Physics Module 3 - Light and Sound

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  • Convex = mirror that bends out 
  • Concave = mirror that bends in
  • A normal image can be seen through a flat (plane) mirror.
  • An image seen in a mirror is due to reflection of light by the mirror.
  • The perependicular line to the mirror is called the normal.
  • The angle of incidence is the angle between the incident ray and the normal.
  • The angle of reflection is the angle between the reflected ray and the normal.
  • Angle of Incidence = Angle of Reflection
  • When you look at a mirror image, light rays that reflect off the mirror into your eye appear to come from the image. 
  • Virtual Image = an image, seen in a mirror (or lens) from which light rays appear to come after being reflected by the mirror (or being refracted by the lens).
  • Real Image = an image formed by a lens or concave mirror that can be projected onto a screen. 

Curved Mirrors

  • When you look closely : an enlarged image of your face.
  • Retreat and the image will look blurred.
  • Keep moving and the image will appear upside down and very small.
  • For a distant point object : the light rays are effectively parallel when they reach the mirror. Then they are focused to the principal focus (focal point), F, of the mirror. A real image is made here.
  • The distance from the mirror to the focal point is the focal length.
  • For an object beyond the focal point : an inverted real image is formed by the reflected rays. The position and size of the image depends on the distance from the object to the mirror. 
  • Magnification of the image : image height / object height.
  • For an object between the focal point and the mirror : the reflected rays form an upright visual image of the object. The image is magnified behind the mirror.

Convex Mirror

  • We use them as rear-view mirrors in cars. The driver has a wider view than a plane mirror. The image of the object viewed in a convex mirror is virtual and smaller than the object. 


  • Refraction = change of direction in which light is passing through. 
  • In refraction, the light ray :
  • - bends towards the normal when it travels from air into glass. the angle of refraction is smaller than the angle of incidence. 
  • - bends away from the normal when it travels from glass into air. The angle of refraction is greater than the angle of incidence.

Explaining Refraction

  • It is a property of all forms of waves including light and sound.
  • A glass plate is submerged in a ripple tank. The water above the glass plate is shallower that the water in the rest of the tank. The waves are slower than in deep water. 
  • They change speed when they cross the boundary, they change direction "
  • - toward the normal when they cross from deep -> shallow + slow down
  • - away from the normal when they cross from shallow-> deep…


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