Unit 10 Light

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Unit 10 Light
Discover PHYSICS for GCE O Level Science

• Refraction

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10.4 Refraction at Plane Surfaces

• Learning Outcomes
• In this section, youll be able to
• Understand the terms used for refraction
• Normal
• Angle of incidence
• Angle of refraction
• Recall that , and solve related problems
• Define refractive index of a medium in terms of
  ratio of the speed of light in vacuum to that in
  the medium

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10.4 Refraction at Plane Surfaces

• In Figure 10.28, we can observe that the pencil
  seems to be bent. What causes this effect?

Figure 10.28
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10.4 Refraction at Plane Surfaces

• Light can travel through transparent materials
  such as glass, water or plastic.
• Figure 10.29 shows a light ray traveling from air
  into glass, and then into air again.
• We can see the bending effect on the light ray
  when it passes from one medium into another. We
  call this refraction.

Figure 10.29
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10.4 Refraction at Plane Surfaces

• Refraction
• Refraction of light is the bending of light ray
  as it passes from one medium into another.

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10.4 Refraction at Plane Surfaces

• What causes refraction?
• Light travels at different speed in different
  media.
• For example, in air its speed is 3.0 108 m s-1,
  while in glass it is 2.0 108 m s-1.
• At the boundary of the two media such as air and
  glass, there is a sudden change in speed of the
  light. This change in speed causes the path of
  the light to bend, resulting in refraction.

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10.4 Refraction at Plane Surfaces

• Laws of Refraction
• The incident ray, the normal and the refracted
  ray all lie in the same plane.
• For two particular media, the ratio of the sine
  of the angle of incidence to the sine of the
  angle of refraction is a constant, i.e.

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10.4 Refraction at Plane Surfaces

• Snells Law
• The equation
• where n constant, is also known as Snells
  Law.

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10.4 Refraction at Plane Surfaces

• Refractive Index
• For the case of a light ray traveling from air
• into a medium such as glass, then

where n is the refractive index of the medium
incident ray
i
air
Medium (glass)
r
refracted ray
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10.4 Refraction at Plane Surfaces

• Refractive Index, n

For light passing from air or vacuum into a
medium, then
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10.4 Refraction at Plane Surfaces

• Table 10.3 Refractive indices of some transparent
• materials

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10.4 Refraction at Plane Surfaces

• Refractive index
• Glass (nglass 1.50) a higher refractive index
  than water
• (nwater 1.33). When a light ray enters glass, it
  will bend
• towards the normal more than compared to when it
  enters
• water.

i
i
air
air
rw
rg
Glass, nglass 1.50
water, nwater 1.33
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10.4 Refraction at Plane Surfaces

• Can reflection and refraction occur
• simultaneously?
• Light is both reflected and refracted at
  air-glass interface. However the amount of light
  reflected is usually smaller than the amount of
  light refracted.

Figure 10.36
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10.4 Refraction at Plane Surfaces

• One-way Mirrors
• A one-way mirror is made of a sheet of glass
  coated with a thin reflecting layer of metal.
  This mirror reflects half the light and allows
  the other half to pass through.

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10.4 Refraction at Plane Surfaces

• Phenomena of Refraction
• Bent Objects when a rod is placed in water,
  it appears bent.

In Figure 10.39, the light rays traveling from
water to air bend away from the normal. However,
our brain tends to tell us that the light rays
travel in straight line. Thus, we tend to
visualize the rod as bent.
Figure 10.39 Ray diagram of the bent image of
a rod in a glass of water
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10.4 Refraction at Plane Surfaces

• Phenomena of Refraction
• Misperception of Depth The effect of refraction
  can make a swimming pool seem shallower than it
  really is.

Figure 10.40 Ray diagram of the image I of a
point O at the bottom of a swimming pool.
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10.4 Refraction at Plane Surfaces

• The Secret of the Archer Fish
• How is the Archer fish able to overcome the
  visual distortion caused by refraction?
• Answer The Archer fish position itself directly
  under the prey. This way, the prey appears the
  least distorted, as the light rays entering the
  water surface perpendicularly are not refracted.

http//www.naturia.per.sg/buloh/verts/archer_fish.
htm
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10.4 Refraction at Plane Surfaces

• News Reader Prompter
• Have you wondered how a news reader is able to
  read the script and yet maintain eye contact with
  the camera lens?

Answer The news reader reads the script off a
partially reflected image, which is formed on a
one-way mirror. The camera is positioned behind
the mirror.
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10.4 Refraction at Plane Surfaces

• Key Ideas
• Refraction occurs because the speed of light
  changes when travelling through different optical
  media.
• The two Laws of Refraction are
• The incident ray, the normal and the refracted
  ray all lie in the same plane.
• For two particular media,
• where i is the angle of incidence in air.

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10.4 Refraction at Plane Surfaces

• Key Ideas
• The refractive index n of a transparent medium is

where c is the speed of light in vacuum or air,
v is the speed of light in the medium
where i is the angle of incidence of light in air
or vacuum
4.
5. Light is both reflected and refracted at the
boundary of two optical media.
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10.4 Refraction at Plane Surfaces

• Test Yourself 10.4
• 1. Draw a diagram to show how the direction of
  light changes when it travels from air into water.

Answer
i angle of incidence, r angle of refraction
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10.4 Refraction at Plane Surfaces

• Test Yourself 10.4
• 2. How is the speed of light in glass related to
  the angle of incidence and angle of refraction?

Answer
i
air
glass
where i angle of incidence in vacuum or
air, r angle of refraction in glass c speed
of light in vacuum or air v speed of light in
glass
r
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10.4 Refraction at Plane Surfaces

• Test Yourself 10.4
• 3. At what angle of incidence in air will light
  pass through another transparent medium without
  being refracted?

Answer When the angle of incidence, i 0 from
the normal, the light ray passes straight through
with refraction or bending.
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10.4 Refraction at Plane Surfaces

• Test Yourself 10.4
• 4. Draw a diagram to show how the eye sees a coin
  at the bottom of a bucket of water.