Introductory physics: reflection

As we saw in a previous post, visible light is simply one portion of the electromagnetic spectrum i.e. visible light consists of electromagnetic waves of a certain frequency travelling at a speed of 3 x 10⁸ m/s (recall also that light can exhibit properties of both waves and particles, a property referred to as quantum wave–particle duality.)

The macroscopic properties of light had been studied for many years before its quantum properties were known. Such properties include transmission, reflection and refraction; the study of these phenomena is known as geometrical optics.

For example, it was realised centuries ago that light travels in straight lines (unlike sound): this can be demonstrated by placing a few pieces of cardboard with pinholes in their centres in a line. On placing a light source in front of A, the light will only be transmitted if the three pinholes are in a straight line.

The light can be seen by the observer if and only if the holes are in a straight line

Using one pinhole, one can form an image of a distant object as shown below: this is the basis of the famous camera obscura.

Rays of light can be convergent, divergent, or parallel. Rays emerging from a source diverge (think of a child’s drawing of the sun); on the other hand, rays arriving at an observer from a distance arrive parallel. Most useful of all, it was soon realised that a good image of an object could be got by causing incoming rays to converge using optical instruments – more on this later.

Reflection

When light falls on a smooth highly polished surface it is reflected i.e. turned back on its path.   A piece of polished metal, or indeed any shiny object makes a good reflector. [One reflecting material that is very much in the news at the moment is ice. The arctic is currently experiencing a global warming more pronounced than anywhere else in the world; this is thought to be caused by the fact that, as the polar ice cap gradually melts to water, it causes a reduction in the reflection of sunlight (water does not relect heat and light very well). This in turn causes further warming, an effect known as a positive feedback loop].

In reflection, a ray of light emerges at the same angle it went in (technically we say the angle of incidence equals the angle of reflection, where both angles are measured relative to the normal to the surface at the point of contact); this makes reflection images rather easy to draw (see below).

Plane Mirror

Glass mirrors have a thin layer of silvering deposited on the back of the glass which is protected.   An IMAGE is produced in the mirror.  The location of the image is got by simply the intersection of the reflected rays. A few trials soon show that the image in a plane mirror is always

– the same size as the object and the same way up

– as far behind the mirror as the object is in front

– laterally inverted

– virtual

Virtual images are images which are formed in locations where light does not actually reach. Light does not actually pass through the location on the other side of the mirror; it only appears to an observer as though the light is coming from this location. (The opposite is a real image; a real image can be focused on a screen, whereas a virtual image can not). In the case of the plane mirror the image is virtual because the rays APPEAR to be diverging from a point behind the mirror.

The reflected rays form a diverging beam which APPEAR to come from A’