Optics: Reflection and Refraction

optics clipart

You may have seen your reflection in a mirror or maybe a rainbow pattern in a puddle of oil and wondered what exactly you were seeing. With the help of optics this can all be explained. Optics is the branch of physics where the behaviour and properties of light are studied. To understand how you can see yourself in the mirror or how the rainbow pattern is achieved in a puddle of soap you first must understand how light works at different surfaces.

What is light?

The light you can see, more accurately defined as visible light, is just an electromagnetic radiation which is a wavelength which our eyes can detect. Although this may seem complicated, the key things to remember are its characteristics. The speed of light, approximately 3.0 m/s in a vacuum, is fixed. Nothing can move faster than the speed of light or even as fast. Also it acts as a particle and a wave, not just one of them. Visible light is emitted and absorbed in what are known as photons which are simply tiny “packets”.


When light strikes a surface, whether it be solid, liquid, or gas, it “bounces” off of the surface. This process is known as reflection. The angle at which the incident, or incoming, light striking the surface makes with the surface’s normal is known as the angle of incidence. The angle of incidence is the same as the angle of reflection, the angle at which is omitted light makes with the normal of the reflective surface. This occurs when light strike anything. It is this property that allows us to see rooms more clearly when we turn a light on.


When light strikes a surface, the light reflects off it it but if the light strikes a surface of a different refractive index, it also undergoes refraction. Refraction is the change in path of the light passing from one surface to another of a different refractive index. This is explained by Snell’s Law which says that the index of refraction, n, multiplied with the sin of the angle of incidence is equal to the refraction index of the transmitted medium multiplied by the sign of the angle of refraction. This is shown using the formula:


The reason why we can see our reflections is a fairly simple phenomena to explain. The light reflects off the smooth, glossy surface of the mirror off of us and into our eyes, allowing us to process it and recognize what we see. You can test this by looking at the mirrors and noting what you can see.
The reason why you can see a rainbow pattern in an oil spot is more involved than that of your reflection in the mirror. The important thing to know is that the reason you see a rainbow is because when the light travels from one medium to another the different wavelengths in that light (ie. The colour spectrum that makes up white light such as from the sun) get refracted which separates them enough so that they don’t all blend together allowing us to view all the different colours. This can also be shown by shining a white light through a prism or another example would be the rainbow you see after it rains.

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This article was written for you by Troy, one of the tutors with Test Prep Academy.