5.3 Total internal reflection

Question 1

What is the critical angle?

Answer 1

» When the incident ray angle reaches the critcal angle, the light ray refracts along the boundary

Question 2

What happens if the angle of incidence is greater than the critical angle?

Answer 2

» The light ray undergoes total internal reflection at the boundary

Question 3

What are the 2 requirements for total internal reflection to take palce?

Answer 3

» The incidence substance has a larger refractive index than the other substance
» The angle of incidence exceeds the critical angle

Question 4

What is the angle of refraction at the critical angle, and why?

Answer 4

» 90 degrees, as the light emerges along the boundary
» Therefore n1.sin(theta1) = n2

Question 5

What happens if incident angle is less than the critical angle?

Answer 5

» Refraction happens

Question 6

What happens when the angle of incidence is exactly equal to the critical angle?

Answer 6

» The light ray refracts along the boundary

Question 7

Why do diamonds sparke so bright?

Answer 7

» Diamond has a very high refractive index - so seperates the colors more than any other substance does
» Critical angle is also low, so a light ray may be totally internally reflect many times before it emerges, which means colors spread out more and more

Question 8

Where are optical fibres used in?

Answer 8

» Meidcal endoscopes
» Communications to carry light signals

Question 9

How do optical fibres work?

Answer 9

» The light ray is totally internally reflected each time it reaches the fibre boundary
» At each point where the light ray reaches the boundary - the angle of incidence exceeds the critical angle of the fibre

Question 10

When does partial internal reflection occur?

Answer 10

» When the angle of incidence is less than or equal to the critical angle

Question 11

What does each fibre consists of ?

Answer 11

» A core surrounded by a layer of cladding of a lower refractive index to reduce light loss from the core
» Light loss will also reduce the amplitude of the pulses

Question 12

What does each fibre consists of ?

Answer 12

» A core surrounded by a layer of cladding of a lower refractive index to reduce light loss from the core
» Light loss will also reduce the amplitude of the pulses

Question 13

Where does total internal reflection take palce in the optical fibres?

Answer 13

» Takes place at the core-cladding boundary,
» At any point where the 2 fibres are in direct contact, light would cross from one fibre to another - if there was no cladding
» Signals would not be secure

Question 14

What is modal dispersion?

Answer 14

» Each beam undergoes TIR, a different number of times
» Therefore each beam reaches the end at a slightly different time, resulsts in pulse broadning

Question 15

Why must the core be very narrow?

Answer 15

» To prevent modal dispersion
» Which would reduce bandwith/cause pulse broadning

Question 16

How is modal dispersion caused in a wide core?

Answer 16

» Light travelling along the axis of the core travels a shorter distance per meter of a fibre
» A pulse of light sent along a wide core would become longer, than it ought to be
» If it was too long, it would merge with the next pulse

Question 17

Why must the light in optical fibres be monchromatic?

Answer 17

» Pulse dispersion can occur if white light is used
» Speed of light in the glass depends on the wavelength
» Violet light travels slower then red lights, causes wavelengths to become longer
» Leads to pulse merging

Question 18

How does a medical endoscope work?

Answer 18

» Contains 2 bundlesof fibres
» Endoscope is inserted into a body cavity - which is illuminated using light
» A lens over the end of the other fibre bundle is used to form an image of the body cavity
» Needs to be a coherent bundle

Question 18

How does a medical endoscope work?

Answer 18

» Contains 2 bundlesof fibres
» Endoscope is inserted into a body cavity - which is illuminated using light
» A lens over the end of the other fibre bundle is used to form an image of the body cavity
» Needs to be a coherent bundle

Question 19

What is a coherent bundle?

Answer 19

» Which means that fibre ends at each end are in the same relative positions

Question 20

What are the 4 properties of lasers?

Answer 20

» Monochromatic
» Collimated
» Coherent
» Polarised

Question 21

What is monochromatic?

Answer 21

» Waves of single frequency/wavelength

Question 22

What is collimated?

Answer 22

» Produces approximately parallel beams

Question 23

What is coherent?

Answer 23

» Waves produced have a constant phase difference

Question 24

What is Polarised?

Answer 24

» Vibrations in one plane only

Question 25

Explain why the fibre bundle needs to be coherent?

Answer 25

» The positions of ends of the fibres at the internal end need to map exactly to the same positions at the viewing end. In other words, the fibres cannot be allowed to “tangle up” as they go through the cable.
» Otherwise, the image would also be “tangled up”.

Question 26

What is one application of optical fibres?

Answer 26

» Communications
» Provide high-speed internet