Goal: To understand how mirrors and lenses work

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Goal To understand how mirrors and lenses work

• Objectives
• To understand how Plane mirrors create an image
• To understand how Convex Mirrors reflect light
• To understand the properties of Concave Mirrors
• To be able to calculate the Focal Length and
  Magnification of a lens or mirror
• To understand the similarities of Lenses and
  Mirrors

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Plane mirrors

• These are the sort of mirrors you find in a
  bathroom.
• They are straight and flat.

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Convex mirrors

• Convex Mirrors curve away from you.

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Concave Mirrors

• Convex mirrors curve towards you

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Some basic properties

• Object distance.
• This is denoted as p.
• This is the physical distance the object is away
  from the mirror.
• Radius of Curvature (C) is what the radius of a
  mirror would be if it was an entire sphere.

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Image distance and focal length

• This is the distance the image appears to be away
  from the mirror (denoted q).
• Focal length (denoted f) is the distance from the
  mirror to the focal point (where all the light
  would come together for a light source very far
  away).

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Mirror Equation

• Finally some math
• 1/p 1/q 1/f
• Yes, you can use that trick I showed you earlier
  with this!
• So, f pq / (pq)
• And p -fq / (f q)
• The minuses due to 1/p 1/f 1/q

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Sample

• An object is place 5 cm away from a mirror which
  has a focal length of 2 cm.
• What is the distance from the mirror to the image?

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Sample

• An object is place 5 cm away from a mirror which
  has a focal length of 2 cm.
• What is the distance from the mirror to the
  image?
• q (on board)
• Note that a negative means that the image is a
  virtual image (is in front of the mirror).
• Positive values of q mean real image (behind the
  mirror).
• p is always positive

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Another

• An object is 10 cm away from a mirror.
• The image in the mirror is 5 cm behind the
  mirror.
• A) is this a real or imaginary image?
• B) what is the focal length of the mirror?

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Magnification

• m h / h
• That is it is the height of the image divided by
  the height of the actual object.
• Also, m -q / p
• Note that when q is positive (real image) m is
  negative.
• This means that the image is inverted (upside
  down).
• What would be true about the image if q is
  negative?

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Lenses

• There are two types of lenses.
• Diverging lenses make light spread out.
• Diverging lenses tend to be concave.
• Not that q and f are BOTH negative for a lens or
  mirror that is diverging.
• Converging lenses make light focus on a point.
• Converging lenses tend to be convex.

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Diverging vs converging

• Diverging (concave) lenses generate an image in
  front of the lens.
• Is this a real or virtual image?

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Diverging vs converging

• Diverging (concave) lenses generate an image in
  front of the lens.
• Is this a real or virtual image?
• Virtual image
• Since you have a virtual image, is q going to be
  positive or negative?

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Diverging vs converging

• Diverging (concave) lenses generate an image in
  front of the lens.
• Is this a real or virtual image?
• Virtual image
• Since you have a virtual image, is q going to be
  positive or negative?
• Negative!
• Since q is negative will the magnification be
  positive or negative?

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Diverging vs converging

• Diverging (concave) lenses generate an image in
  front of the lens.
• Is this a real or virtual image?
• Virtual image
• Since you have a virtual image, is q going to be
  positive or negative?
• Negative!
• Since q is negative will the magnification be
  positive or negative?
• Positive (upright image)

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Converging lens

• q is usually, but not always positive.
• This means the magnification will be negative
  (inverted image).
• However, q can be negative
• q -fp / (f p)
• So, if f is gt p then q is actually negative.
• That is if the object is closer to the lens than
  the focal length you get a virtual image
  otherwise you get a real image.

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Conclusion

• We learned about the different mirror and lens
  types.
• We learned how to find object distance, image
  distance, and focal length.
• We learned 2 ways to calculate magnification.
• We learned the differences between real and
  virtual images and how they translate to inverted
  or upright images.