Principles behind solar cooking (1)

Refraction

Another property of light is that it refracts, which means that it bends when passing from one medium to another. Moreover, when light enters a more dense medium from one that is less dense, it bends towards a line normal to the boundary between the two media. This is illustrated in the figure below.

The greater the density difference between the two materials, the more the light bends. One place where this is used is in lenses for a variety of optical devices, such as microscopes, magnifying glasses, and glasses for correcting vision. An example of an image formed from a lens is shown below.

In this case the light from the object passes through the lens and is bent, forming an image on the other side of the lens which is magnified and inverted.

Many types of optical illusions are due, at least in part, to the refraction of light. One such example is the fact that if you look down while standing in a swimming pool, your feet appear closer to the surface than they actually are. This is due to the fact that light is bent when passing from water to air, as indicated below. Note that since air is less dense than water, the light bends away from the normal as it emerges.

The illusion comes from the fact that our eye doesn’t know that the light has been refracted when it comes from water into air, and so thinks that it has originated from a point closer to the surface.

No Author. (1999). Refraction. [on-line] Available from: http://theory.uwinnipeg.ca/mod_tech/node113.html (Last Accessed 1st March 2008)

Total internal reflection

An effect that combines both refraction and reflection is total internal reflection. Consider light coming from a dense medium like water into a less dense medium like air.

 

When the light coming from the water strikes the surface, part will be reflected and part will be refracted. Measured with respect to the normal line perpendicular to the surface, the reflected light comes off at an angle equal to that at which it entered at, while that for the refracted light is larger than the incident angle. In fact the greater the incident angle, the more the refracted light bends away from the normal. Thus, increasing the angle of incidence from path “1” to “2” will eventually reach a point where the refracted angle is 90^o, at which point the light appears to emerge along the surface between the water and air. If the angle of incidence is increased further, the refracted light cannot leave the water. It gets completely reflected. The interesting thing about total internal reflection is that it really is total. That is 100% of the light gets reflected back into the more dense medium, as long as the angle at which it is incident to the surface is large enough.

Fiber optics uses this property of light to keep light beams focussed without significant loss.

The light enters the glass cable, and as long as the bending is not too sudden, will be totally internally reflected when it hits the sides, and thus is guided along the cable. This is used in telephone and cable TV cables to carry the signals. Light as an information carrier is much faster and more efficient than electrons in an electric current. Also, since light rays don’t interact with each other (whereas electrons interact via their electric charge), it is possible to pack a large number of different light signals into the same fibre optics cable without distortion. You are probably most familiar with fibre optics cables in novelty items consisting of thin, multi-coloured strands of glass which carry light beams.

No Author (1999). Total Internal Reflection. [on-line]. Available from: http://theory.uwinnipeg.ca/mod_tech/node114.html (Last accessed 1st March 2008)