Unit 10 Light

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Unit 10 Light
Discover PHYSICS for GCE O Level Science

• Thin Converging Lens

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Unit 10.6 Converging Lens

• Learning Outcomes
• In this section, youll be able to
• Describe the action of a thin converging lens on
  a beam of light
• Define the term focal length for a converging lens

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Unit 10.6 Converging Lens

• What is a lens?
• A lens is a piece of clear plastic or glass with
  curved surfaces.
• Lenses are widely used in spectacles, cameras,
  projectors and many other optical instruments.

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Unit 10.6 Converging Lens

• How does a lens refract light?
• A lens can be thought of as a set of blocks and
  prisms (see Fig 10.53 and 10.54).

Fig 10.53 Converging Lens
Fig 10.54 Diverging Lens
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Unit 10.6 Converging Lens

• How does a lens refract light?
• Parallel light rays will incident different parts
  of the lens at different incident angles.
• Light rays refract the most at the outermost part
  of the lens, while less or no refraction occurs
  in the middle portion.

Fig 10.53 Converging Lens
Fig 10.54 Diverging Lens
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Unit 10.6 Converging Lens

• How does a lens refract light?
• As a result, the light rays will converge (Fig
  10.53) or diverge (Fig 10.54) behind the lens.

Fig 10.53 Converging Lens
Fig 10.54 Diverging Lens
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Unit 10.6 Converging Lens

• Types of Lenses Converging and Diverging
• For converging lenses, parallel light rays are
  brought to focus at a point.
• For diverging lenses, parallel light rays tend to
  be spread out.

Insert table 12.4
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Unit 10.6 Converging Lens

• Thin converging lens and its main features

1. Optical centre C
2. Principal axis
3. Focal point F All parallel rays to the
   principal axis converge at the focal point F.
4. Focal length, f The distance between the
   optical centre C and the focal point F.
5. Focal plane

Fig 10.58 Converging lens
Fig 10.59 Converging lens
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Unit 10.6 Converging Lens

• Tracing path of light through a thin converging
  lens

Insert fig for path 1,2 and 3 on pg 247
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Unit 10.6 Converging Lens

• Key Ideas
• Lenses are used to converge and diverge a beam of
  light.
• The main features of a thin converging lens are
• Optical centre C
• Focal point F
• Focal length f
• Principal axis

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Unit 10.6 Converging Lens

• Key Ideas
• Three definite paths for light rays passing
  through a thin converging lens

Insert Fig for path 1, 2 and 3 on pg 247
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Unit 10.6 Converging Lens

• Test Yourself 10.6
• Fig 10.61 shows light rays passing through a
  converging lens. Is the ray diagram correct? Give
  your reasons.
• Answer
• For a converging lens, light rays should be
  refracted in such a way that they tend to
  converge. So the correct ray path is

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Unit 10.7 Ray Diagram for Lenses

• Learning Outcome
• In this section, youll be able to
• Draw ray diagrams to illustrate the formation of
  real and virtual images of an object formed by a
  thin converging lens.

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Unit 10.7 Ray Diagram for Lenses

• How to locate the position of an image?
• 3 steps to locate the image
• Step 1 Set up the lens and the ray diagram.
• Step 2 Placing the object.
• Step 3 Trace the light rays using paths 1, 2 or
  3 as learned on pg 189.

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Unit 10.7 Ray Diagram for Lenses

• How to locate the position of an image?
• Step 1 Set up the lens and the ray diagram
• Draw principal axis
• Draw the lens
• Mark optical centre C
• Mark the focal point F

Fig 10.62 Step 1
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Unit 10.7 Ray Diagram for Lenses

• How to locate the position of an image?
• Step 2 Placing the object.
• Place the object O to the left of the lens.
• Mark the object distance as u.

Fig 10.63 Placing the object
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Unit 10.7 Ray Diagram for Lenses

• How to locate the position of an image?
• Step 3 Trace the light rays and draw the image.
• Draw 2 of the 3 definite paths e.g. Path 1 and 2.
• The point where the two light paths intersect is
  the position of the image.

Fig 10.64 Draw paths and locate image
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Unit 10.7 Ray Diagram for Lenses

• How to locate the position of an image?
• In this example, the image is said to be a real
  image.
• A real image is formed when the light rays
  converge at the point of the image. If a screen
  is place at this position, the image will be
  captured clearly on screen.

Fig 10.64 Formation of a real image
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Unit 10.7 Ray Diagram for Lenses

• Virtual image formed by a converging lens
• When the object O is placed near to the lens such
  that the object distance u is less that the focal
  length f, then a virtual image is formed.
• A virtual image cannot be captured on screen.

Fig 10.65 Virtual image is formed when u lt f
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Unit 10.7 Ray Diagram for Lenses

• Table 10.5 Types of images formed by a
  converging lens with different values of object
  distances u.

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Unit 10.7 Applications of Converging Lenses

• Converging Lenses can be used in various optical
  instruments
• Magnifying glass
• LCD projector
• Camera
• Visual correction for long-sightedness

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Unit 10.7 Applications of Converging Lenses

• Magnifying glass See Worked Example 10.11
• When the object is placed such that u lt f, image
  is magnified, virtual and upright.

Fig 10.67 Magnifying glass
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Unit 10.7 Applications of Converging Lenses

• LCD Projector In the LCD projector the object
  is placed between f and 2f such that the image
  formed is magnified, real and inverted.

Fig 10.68 LCD Projector
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Unit 10.7 Applications of Converging Lenses

• Camera Lens For the camera lens, the image is
  diminished, real and inverted.

Fig 10.69 Camera Lens
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Unit 10.7 Applications of Converging Lenses

• Visual Correction for Long-sightedness
• People with long-sightedness are unable to focus
  a clear image of near objects on the retina.

Fig 10.70(a) Long-sightedness
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Unit 10.7 Applications of Converging Lenses

• Visual Correction for Long Sightedness

Spectacles with converging lenses can partially
converge the light rays, thereby helping to form
a sharp image on the retina.
Fig 10.70(b) Converging lens helps a long
sighted eye to focus a clear image on the retina
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Unit 10.7 Ray Diagrams for Lenses

• Key Ideas
• Ray diagrams can be constructed to locate the
  position and type of image formed by a thin
  converging lens.
• Light paths using Path 1 and Path 2 to construct
  the ray diagram.
• A real and inverted image is formed when object
  distance u gt focal length f. The image is formed
  on the other side of the lens.

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Unit 10.7 Ray Diagrams for Lenses

• Key Ideas
• 4. A virtual and upright image is formed when the
  object distance u lt focal length f. The image is
  formed on the same side of the lens as the
  object.
• Converging lenses can be found in a wide variety
  of optical instruments such as the camera and
  projector.

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Unit 10.7 Ray Diagrams for Lenses

• Test Yourself 10.7
• How far from a thin converging lens must an
  object be placed to produce a magnified image?
• Answer
• When object distance u lt focal length f, the
  image is magnified, virtual and upright.
• (See Worked Example 10.11). Eg. Magnifying Glass
• When object distance u is between f and 2f, the
  image is magnified, real and inverted.
• (See Worked Example 10.12). Eg. Projector.

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Unit 10.7 Ray Diagrams for Lenses

• Test Yourself 10.7
• State two applications of converging lenses.
• Answer
• Any two of the following
• Use as magnifying glass
• In LCD projector
• Camera lenses
• Use for visual correction for long sightedness

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