Reflection of Light (CIE IGCSE Physics)

• Angles are measured between the wave direction (ray) and a line at 90 degrees to the boundary
  • The angle of the wave approaching the boundary is called the angle of incidence (i)
  • The angle of the wave leaving the boundary is called the angle of reflection (r)

• The line at right angles (90°) to the boundary is known as the normal

• When drawing a ray diagram an arrow is used to show the direction the wave is travelling
  • An incident ray has an arrow pointing towards the boundary
  • A reflected ray has an arrow pointing away from the boundary

• The angles of incidence and reflection are usually labelled i and r respectively

A ray diagram for light reflecting at a boundary, showing the normal, angle of incidence and angle of reflection

• The law of reflection states that these angles are the same:

Angle of incidence (i) = Angle of reflection (r)

Reflection of a wave at a boundary, i = r

• When an object is placed in front of a mirror, an image of that object can be seen in the mirror
• The image will be:
  • The same size as the object
  • The same distance behind the mirror as the object is in front of it
  • Virtual

• The formation of this image can be understood by drawing a ray diagram

Diagram showing the formation of an image in a mirror by the reflection of light

• Light from the object hits the mirror, reflecting from it (i=r)
• To an observer, the reflected ray appears to have come from the right-hand side of the mirror
• The reflected ray can be traced back in this directions, forming a virtual ray

• This can be repeated for another ray travelling in a slightly different direction
• An image of the object will appear where these two virtual rays cross
• The type of image formed in the mirror is called a virtual image
• A virtual image is formed by the divergence of rays from the image, and cannot be projected onto a piece of paper (because the rays don’t actually go through the image)