1 LIGHT Sec 3 Exp

2 BTEC, you should be able to:  Define terms used in reflection: normal, incident angle, reflected angle  State Law of Reflection  Use this law in constructions, measurements and applications  Define terms used in refraction: normal, incident angle, refracted angle  Recall equation sin i /sin r = constant  Use equation above to solve related problems  Define refractive index as the ratio of speed of light in vacuum and a medium  Define critical angle and total internal reflection  Understand internal reflection and how this idea in the use of optical fibres for communication purposes

3 Recall Basic Ideas of Light Light travels in a straight line. Light is a form of energy that allows us to see things Light is transmitted from one place to another, in the form of waves. Light waves are transverse in nature. It is part of the e.m. wave spectrum. A bundle of light rays are called light beam. Light travels at speed = 3 x 10 8 m/s

4 Luminous & Non-luminous object Luminous objects  Are those that give out light  Can be seen directly  Ex: Sun, light bulb, traffic light, some chemicals like phosphurus Non-luminous objects  Are those that cannot give out light by itself  It can only be seen when light from luminous object reflects from it surfaces

5 Which of the above object is considered: luminous : lamp non-luminous : picture, eye

6 Reflection Light can bounce off a surface, like a basketball bouncing from a floor. This property of light is known as reflection of light. Unlike a basketball, when light bounces off a surface, it does not slow down. Reflected light remains at the same speed.

7 Reflection of Light Surface i r Incident Light ray Reflected Light ray Normal line

8 Terms used in Reflection Normal Line : An imaginary line drawn 90 o (perpendicular) to surface at the point where light ray is reflected away Incident Ray: Ray of light striking surface Reflected Ray: Ray of light reflected from surface i = angle of incidence: Angle between incident ray and normal line r = angle of reflection: Angle between reflected ray and normal line

9 Law of Reflection States that angle of incidence is equal to angle of reflection (i = r) Angle of reflection = 30 o Surface 30 o r Incident Light ray Reflected Light ray Normal line

10 Determining Angles of Reflection Apparatus: Protractor, Ruler, Pencil First draw normal line at where light ray strikes surface. Place protractor as shown. Next, measure angle from light ray to normal line.

11 Image of object on Plane Mirror A mirror will show an image of an object placed in front of it. This image is formed due to reflection of light. Properties of this image are:  Same size as object  Image distance to mirror is the same as object distance to mirrror  Left-right inverted  Upright  Virtual (cannot be formed on a screen)

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13 Ray Diagram You must learn how to draw image of an object as seen by an observer, when object is placed in front of one or more mirrors. To do this successfully, you will need to understand how to draw a ray diagram. A ray diagram obeys Law of Reflection. Apparatus: protractor, ruler and pencil.

14 An example of a ray diagram of cylinder object Image as seen by both Johnny and Suzie Note that the light rays are drawn according to Law of Reflection

15 Image formation

16 Steps to drawing a ray diagram 1.Pick a point on object to start. (Corner of object.) 2.Draw a line from corner of object and perpendicular to mirror. 3.Measure distance of object to mirror. Use this distance and make a mark as shown. 4.Draw a line from mark to middle of eye. 5.Draw a line perpendicular to surface at spot where line in step 4 cuts mirror. This is the normal line. 6.Draw reflected ray from normal to eye. 7.Draw incident ray from corner of object to normal. 8.Draw image of object (Must be same size) Mirror Object Protractor is needed if asked to measure angles of incidence reflection.

17 Ray diagram for slanted object For slanted objects like this, the idea Behind drawing of ray diagram is Still the same. Start first with the Distance of image followed by ray to eye. Just repeat for next point.

18 Ray diagram - multiple images Removed from ‘O’ Levels syllabus

19 Uses of reflection Periscope Rear view mirror of vehicles Make-up mirror

20 Short Quiz 1 1.A ray of light is reflected by a mirror. If the angle between incident and reflected light rays is 76 o, what is the angle of incidence? A. 14 o B. 38 o C. 76 o D. 104 o 2.A red light ray traveling at speed of 3x10 8 m/s is incident on a mirror and is reflected away. What is the most likely speed of the reflected red light ray? A x10 8 m/sB. 3x10 8 m/sC x10 8 m/sD. unknown 3.The image formed by a plane mirror is always i. uprightii. virtualiii. same size as the object A. i only.B. iii only.C. ii and iii.D. i, ii and iii. 76 o

21 4a.Which of the angles is: i. Angle of incidence: _____ ii. Angle of reflection: _____ 4b.If angle a = 35 o, what is the angle of reflection? ______ 4c.According to the law of reflection, which angle must be equal? ___________ 5.A car plate has the numbers SFA 37Z. Write down the number as seen by another driver from his rear-view mirror. 6.There are 11 letters in the word EXAMINATION. How many of these are not changed when the word is seen by reflection? Write down the letters that are laterally inverted. a bc d

22 7.A person standing at pt A in the figure below sees the reflection in a shop window of a person standing at pt B. a)On the figure, draw a ray of light to show how, by reflection, the person at A sees the person at B. b)On the figure, mark with a letter I, the position of the image of B formed by reflection in the shop window. c)Measure and write down the angle of reflection. d)(i)Person at A moves away from B towards Y in the direction shown in the figure. Mark with a letter X the furthest position along Y to which the person A can moves so that the two people will still be able to see each other by reflection in the shop window. (ii)Explain how you decided on the position of X. wall Shop window BAY

23 Optical Density Optical density of an object can be understood as how transparent an object is. Objects with low optical density is highly transparent. These objects or substances allows light to travel through at very fast speed. Example: Air Objects with high optical density is not very transparent. These objects or substances allows a small amount of light to travel through it, at slow speed. Example: Red plastic bag used to pack food

24 Refraction Refraction is the bending of light when it moves from one substance to another substance of different optical density. The bending of light is caused by changes in speed of light. Speed of light changes in different medium.  In vacuum or air, speed = 3 x 10 8 m/s  In water, speed < 3 x 10 8 m/s In general, medium with high optical density slows down speed of light greatly.

25 Refraction When light travels from a lower dense to a higher dense medium, it slows down, causing it to bend towards the normal. When light travels from a higher dense to a lower dense medium, it speeds up, causing it to bend away from the normal. water Incident light ray Normal line Air Refracted light ray glass Incident light ray Normal line Air Refracted light ray

26 Refractive Index Refractive index is an indication of bending of light when it travels from one medium to another. It has a symbol, n and has no unit. If a medium has value  n just > 1 means that light is bent slightly  n >> 1means that light is bent greatly  n = 1 means that light is not bent

27 Refractive Index High values of n means that the medium has caused light to be bent greatly. There are 2 ways to find n of a medium. diamondglasswaterair Refractive index, n

28 Finding Refractive Index, n (1) Using angles:  n of a medium = sin ii, r are in degrees sin r  Note that light ray must be from air to medium Medium Air Incident light ray i r i : Angle of Incidence r : Angle of Refraction

29 Finding Refractive Index, n (2) Using speed of light:  n of a medium = c c = speed of light in air v v = speed of light in medium  Note again, it is assumed that light ray traveled from air to medium.

30 Law of Refraction (Snell’s Law) States that for a particular medium, the value of = constant. This constant is the refractive index, n. For a particular medium, its refractive index is always the same. Note that r here refers to angle of refraction, not reflection. Both symbols are the same.

31 Conditions for No Refraction No refraction will occur if light ray makes a incident angle of 0 o with surface. Glass

32 Principle of Reversibility of Light The equation sin i/sin r is meant for light travelling from air to a denser medium How is n calculated when light travels from other medium to air? We have to use the principle of reversibility (P.o.R) here. The idea here is that a light ray moving from A to B, is the same as from B to A, since light move only in straight line and is bent in the same manner.

33 If case A is given, applying P.o.Reversibility, we can assume that light travels in the opposite direction, as shown in B. As a result, i A = r B and r A = i B. n for glass = Glass i r r i By Principle of Reversibility AB

34 Ray Diagram for Refraction You are required to calculate angle of refraction and draw refracted ray Apparatus: Protractor, ruler, pencil Steps:  Calculate refractive index from given information  Use ruler to draw normal line (if needed)  Use protractor and mark on angle of refraction  Draw a straight line to indicate refracted ray

35 Calculate angle of refraction Marking with protractor at 35.3 o (3 s.f.) Draw refracted ray 60 o Glass, with n = 1.5

36 Effect of Refraction ‘Appearing and disappearing’ act of objects Movie

37 Bent objects in liquid. A stick partially submerged in water appears bent. The refraction makes the part of the stick under water appear shallower.

38 Fish appears nearer to surface Fish appears nearer to surface of water than it actually is. Refraction causes a distortion of depth. From the given information of apparent depth and actual depth, can you calculate refractive index of water?

39 Short Quiz 2 1.Light can travel from one end of an optical fibre to the other end, even if the fibre shows a curved path. This is due to ____. A. dispersion of lightB. refraction of light C. the use of plane mirrorD. total internal reflection 2.Find the refractive index of medium Z given a light ray from air enters the medium with angle of incidence and refraction of 50 o and 32 o respectively. A. 0.64B. 0.69C. 1.45D Given that the index of refraction of a tinted glass is 1.6, find the critical angle of this glass. A o B o C o D o

40 4.(a)A ray of light is traveling from air to glass as shown in the figure. Write down the refractive index of glass in terms of angles a, and g. (b)If a = 64 o and g = 41 o, calculate n of glass. 5.(a)A ray of light is incident on surface of water at an incident angle of 50 o. If n of water is 1.34, calculate the angle of refraction. (b) Draw a ray diagram to scale to show how the light ray above is refracted when entering water from air. air glass a g air water

41 6.Find the index of refraction of polished glass. 7.A swimming pool is lit by a lamp in the bottom of the pool. The direction of 3 rays from the lamp are shown. (a) Which of the marked angles is the critical angle of water. (b) Complete the ray diagram for the light ray labeled Y. Polished glass air 33 o 65 o A B C D Y