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 θi and angle of reflection θr 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:
θi = θr
θi = = the angle of incidence
θr = 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
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