LIGHT

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LIGHT

• Sec 3 Exp

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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

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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 108 m/s

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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

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

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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.

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Reflection of Light
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Terms used in Reflection

• Normal Line An imaginary line drawn 90o
  (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

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Law of Reflection

• States that angle of incidence is equal to angle
  of reflection (i r)
• Angle of reflection 30o

Normal line
Reflected Light ray
Incident Light ray
Surface
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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.

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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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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.

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• 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

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Image formation
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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.
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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.
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Ray diagram - multiple images
Removed from O Levels syllabus
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Uses of reflection

• Periscope
• Rear view mirror of vehicles
• Make-up mirror

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Short Quiz 1

• 1. 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. 14o B. 38o C. 76o D. 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/s B. 3x108 m/s C. 3.012x108
  m/s D. unknown
• 3. The image formed by a plane mirror is always
• i. upright ii. virtual iii. same size as
  the object
• A. i only. B. iii only. C. ii and iii. D. i, ii
  and iii.

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• 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.

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• 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.

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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

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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 108 m/s
• In water, speed lt 3 x 108 m/s
• In general, medium with high optical density
  slows down speed of light greatly.

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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.

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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 gt 1 means that light is bent slightly
• n gtgt 1 means that light is bent greatly
• n 1 means that light is not bent

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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.

diamond glass water air
Refractive index, n 2.4 1.5 1.33 1
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Finding Refractive Index, n (1)

• Using angles
• n of a medium sin i i, r are in degrees
• sin r
• Note that light ray must be from air to medium

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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.

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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.

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

Glass
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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.

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A
B

• 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

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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

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60o
Glass, with n 1.5

• Calculate angle of refraction
• Marking with protractor at 35.3o (3 s.f.)
• Draw refracted ray

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Effect of Refraction
Movie

• Appearing and disappearing act of objects

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Bent objects in liquid.
A stick partially submerged in water appears
bent. The refraction makes the part of the stick
under water appear shallower.
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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?
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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 light B. refraction of light
  C. the use of plane mirror D. 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 50o and 32o
  respectively.
• A. 0.64 B. 0.69 C. 1.45 D. 1.56
• 3. 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

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• 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.

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• 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.