Reflection of Light (Cambridge (CIE) IGCSE Physics): Revision Note

Ray diagrams

• In optics, a normal line is drawn at right angles to the boundary between two media

• In reflection, angles are measured between the ray (showing the wave direction) and the normal line 

  • The angle between the incident ray and the normal is called the angle of incidence ($i$)

  • The angle between the reflected ray and the normal 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

The law of reflection

• The law of reflection states that the angles of incidence and reflection are equal:

$i=r$

• Where:

  • $i$ = angle of incidence in degrees (°)

  • $r$ = angle of reflection in degrees (°)

• This law of reflection is used in many devices such as:

  • mirrors

  • cameras

  • optical fibres

  • periscopes

Reflection in a plane mirror

• When an object is placed in front of a 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

  • virtual

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

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

• 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)