It can be tricky to wrap one’s head around how clear objects can be visible but also transparent. As you may recall from your O-level physics tuition classes, four things take place once light hits an object:
1. Diffuse Reflection
Diffuse reflection is the type of reflection wherein light gets reflected by irregular surfaces or rough surfaces. Since the angle of incident is variable, the light then reflects in different directions instead of staying parallel.
2. Specular Reflection
An example of specular reflection is the kind of reflection that takes place in mirrors. The mirror’s properties allow for parallel rays of light that strike its surface to stay parallel after being reflected.
3. Refraction
A good example of refraction is a clear drinking glass wherein light travels straight through it, but its direction bends upon entering and leaving the object.
4. Absorption
Absorption is when light gets absorbed by an object and converted into heat energy.
Everything we see interacts with light in all these four ways. Take the hood of a red supercar, for example, which reflects light specularly, diffusely, absorbed, and refracted. This leads us to see all sorts of things like glare spots, reflections, and the car’s red colour, which is the only wavelength that reaches our eyes since the yellow, orange, green, blue, and violet light are all absorbed. Had these not been absorbed, the car would appear white instead of red.
Moreover, every material typically has one dominant way of interacting with light such that the other ways become so insignificant that they can be ignored. An example is water, which, despite absorbing red light and reflecting some light, is mainly a clear material because transmission/refraction dominates over the others.
The most interesting thing about the four interactions above is how they alter light. Our brains can detect this alteration of light, which grants information that helps us deduce an object’s shape and presence. Hence, we can perceive clear objects because we see the light bending as it passes through them. Let us return to the drinking glass example. If you take one from your kitchen and look closely at it, you will see a distorted version of whatever lies behind the cup. Refraction is the dominant interaction at work here, and our brains are smart enough to deduce the shape of the glass simply by seeing how the background gets distorted.
If that is the case, that would mean clear objects can become truly invisible if their refraction properties can be cancelled. One such way to do so is by shaping a clear material into a smooth and flat shape with parallel surfaces. Once light enters this material, it still bends, but this time it bends by the same amount upon leaving out the other side. The resulting image from the other side will thus no longer be distorted, making the flat and clear material effectively invisible. As you can probably tell by now, this is essentially the principle behind windows. That being said, windows are not entirely invisible because they still reflect some light that can be seen under certain conditions.
Conclusion
We encounter plenty of interesting physics phenomena in our everyday lives that are worth taking a closer look at, such as how clear objects can be transparent and visible at the same time. Hopefully, you found the contents above interesting and helped you better understand how light and objects interact in our world.
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