Unit Conclusion

Unit C Conclusion

In Module 5 you learned about the roles that electricity and magnetism play in the production of EMR as you explored many optical systems. First, you were introduced to the electromagnetic spectrum by observing a rainbow, a natural example of the visible light spectrum. Visible light represents only a small portion of the entire electromagnetic radiation spectrum, which is organized by wavelength and frequency.

Next, you investigated significant scientific discoveries in the field of EMR. Maxwell’s electromagnetic theory predicted that perpendicular, oscillating, electric and magnetic fields propagated in the form of an electromagnetic wave. He also concluded that all electromagnetic waves travel through a vacuum at a common speed and obey the universal wave equation. This was later proven by Hertz using a spark gap and observing the resulting spark across the gap in a receiving antenna wire.

You then learned how to measure the speed of light in a few different contexts. The ability to measure the speed of light led to the discovery of precise technologies for measuring both terrestrial and astronomical distances.

After exploring the basic properties of EMR, you learned that ray diagrams can identify the path of light based on the law of reflection and can be used to predict the position and characteristics of images in both plane (flat) and curved mirrors.

You then investigated how many modern technologies, such as digital light processing, telescopes, and satellite dishes, are all excellent applications of the law of reflection.

Next, you learned about Snell’s Law as it applies to refraction, which is a change in the direction of a light caused by a change in its speed as it passes at an angle from one medium to the next. You also investigated the index of refraction and the critical angle, both of which are important properties of optical systems.

You also observed dispersion as the refraction of white light as it enters and exits a prism or a water droplet and produces a spectrum of colours quite similar to a natural rainbow. You concluded that it supports the wave model of light.

Next, you learned to distinguish between a converging lens and a diverging lens and how to use ray diagrams and the thin lens equation to identify image characteristics for both types of lens.

You concluded Module 5 by investigating that diffraction occurs when any wave front bends or changes direction as it passes by the sharp edge of an obstacle or through a small opening in the obstacle. You also discovered that if light is shone through two small openings (double slit), the diffracted waves form an interference pattern characterized by a repeating pattern of constructive and destructive interference, explained by assuming light has wave-like characteristics.

Lastly, you investigated the reason why diffraction gratings have a large number of equally spaced, parallel lines and that you can apply the same equations that describe a two slit experiment to gratings, since they only rely on the spacing between any two slits, or grooves, such as those making up the track on a CD or DVD.

All of the concepts learned thus far helped you understand that EMR exhibits both wave and particle characteristics.

In Module 6 you learned that a blackbody refers to a hypothetical object that absorbs all of the electromagnetic radiation that falls on it. You also investigated blackbody radiation curves and how they are used to observe the emitted energy distribution in terms of wavelength (or frequency) versus intensity for a blackbody at various temperatures. The curves can be explained by assuming that energy is emitted from hot objects in discrete bundles, or photons, each capable of transferring a minimum quantum of energy associated with its wavelength and frequency according to Planck’s formula. Applying the formula to the electromagnetic spectrum reveals the energy of the photons in all the general classifications of the spectrum.

It was at this point that you discovered that, for physics in general, the idea of the quantum marked the end of classical physics and the beginning of quantum physics.

Next, you learned about the photoelectric effect and how it is characterized by the following general observations, which could be demonstrated graphically and were explained by Einstein and verified in Millikan’s famous experiment:

• Photoelectrons are emitted instantly when EMR is incident on the surface.
  
  
• There is a threshold frequency of EMR required to cause the emission of photoelectrons. If the light shining on the photoelectric surface is below a certain frequency, there is no photoelectron emission, regardless of the intensity, or brightness, of the light. If the frequency is higher than the threshold frequency, the photoelectron current is proportional to the intensity.
  
  
• Each type of metal has its own characteristic threshold frequency. When the frequency of the incident EMR increases beyond the threshold frequency, the kinetic energy of the released photoelectrons shows a corresponding increase.
  
  
• If the light is at or above the threshold frequency, increasing the intensity will increase the number of photoelectrons, but not the energy of any individual photoelectron.

You then concluded that the photoelectric effect in combination with the wave-like characteristics of EMR from other experiments supported the notion that EMR has both particle- and wave-like characteristics, thereby promoting the notion of wave-particle duality.

Finally, you learned that the Compton Effect is an increase in wavelength, hence a decrease in energy, of an X-ray as a result of its interaction with matter, and that it obeyed the law of conservation of momentum. This was supporting evidence for the particle nature of EMR.

Throughout this unit, you built on your knowledge from the previous unit of how electrical and magnetic fields interact to produce EMR. By exploring optical systems, you discovered EMR exhibits both wave and particle characteristics. This led to the discovery of quantum physics, which is where you are headed in Unit D.