31

32 Ray Diagrams of convex lens u < f u = f 2 2 f < u < 2f

33 u = 2f u > 2f u = bject distance u = u > 2f u = 2f f < u < 2f Characteristics of the image: Diminished, inverted, real Diminished, inverted, real Same size, inverted, real Magnified, inverted, real u = f Magnified, upright, virtual u < f Magnified, upright, virtual 33

34 Ray Diagrams of concave lens f < u < 2f R1 bject 2 Image R3 34

35 Ray Diagrams of concave lens u = 2f u < f 2 2 bject distance u = u > 2f u = 2f f < u < 2f u = f u < f Characteristics of the image: 35

36 Power of Lenses 1. The power of a lens is a measure of its ability to converge or to diverge an incident beam of light. 2. SI unit = m -1 or Diopter (D). 3. Power for a convex lens is positive. Power for a concave lens is negative. f in m f in cm Lens ormula m < 1 1 f = focal length u = object distance v = image distance m = 1 m > m = Linear magnification h I = size of image h 0 = size of object Example 1 ind the power: a) convex lens, f = 20 cm, b) concave lens, f = -5 cm. 36

37 Exercise An object is placed in front of a convex lens with focal length of 10 cm. ind the nature, position and magnification of the image formed when the object distance is 15 cm. 37

38 2. An object is placed 20 cm from a concave lens of focal length 15 cm. Calculate the image distance. State the characteristics of the image formed. 38

39 3. A convex lens with focus length of 15 cm formed an image which is real, inverted and same size with the object. What is the object distance from the lens? 39

40 4. When an object of height 3.0 cm is placed 20 cm from a concave lens of focal length 30cm, what is the height of the image formed? 40

41 Applications of Lenses Simple Microscopes Application : to magnified the image Lens : a convex lens bject distance: less than the focal length of the lens, u < f Characteristics of image: virtual, upright, magnified The magnifying power increases if the focal length of the lens is shorter. object eye 41

42 Telescope Light ray from distant object u 1 = bjective lens f o f e Eyepiece lens o e o e u 2 = f e inal image formed at infinity Application : view very distant objects like the planets and the stars. Made up of two convex lenses :bjective lens and eyepiece lens ocal length f o for objective lens is longer than the focal length for eyepiece lens, f e The objective lens converges the parallel rays from a distant object and forms a real, inverted and diminished image at its focal point. The eyepiece lens is used as a magnifying glass to form a virtual, upright and magnified image. At normal adjustment the final image is formed at infinity. This is done by adjusting the position of the eyepiece lens so that the first real image becomes the object at the focal point, e of the eyepiece lens. Normal adjustment: The distance between the lenses is f 0 + f e 42

43 Compound Microscope bjective lens Eyepiece lens f o < u 1 < 2f o object o f o e o 2 o 1 st image f e e inal image u 2 < f e Application: to view very small objects like microorganisms Uses 2 powerful convex lenses (bjective lens, Eyepiece lens ) of short focal lengths. ocal length f o for objective lens is shorter than the focal length for eyepiece lens, f e bject to observed must be placed between 0 and 2 0 Characteristics of 1 st image: real, inverted, magnified. 43

44 The eyepiece lens is used as a magnifying glass to magnify the first image formed by the objective lens. The eyepiece lens must be positioned so that the first image is between the lens and e, the focal point of the eyepiece lens. Characteristics of final image formed by the eyepiece lens: virtual, upright and magnified. Normal Adjustment: The distance between the lenses is greater than the sum of their individual focal length (f o + f e ) 44