6091 Chapter 14 Light - Refraction , TIR & Lenses

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By Thou Reaksmey

Draw

Answer the question and show your workings clearly

Draw

Answer the question and show your workings

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Let's unpack the term "Total Internal Reflection"

Total : All the light

Internal: Within the medium eg water, air

Reflection: Light is reflected


Together - All the light is reflected within the medium

• Go to "PhET simulation - Light" website through Google

• Select "Intro"

Explore Total Internal Reflection using Simulation

• Switch on the torch

• Adjust the position of the torch

  
  Notice that as most of the light gets refracted in the water, there are some light that is reflected back into the air.
  
  Can you get all the light to be reflected within one medium?
  

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Scenario 1: Light passing from optically less dense air to optically denser water

• Change the material on top to "Water"

  Change the material at the bottom to "Air"

• Switch on the torch

• Adjust the position of the torch
  
  

  Can you get all the light to be reflected within one medium?
  

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Scenario 2: Light passing from optically denser water to optically less dense air

Is this Total Internal Reflection?

Open Ended

From the exercise done on the Simulation, light is not internally reflected when it goes from air into water but it is internally reflected when it goes from water into air.

Given that water is optically denser than air, suggest one condition that must be met for Total Internal Reflection to occur.

Multiple Choice

Light is directed on a glass block and it travels into the glass before travelling out of the glass. Which angle shown is the critical angle of glass?

• A

• B

• C

• D

Multiple Choice

Which diagram is the correct representation of critical angle of water?

• A

• B

• C

• D

Multiple Choice

A ray of light travels from glass to air with an angle of incidence of 35^o. If the critical angle is 42^o what will happen at the boundary?

• light refracts out into the air

• total internal reflection

• light travels along the boundary

• light is reflected into the air

Draw

Answer the question. Show your working

Multiple Choice

Diamond has a higher refractive index than glass. Which of the following is true about their critical angles?

• They are equal

• Diamond has a greater critical angle than glass

• Diamond has a lower critical angle than glass

Multiple Choice

Diamond has a higher refractive index than glass, hence a lower critical angle than glass. What can you say about the total internal reflections occurring in both media?

• It is less likely to occur in diamond than glass because there is a smaller range of angle i greater than critical angle in diamond

• It is more likely to occur in diamond than glass because there is a greater range of angle i greater than critical angle in diamond

• Total internal reflections are equally likely to occur in both media since the light source is the same.

Question

If glass and diamond are cut exactly the same way and same size for use as ornaments, diamond will be visibly more sparkly.

Given that diamond has a higher refractive index and by comparing the critical angles of diamond and glass, explain why diamond is more sparkly.

•Total internal reflection is the complete reflection of a light ray in an optically denser medium at the boundary with an optically less dense medium.





•Critical angle θ c is defined as the angle of incidence in an optically denser medium for which the angle of refraction in the less dense medium is 90°.

Summary of Total Internal Reflection

Conditions for total internal reflection to occur:

1.The incident ray must travel from an optically denser medium to an optically less dense medium.

2.The angle of incidence must be greater than the critical angle.

Summary of Total Internal Reflection

• Relationship between refractive index and critical angle

• Greater n => Smaller sin c => smaller c

• Optical fibres, which are thin and flexible, rely on total internal reflection of light to transmit signals in telecommunications and medicine.

• Advantages of optical fibres are they are faster , they result in less signal loss

  ,and they are lighter and cheaper
  
  

Summary of Total Internal Reflection

Draw

Draw light rays to complete the ray diagram

Draw

Draw light rays to complete the ray diagram

Multiple Choice

Which distance is equal to the focal length of a lens?

• The distance between a distance object and its image.

• The distance between the image of a close object and the centre of the lens.

• The distance between the image of a distant object and the centre of the lens.

• The distance between two principal foci.

Draw

Draw light rays to find the position of the image.

Draw the image.

Draw

Draw light rays to find the position of the image.

Draw the image.