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Study Notes: Reflection and Mirrors

When light rays shine on a smooth metal surface they are reflected in a way that produces a clear image. This is called specular reflection.

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You can see your own reflection in a mirror. The image that you see is very clear because the light rays bounce off the smooth mirror in a precise way.

When light shines on a rough surface the rays are reflected at many different angles and the image becomes distorted. This is called diffuse reflection.

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A coin is a good example of diffuse reflection since you cannot see your own image reflected from its surface.

Plane mirrors
A plane mirror is flat and forms images by specular reflection.

When light strikes a plane mirror we find that the light is always reflected at the same angle as it strikes the mirror.

That is, the angle of incidence (i) is equal to the angle of reflection (r). We measure these angles relative to an imaginary line called the normal. The normal is always at right angles (90°) to the surface where the light strikes it.

i = r
angle of incidence = angle of reflection

The diagram shows a light ray reflecting from a plane mirror.  The angle of incidence is equal to the angle of reflection r.

Changing the angle of incidence always changes the angle of reflection by the same amount.

Use your mouse to move the incident light ray below to view the change in the reflected ray.Skip flash movie

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Some other effects of reflection from a plane mirror are summarized below. If you look at an image of an object in a plane mirror you will notice that the image is:

  • upright - the image is the same way up as the object
  • laterally inverted - the left and right sides of the image are on opposite sides to the left and right sides of the object
  • the same size as the object, although it will appear to be the same distance behind the mirror as the object is in front.

Curved mirrors

A concave mirror can be used to focus a wide light beam into a narrow bright spot.

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A concave mirror is formed on the inside of the shape of a sphere.

Each light ray from a beam is reflected back through a single point called the focus before the light rays spread out again.

The light rays are said to converge (come together) at the focus.

The radius of the spherical curvature of the mirror will determine how far the focal point (focus) is from the surface. The focal point of a spherical mirror is half way between the mirror and the centre of the sphere.

A spherical mirror will not properly focus a wide beam of light. A mirror formed in the shape of a parabola has a sharper focus. It is almost spherical in shape but the edges are a little straighter to allow better focusing of wider beams of light.

A reflector does not have to be a polished metal surface like a mirror. All objects (except black) reflect light.

The smooth surface of a block of glass can reflect light. Sometimes it reflects all of the light (if the angle of incidence is large enough) and sometimes it will reflect some of the light and the rest passes through the surface.

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