nature of reflection / from curved surfaces

Nature of Reflection

Reflection is the abrupt change in direction of a wave front at an interface between two dissimilar media so that the wave front returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves.

When parallel light rays strike a smooth, plane surface the reflected rays are parallel to each other. This is known as specular reflection. Reflection off an irregular surface is known as diffuse reflection. In this case, incident light is reflected at a variety of different angles.

Let us now consider a single light ray incident upon a shiny surface (such as a mirror). This light ray is known as the incident ray. The angle of the incident ray will equal the angle of the reflected ray.

The angle of incidence θ_i and angle of reflection θ_r are both taken with respect to the normal. The normal is an imaginary line perpendicular to the surface at the point of incidence.

The diagram below shows this.

Thus, the law of reflection for light incident upon a shiny surface is that the angle of incidence is equal to the angle of reflection. That is:

θ_i = θ_r

Where:

θ_i = = the angle of incidence
θ_r = the angle of reflection

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Reflection from Curved Surfaces

Surfaces which curve inwards (like a cave) are known as concave surfaces. Surfaces which curve outwards are called convex surfaces. The focus of a curved surface is the point on the principle axis, midway between the curve and its centre of curvature.

When electromagnetic rays hit a concave surface they are reflected so that they pass through the focus of the curve. The ray travelling along the principle axis is reflected straight back (it hits the curved surface at 90°). Any reflected ray which then hits the surface will be reflected again to travel parallel to the principle axis. It is this property of concave surfaces that makes them useful for directing light rays ahead in torches and car headlights.

Concave surfaces are converging surfaces as they cause the rays to reflect and move closer together until they pass through the focus. It is this property of a concave surface which we use in solar cookers (we place the meat on a skewer at the focus) and in collecting data from radio and microwave signals from space (the receiver is suspended at the focus of the collecting dish).

Convex surfaces reflect rays so that their backwards extension passes through the focus of the curve. Convex surfaces are called diverging surfaces as they reflect the rays so that they travel further apart. This makes them useful in rear vision mirrors or driveway mirrors to give drivers and oncoming traffic a wider view of the area behind them or around a corner respectively.

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download activity 8.2.3 - reflection

download answers to activity 8.2.3 - reflection

Syllabus References

These references relate to the content covered on this page and can be found in Section 8.2.4 of the syllabus.

4. Many communication technologies use applications of reflection and refraction of electromagnetic waves.

Students:

• describe and apply the law of reflection and explain the effect of reflection from a plane surface on waves.

• describe ways in which applications of reflection of light, radio waves and microwaves have assisted in information transfer.

• describe one application of reflection for each of the following:
  
  – plane surfaces.
  – concave surfaces.
  – convex surfaces.
  – radio waves being reflected by the ionosphere.

Students learn to:

• perform first-hand investigations and gather information to observe the path of light rays and construct diagrams indicating both the direction of travel of the light rays and a wave front

• present information using ray diagrams to show the path of waves reflected from:
  
  –   plane surfaces
  –   concave surfaces
  –   convex surface
  –   the ionosphere

Textbook References

Taken from:

Heffernan, D., Parker, A., Pinniger, G. & Harding, J. (2002) Physics Contexts 1, Pearson Education, Melbourne

• Section 4.1 on pp. 162 - 169