*LIGHTSec 3 Exp

*BTEC, you should be able to:Define terms used in reflection: normal, incident angle, reflected angleState Law of ReflectionUse this law in constructions, measurements and applicationsDefine terms used in refraction: normal, incident angle, refracted angleRecall equation sin i /sin r = constantUse equation above to solve related problemsDefine refractive index as the ratio of speed of light in vacuum and a mediumDefine critical angle and total internal reflectionUnderstand internal reflection and how this idea in the use of optical fibres for communication purposes

*Recall Basic Ideas of LightLight travels in a straight line.Light is a form of energy that allows us to see thingsLight 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 108 m/s

*Luminous & Non-luminous objectLuminous objectsAre those that give out lightCan be seen directlyEx: Sun, light bulb, traffic light, some chemicals like phosphurus

Non-luminous objectsAre those that cannot give out light by itselfIt can only be seen when light from luminous object reflects from it surfaces

*Which of the above object is considered:luminous : lampnon-luminous : picture, eye

*ReflectionLight 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.

*Reflection of Light

*Terms used in ReflectionNormal Line : An imaginary line drawn 90o (perpendicular) to surface at the point where light ray is reflected awayIncident Ray: Ray of light striking surfaceReflected Ray: Ray of light reflected from surfacei = angle of incidence: Angle between incident ray and normal liner = angle of reflection: Angle between reflected ray and normal line

*Law of ReflectionStates that angle of incidence is equal to angle of reflection (i = r)

Angle of reflection = 30oSurfaceIncidentLight rayReflectedLight rayNormal line

*Determining Angles of ReflectionApparatus: Protractor, Ruler, PencilFirst draw normal line at where light ray strikes surface.Place protractor as shown.Next, measure angle from light ray to normal line.

*Image of object on Plane MirrorA 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 objectImage distance to mirror is the same as object distance to mirrrorLeft-right invertedUprightVirtual (cannot be formed on a screen)

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*Ray DiagramYou 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.

*An example of a ray diagram of cylinder objectImage as seen by both Johnny and SuzieNote that the light rays are drawn according to Law of Reflection

*Image formation

*Steps to drawing a ray diagramPick a point on object to start. (Corner of object.)Draw a line from corner of object and perpendicular to mirror.Measure distance of object to mirror. Use this distance and make a mark as shown.Draw a line from mark to middle of eye.Draw a line perpendicular to surface at spot where line in step 4 cuts mirror. This is the normal line.Draw reflected ray from normal to eye.Draw incident ray from corner of object to normal.Draw image of object (Must be same size)MirrorObjectProtractor is neededif asked to measure angles of incidencereflection.

*Ray diagram for slanted objectFor slanted objects like this, the ideaBehind drawing of ray diagram isStill the same. Start first with theDistance of image followed by rayto eye. Just repeat for next point.

*Ray diagram - multiple imagesRemoved from O Levels syllabus

*Uses of reflectionPeriscopeRear view mirror of vehiclesMake-up mirror

*Short Quiz 11.A ray of light is reflected by a mirror. If the angle between incident and reflected light rays is 76o, what is the angle of incidence?A. 14oB. 38oC. 76oD. 104o

2.A red light ray traveling at speed of 3x108 m/s is incident on a mirror and is reflected away. What is the most likely speed of the reflected red light ray?A. 2.998x108 m/sB. 3x108 m/sC. 3.012x108 m/sD. unknown

3.The image formed by a plane mirror is always i. uprightii. virtualiii. same size as the objectA. i only.B. iii only.C. ii and iii.D. i, ii and iii.

*4a.Which of the angles is:i. Angle of incidence: _____ii. Angle of reflection: _____4b.If angle a = 35o, 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.

*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.

*Optical DensityOptical 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: AirObjects 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

*RefractionRefraction 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 108 m/sIn water, speed < 3 x 108 m/s

In general, medium with high optical density slows down speed of light greatly.

*RefractionWhen 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.

*Refractive IndexRefractive 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 valuen just > 1 means that light is bent slightlyn >> 1means that light is bent greatlyn = 1 means that light is not bent

*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.

diamondglasswaterairRefractive index, n2.41.51.331

*Finding Refractive Index, n (1)Using angles: n of a medium = sin ii, r are in degreessin rNote that light ray must be from air to medium

*Finding Refractive Index, n (2)Using speed of light:n of a medium = c c = speed of light in airv v = speed of light in medium

Note again, it is assumed that light ray traveled from air to medium.

*Law of Refraction (Snells 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.

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

*Principle of Reversibility of LightThe equation sin i/sin r is meant for light travelling from air to a denser mediumHow 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.

*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, iA = rB and rA = iB.

n for glass = AB

*Ray Diagram for RefractionYou are required to calculate angle of refraction and draw refracted rayApparatus: Protractor, ruler, pencilSteps:Calculate refractive index from given informationUse ruler to draw normal line (if needed)Use protractor and mark on angle of refractionDraw a straight line to indicate refracted ray

*Calculate angle of refraction

Marking with protractor at 35.3o (3 s.f.) Draw refracted ray60oGlass, with n = 1.5

*Effect of RefractionAppearing and disappearing act of objectsMovie

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

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

*Short Quiz 21.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 reflection2.Find the refractive index of medium Z given a light ray from air enters the medium with angle of incidence and refraction of 50o and 32o respectively.A. 0.64B. 0.69C. 1.45D. 1.563.Given that the index of refraction of a tinted glass is 1.6, find the critical angle of this glass.A. 35.0o B. 36.9o C. 38.7o D. 45.0o

*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 = 64o and g = 41o, calculate n of glass.5.(a)A ray of light is incident on surface of water at an incident angle of 50o. 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.

*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.

Name: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*Add examples of problems(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*In fact light is something special. It behaves as both a wave and a particle. This is what we call the Duality Theory of Light(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*Need to re-organize quiz 21. Split into Quiz 2 32. Quiz 2 Refraction, P.O.Reversibility and Drawing of Ray Diagram to show effect of refraction

3. Quiz 3 Total internal reflection and Critical Angle(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David OonName: _____________________ ( )Name: _____________________ ( )Class: ______ Date: ___________(c) Canberra Sec School - David Oon*(c) Canberra Sec School - David Oon