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Chapter 25: Optics

25.A: Reflection and Mirrors

Reflection
The change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated.
Law of Reflection
The principle that the angle of incidence is equal to the angle of reflection.
Plane Mirror
A mirror with a flat reflective surface.
Concave Mirror
A mirror with a reflective surface that curves inward.
Convex Mirror
A mirror with a reflective surface that curves outward.

Law of Reflection

  • The angle of incidence (θi) is equal to the angle of reflection (θr).
  • Both angles are measured relative to the normal line, which is perpendicular to the surface at the point of incidence.

Plane Mirrors

  • Plane mirrors produce virtual images that are the same size as the object and are located the same distance behind the mirror as the object is in front.
  • The image is laterally inverted (left-right reversed).

Concave Mirrors

  • Concave mirrors can produce real or virtual images depending on the object's position relative to the focal point.
  • When the object is beyond the focal point, the mirror produces a real, inverted image.
  • When the object is between the focal point and the mirror, the mirror produces a virtual, upright, and magnified image.

Convex Mirrors

  • Convex mirrors always produce virtual, upright, and reduced images.
  • The image appears to be located behind the mirror.

Example: Image Formation by a Concave Mirror

An object is placed 30 cm from a concave mirror with a focal length of 10 cm. Determine the position and nature of the image.

Using the mirror equation: 1/f = 1/do + 1/di

1/10 cm = 1/30 cm + 1/di

1/di = 1/10 cm - 1/30 cm

1/di = 3/30 cm - 1/30 cm

1/di = 2/30 cm

di = 15 cm

The image is real, inverted, and located 15 cm in front of the mirror.

Questions for Students

  1. Define reflection and state the law of reflection.
  2. Describe the characteristics of images formed by plane mirrors.
  3. Explain the difference between real and virtual images formed by concave mirrors.
  4. What type of images do convex mirrors produce?
  5. Calculate the image position for an object placed in front of a concave mirror using the mirror equation.