# Light - Kent School District

Light Big Idea: Electromagnetic Radiation, which includes light, is a form of radiant energy possessing properties of both waves and zero-mass particles called photons. Photons vary in their energy, which causes them to vary in their frequencies and wavelengths as well. EM radiation can be bent (refracted) or reflected by certain materials, allowing us to manipulate how it travels and what kinds of images it produces. Topic 7.1: Light basics and the EM spectrum Topic 7.2: Refraction Topic 7.3: Reflection and Polarization Topic 7.4: Diffraction and Wave-Particle Duality Learning Goal: You will be able to describe how light behaves when it is reflected off of mirrors

of different shapes. Success Criteria: You will know you have met the learning goal when you can truthfully say: 1. I can describe what light is and how it is created. 2. I can describe how diffraction produces wave interference patterns. Image(s) from Bing Success Criteria 1: I can describe what light is and how it is created. Weve all learned a thing or two about light and the electromagnetic spectrum before. From one point of view, it is a form of energy propagated as waves that are generated from electrons changing energy levels. When an electron changes back and forth between energy levels, it causes the electric field produced by the electron to oscillate. The amplitude of the wave determines the brightness and the frequency determines the color or type of radiation.

Image(s) from Bing Success Criteria 1: I can describe what light is and how it is created. From another point of view, light is not a wave, but a zero-mass particle. When an object is heated, it emits some of the kinetic energy it absorbs as photons. A photon is a packet of energy (a quantum of energy) that is created when an electron jumps from an excited state to a lower energy state. This light can then be absorbed by electrons in other atoms, which causes those electrons to jump to higher energy levels, which can then emit light of their own. This basic process can be summarized as:

Heat or light absorbed Light emitted Light absorbed Energy reemitted as light or remains in atom as heat Image(s) from Bing Success Criteria 1: I can describe what light is and how it is created. So which is it? Is light a wave phenomenon like sound or is it a collection of particles being emitted by atoms? Our best explanation is that it is both. This is called wave-particle duality, and has no good analogy with which we can compare it to. Image(s) from Bing Success Criteria 1: I can describe what light is and how it is created.

Task 7.4.1 (4 points): Watch these videos about wave-particle duality: https://www.youtube.com/watch?v=h1tflE-L 2Dc https://www.youtube.com/watch?v=v-1zjdUT u0o a) Give one piece of evidence that light is a wave and explain why it supports the wave theory of light. b) Give one piece of evidence that light is a particle and explain why it supports the particle theory of light.

Image(s) from Bing Success Criteria 2: I can describe how diffraction produces wave interference patterns. Another optical phenomenon that involves the bending of light is diffraction. Diffraction is the bending of a wave when it passes by the edge of a surface. Different wavelengths are bent to differing degrees, with shorter wavelengths being bent more than longer wavelengths. Image(s) from Bing Success Criteria 2: I can describe how diffraction produces wave interference patterns.

Why this happens is somewhat complicated. From the particle perspective, think of what would happen if a metal ball were to roll passed a magnet: It would bend the path of the ball. Since light is an electromagnetic wave, when it passes by the edge of something, the electromagnetic fields of the atoms in the material alter the path of the light. The longer the wavelength, the more it is bent. Thus, red light is bent more than blue light, and white light is split

into its constituent rainbow of colors. From the wave perspective, it comes down to interference. Locations where wavelengths of red light are in phase appear red. Locations where blue light waves are in phase appear blue. The Image(s) from Bing Success Criteria 2: I can describe how diffraction produces wave interference patterns. Task 7.4.2: Say which of each pair

would be diffracted at a greater angle. Light through a narrow slit Light through a wide slit Blue light Red light A 200 lines/mm diffraction grating A 300 lines/mm diffraction grating 450 nm light 600 nm light Image(s) from Bing Success Criteria 2: I can describe

how diffraction produces wave interference patterns. Task 7.4.3 (4 points): a) Explain why the wave view of light explains diffraction. Include a drawing along with your explanation. b) How is a diffraction grating used to analyze which elements are present in a light sources (this may require some internet research).

Image(s) from Bing Task 7.4.4 (4 points): Write at least 8 things you learned in this topic (1/2 point each). If you do this in your notebook, please do it in list form rather than paragraph form. Image(s) from Bing

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