Reflection of Light (Cambridge International IGCSE Science)

The law of reflection

• In optics, a normal line is defined as: An imaginary line drawn at right angles to the boundary between two media

• A medium is a transparent material

• Media is more than one transparent material

• As shown in the diagram in reflection, angles are measured between the ray (showing the wave direction) and the normal line

  • The normal is an imaginary line drawn at a right angle 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)

• 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

Ray diagram of reflection

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

• For reflection, the law of reflection states that the angle of incidence is equal to the angle of reflection:

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

Reflection in a plane mirror

• When an object is placed in front of a vertical plane 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
  • Laterally inverted where the left and right sides of the image appear to be the opposite of those of the object

• A plane mirror defines a flat, smooth and polished surface

Exam Hint When drawing light waves being reflected take care to get the angle about right.

If they are slightly out it won’t be a problem, but if there is an obvious difference between the angle of incidence and the angle of reflection then you will probably lose a mark.

Formation of an optical image

Spec point — P4.2.1

Extended Tier Only

• When an object is placed in front of a plane mirror, a virtual image of that object can be seen in the mirror
• The formation of this virtual image can be understood by drawing a ray diagram

Ray diagram showing reflection in a plane mirror

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

• Each incident ray on the diagram above can be drawn following these steps:

  • Light from the object hits the mirror, reflecting from it (i=r)
  • To an observer, the reflected ray appears to have come from behind the mirror
  • The reflected ray can be traced back in this same direction behind the mirror, forming a virtual ray
  • This process is 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 because of the divergence of the rays from the image

  • It cannot be projected onto a piece of paper (because the rays don’t go through the image)

Video of Reflection