Download Refraction – Learning Outcomes

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Refraction – Learning Outcomes
 Define refractive index.
 Demonstrate refraction.
 State the Laws of Refraction.
 Solve problems about refraction.
 HL: Solve problems about refractive index in terms of
relative speeds.
 Give examples of refraction in nature.
 Define critical angle and total internal reflection.
 Demonstrate total internal reflection.
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Refraction – Learning Outcomes
 Solve problems about total internal reflection.
 Give uses and natural occurrences of refraction.
 Discuss transmission of light in optical fibres.
 Give uses of optical fibres.
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Refraction
 Refraction is the bending of light as it passes from one
medium to another.
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To Demonstrate Refraction
1. Aim a narrowed beam from a ray box at the side of a
block of glass.
2. Vary the angle of incidence and note that the angle of
refraction increases with angle of incidence.
3. Note that the ray exiting the block is parallel to the
incident ray.
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Laws of Refraction
1. The incident ray, the normal at the point of incidence,
and the refracted ray all lie in the same plane.
2. The ratio of the sine of the angle of incidence to the
sine of the angle of refraction is a constant.
 The second law is also called “Snell’s Law” which we
must verify experimentally.
 The constant in the second law is the refractive index
between the two media, xny, i.e. the ratio of the
absolute refractive indices.
sin 𝑖
 Formula: 𝑥𝑛𝑦 = sin 𝑟
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Refractive Index
 The refractive index of a medium is the ratio of the sine
of the angle of incidence to the sine of the angle of
refraction when light travels from a vacuum into that
medium.
Material Refractive Index
sin 𝑖
 𝑛 = sin 𝑟
Vacuum 1 (by definition)
Air 1.0003
Water 1.33
Glass ~1.5 (varies with glass)
Diamond 2.4
Germanium 4.1
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Snell’s Law
 e.g. A ray of light enters glass from air. The angle of
incidence is 30o and the angle of refraction is 19o. What
is the refractive index of the glass?
 e.g. A ray of light enters water from air. If the angle of
incidence is 40o, find the angle of refraction if the
refractive index of water is 1.33.
 e.g. Light enters water from glass. If the angle of
incidence is 40o and the angle of refraction is 46.3o,
what is the refractive index between glass and water?
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Depth
 Due to refraction, objects
immersed in a fluid will
appear to be closer to the
surface than they really are.
 This is given by:
 Formula: 𝑛 =
𝑟𝑒𝑎𝑙 𝑑𝑒𝑝𝑡ℎ
𝑎𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑑𝑒𝑝𝑡ℎ
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Depth
 e.g. Sorcha draws a mark on a sheet of paper and
places a glass block with thickness 8 cm over it. When
viewed from above the glass, the mark appears to be
5.33 cm from the surface. What is the refractive index of
the glass?
 e.g. A pool of water is 12 m deep. If the bottom of the
pool is viewed from the air, how deep does it appear?
nwater = 1.33
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Refraction in Nature – Bears
 Bears have to recognise refraction when fishing – the fish
appears to be higher up than it really is.
Real fish by unknown artist – public domain
Bear by Joseph Smit – public domain
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HL: Speed of Light
 Light travels at different speeds in different media. The
ratio of speeds between two media is the refractive
index between them.
𝑐
 Formula: 𝑛 = 𝑐1
2
 For any medium, it follows that:
 Formula: 𝑛 =
𝑐 𝑖𝑛 𝑎𝑖𝑟 𝑜𝑟 𝑣𝑎𝑐𝑢𝑢𝑚
𝑐 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚
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HL: Speed of Light
 e.g. The refractive index of water is 1.33. If the speed of
light in air is 3 × 108 𝑚 𝑠 −1 , what is the speed of light in
water?
 e.g. Light enters glass from air. The angle of incidence is
35o and the angle of refraction is 22o. If the speed of light
in glass is 2 × 108 𝑚 𝑠 −1 , calculate the speed of light in air.
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Total Internal Reflection
 When light travels from a denser to a rarer medium, the
critical angle, C is the angle of incidence which gives an
angle of refraction of 90o.
 Total internal reflection (TIR) occurs when light travelling
from a denser to a rarer medium is incident at an angle
greater than the critical angle.
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To Demonstrate TIR
1. Aim a narrowed beam from a ray box at a semi-circular
slab of glass
2. Aim the beam so that it is incident on the flat face of
the slab internally.
3. Starting with a small angle of incidence, increase this
angle.
4. When the critical angle is reached, the refracted ray
skims along the flat face of the glass.
5. For higher angles of incidence, the refracted ray
changes to a totally internally reflected ray.
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To Demonstrate TIR
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Total Internal Reflection
 Given 𝑥𝑛𝑦 =
sin 𝑖
,
sin 𝑟
we can set up the equation for TIR.
 If the rarer medium is a vacuum, then:
1
sin 𝐶
90𝑜
 𝑛 = sin
⇒𝑛=
1
sin 𝐶
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Total Internal Reflection
 e.g. The critical angle for a certain medium is 50o. Find its
refractive index.
 e.g. The refractive index of glass is 1.5. What is the critical
angle of glass?
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TIR in Nature – Mirages
 The refractive index of air changes with temperature.
 On hot days, light from the sky can bend away from a
road towards your eye, creating a “puddle” image.
By Brocken Inaglory from Wikipedia – CC-BY-SA-3.0
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TIR in Nature – Mirages
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TIR in Nature – Snell’s Window
 Looking up while underwater, only light from within a
certain radius will reach you – the rest is totally internally
reflected from underwater. This is called Snell’s window.
by Jayme Pastoric – public domain
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TIR in Nature – Snell’s Window
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Uses
 Prism reflectors are used in
road signs to ensure that
light from headlights
reflects back at the driver.
 Safety reflectors on bikes
and cars use the same
effect.
 Many modern devices use
round reflectors, which
work on the same
principle.
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Uses – Optical Fibres
 Optical fibres are thin transparent glass rods that can
transmit light via total internal reflection.
by Timwether – CC-BY-SA-3.0
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Uses – Optical Fibres
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Optical Fibres
 Optical fibres are used to transmit telephone, television,
and internet signals as pulses of light.
 It is better than the old copper cables in pretty much
every way – lower loss, lower size, lower interference.
 They are also used in medicine as flexible cameras,
called endoscopes.