What is reflection
We can see objects because light reflects off that object and into our eyes. But we didn’t always understand it this way.
Pythagoras (about 500 years BCE), best known for the theorem of the right-angled triangle, proposed that vision resulted from light rays emerging from a person’s eye and striking an object. Epicurus argued the opposite: Objects produce light rays, which then travel to the eye. Other Greek philosophers — most notably Euclid and Ptolemy — used ray diagrams quite successfully to show how light bounces off a smooth surface or changes direction as it passes from one transparent medium to another.
Epicurus was honing in on the correct idea, but it took until the 9th and 10th centuries before we started to get close to what really happens. Abu Ali Mohammed Ibn Al Hasn Ibn Al Haytham (Ibn al-Haytham), who lived in present-day Iraq between A.D. 965 and 1039, identified the optical components of the human eye and correctly described vision as a process involving light rays bouncing from an object to a person’s eye. A key difference with Ibn al Haytham’s conclusion was that he determined it by performing an actual scientific experiment where he shone two lanterns through two pin holes at different heights into a dark room. In the wall of the dark room he saw two light spots. When he removed one lantern, the light spot that corresponded to that lantern disappeared. This finding enabled him to conclude that rather than light emanating from the eye, it is reflected off objects in a straight line.
The way light reflects off a surface is predictable and follows the law of reflection. Essentially, if light hits a mirror at one angle it will be reflected from the mirror at the same angle. See Figure 1.
Or in more mathematical terms the angle of incidence (i) is equal to the angle of reflection (r). This is easily demonstrated with a laser light, a mirror and a protractor to measure angles. See Figure 1. where I is the incident (incoming) ray; R is the reflected ray; N is the normal line, which is an imaginary line that is perpendicular to the surface the light reflects off (a mirror in this case). The angle of incidence (i) is the angle between the incident ray and normal; the angle of reflection (r) is the angle between the reflected ray and normal.
Figure 1 Reflection of light off a mirror, with incident (I) and reflected (R) rays; and the angles of incidence (i) and reflection (r). i=r
To explore the nature of reflection and how it enables us to see, get students to do Activity 3, Light Box.
Pepper’s Ghost is another intriguing and visually appealing example of reflection. Students can examine the physics of reflection and how it is used to create haunted houses and theatrical effects to create virtual and ghostly images that appear to float in the air. See Activity 4, Reflection of Pepper’s Ghost. Check the video below of what one version of Pepper’s Ghost illusion looks from from your creation in Activity 4.
Absorption
When an object absorbs light it transforms a proportion of that light into heat energy. Depending on the material, the light may also get transformed and re-emitted as a form of bioluminescence or phosphorescence. See the following story on glow-in-the-dark wombats, platypuses and other animals.
For visible light, coloured objects will reflect the colour we see, but absorb all the other colours in the visible spectrum.
What is refraction?
While light travels in a straight line and passes from one medium into another, for example from air to water, it will change speed and change direction. This is called refraction. The speed of light will change depending on the medium it travels through. The constant (the c in E = mc2) is the speed of light in a vacuum.
Technically the speed of light does not change, because as it passes through the medium it interacts with the atoms in that different media, changing direction with each interaction, which affects the time it takes to pass through the particular media. See Figure 2.
To introduce students to the concept of refraction and perform a magic trick, check Activity 5, Appearing coin.
Make your own magnifying glass. The concept of refraction is employed to make magnifying glasses, microscopes, telescopes, cameras and corrective lenses for our glasses to help us see. You can make you own magnifying glass using a droplet of water. A water droplet has a spherical shape above the surface it is resting on that will act as a lens. This creative site has a easy make-at-home version of the water droplet magnifying glass.
Figure 2. As light passes from one medium to another it will change direction (refracts) and slow down, or more accurately take a longer route from its point of entry to the point of exit and therefore appear to have slowed down. In this case because water is denser than air it refracts toward the normal line. As it exits the water into the air it will refract away from the normal line.
Snell’s Law
Willebrord Snellius (1580–1626) is credited with working out the laws of refraction (Snell’s Law), although many philosophers and scientists as far back as Ptolemy had worked out that the amount light changed direction was determined by the medium it passed through, but they had not worked out the math to explain it.
Again, it was Ibn al-Haytham that did much of the groundwork on refraction. He explained how a lens could magnify objects and that this was caused by the light changing direction (refracting) as it entered the glass. Before Ibn al-Haytham, however, another Islamic scientist, Ibn Sahl, discovered how lenses ‘bend’ and focus light and it was this work that Willebrord Snellius based his laws on. What Snell’s Law demonstrates is that light will change direction though a media at a specific ratio relative to the angle it hits that media. Snell’s law will determine the direction of light rays through a media that will refract light and is the simple equation, n₁sin(θ₁) = n₂sin(θ₂), where n₁ and n2 are the angle of incidence and angle of refraction.
We have used this understanding of how much light will refract in any given substance, to invent microscopes, telescopes and magnifying glasses. We have learned how the eye works and can now design lenses to correct vision. We have even begun development of a bionic eye
And we understand why we get rainbows, one of nature’s most impressive sights. See Activity 6, How to find a rainbow, to give students practical experience at using Snell’s Law. As a bonus, you learn how to find a rainbow.
As with the law of reflection, to measure refraction, all angles are measured from an imaginary line (the normal) drawn at 90° to the surface of the two media.
The amount light will slow down and the degree it will change direction as it enters the different medium is determined by the refractive index of the medium it enters. Essentially the denser the medium, the greater the refractive index. For instance, glass is denser than water, and water is denser than air.
If light enters any medium that has a higher refractive index, (such as from air into glass) it slows down and changes direction towards the normal line. If light enters into a substance with a lower refractive index (such as from water into air) it changes direction away from the normal line.
Light does not bend, technically. You may see references to light bending when it refracts. This refers to light changing direction. Light does not actually bend like a hose or length of wire might. Light travels in a straight line.
For a deeper exploration of refraction, see Fermilab’s two videos here and here
Diffraction
Diffraction is light as a wave changing direction as it passes through a gap or around objects. Light waves appear to bend because of how the peaks and troughs of waves interact with each other once they go through a gap or around an object. When a wave goes around an object, however, it will split into two waves to pass around the object. As the two waves interact on the other side of the object they form what is known as a resultant wave. In the interaction of these two waves, where the peak of a wave overlaps with another peak they add themselves together to make a bigger wave (increase in amplitude). Where a peak meets a trough they will cancel each other out and you get a smaller or flatter wave (or decrease in amplitude). This results in the classic interference pattern that is indicative that light has a wave-like nature. The interference pattern will have bright blobs of light, which are where the peaks have overlapped. The dark blobs are where the peak and trough have cancelled each other out. This is also the pattern you would see if a wave passed through two gaps rather than one and is what is replicated in the classic double slit experiment. The waves passing through each gap would bend and then interfere with each other producing a similar interference pattern. The Exploratorium also has a home science version to demonstrate this.
For a neat in-depth demonstration and explanation of how light behaves when it goes around an object see this clip.
But why does the wave bend in the first place? This is where it gets a bit technical, but for any year 9-10 students who want to extend their thinking check Parth G’s video out on diffraction below – handy stuff.