Reflection of Light

Specular Reflection

• Angle of incidence: the angle at which light hits a reflecting surface
• Angle of reflection: the angle at which light bounces off a reflecting surface
• Specular Reflection: the angle of incidence is equal to the angle of reflection
  • Seen on smooth, shiny, mirror-like surfaces
• If you want to measure these angles, imagine a perfectly straight line at a right angle of 90 deg to the reflective surface (this imaginary line is called ‘normal’).
• With a flat mirror, if you measure the angle of incidence and the angle of reflection against the normal, they are exactly the same.
• It is easy to show that the angle of reflection is the same as the angle of incidence.

Diffuse Reflection

• Diffuse reflection of light: results in light rays reflected at many angles, rather than at just one angle as in the case of specular reflection; angle of incidence is not equal to the angle of reflection
  • Seen on rough surfaces
• The reflected light from each individual incident ray follows the law of reflection.
• However, the roughness of the material means that each individual ray meets a surface that has a different orientation.
• The normal line at the point of incidence is different for different rays.
• Subsequently, when the individual rays reflect off the rough surface according to the law of reflection, they scatter in different directions.

Water

• Water is a reflective surface.
• When the water in a lake or sea is very still, the reflection of the landscape is perfect, because the reflecting surface is very flat.
• However, if there are ripples or waves in the water, the reflection becomes distorted.
• This is because the reflecting surface is no longer flat and may have humps and troughs caused by the wind.

Concave Mirrors

• Reflection of light on a concave mirror: when parallel light rays hit a concave mirror they reflect inwards towards a focal point.
• Each individual ray is still reflecting at the same angle as it hits that small part of the surface.
• Concave mirrors are used in certain types of astronomical telescopes called reflecting telescopes.
• The mirrors condense lots of light from faint sources in space onto a much smaller viewing area and allow the viewer to see far away objects and events in space that would be invisible to the naked eye. Also used for make-up mirrors.

Convex Mirrors

• Reflection of light on a convex mirror: when parallel light rays hit a convex mirror they reflect outwards and travel directly away from an imaginary focal point .
• Each individual ray is still reflecting at the same angle as it hits that small part of the surface.
• Parallel rays of light strike the convex mirror and are reflected outwards.
• If imaginary lines are traced back, they appear to come from a focal point behind the mirror.
• Convex mirrors are useful for shop security and rear-view mirrors on vehicles because they give a wider field of vision.