REFRACTION

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PhET Refraction Simulation

Advice from Esther, 9 August 2020

The PhET Simulation is a very nifty device that is used to measure the angle of refraction or reflection from the angle of incidence.

To begin, simply press the intro button. Then, click the red button on the laser. You should be able to see a red line which represents a ray of light. You could move the laser around in a 90 degree arc to either increase or decrease the size of the angle of incidence. (For your information, an angle of incidence refers to the angle which is made from the ray of incidence with an imaginary line which is perpendicular to the surface at its point of incidence or at the point where the ray hits another material which reflects it.) There would be a dotted line at the center of the simulation, that's the imaginary perpendicular line. Now you could measure the angles made by the rays using a yellow protractor which is located at the bottom left of your simulation. Just drag to move it around. To the right, you will see two grey rectangles which, in default, would be set to air on the 1st rectangle, and water on the 2nd. These two rectangles will determine what atmosphere/medium your ray is in. The 1st grey rectangle corresponds to the upper half of the simulation while the 2nd grey rectangle corresponds to the bottom half of the simulation. There are 3 main ways to change the mediums of both these sides: First, is by changing the index of refraction (the ratio of speed of light in vacuum to the speed of light in the specific medium) numerically, second, is using the small blue rectangle to change the medium between air, water or glass. Or, you could just simply press the arrow key located to the right of the bolded word 'Material' and it will show you an assortment of mediums to change it into.

In order to use this simulation, you will need to determine what material your ray of incidence will have to pass or reflect from. Let's say you have chosen air and water and you have placed your laser 45 degrees to the point of incidence. There will be two rays that emerge from your incidence ray to the right of the dotted line. The ray which bounces off of the second medium is your reflected ray, while the one which penetrates the second medium which undergoes a change in direction is your refracted ray. Take note that no reflection or refraction would happen if both your mediums are the same or both the medium's refractive index is equal, for e.g. air and air. You might notice that if you put your laser directly above the dotted line perpendicular to the second medium's surface, the light will not reflect nor refract. Because if the angle of incidence is zero then it means the light ray is travelling perpendicular to the medium. Hence there would be no refraction and the angle of refraction would also be zero since the ray enters the second medium in the same direction but with different speed. And also if a straight line is formed at the left of the dotted line, it means you have found the medium's critical angle. Again, place your protractor on the horizontal line separating your two mediums and measure the angles you have made. That basically sums up the use of the Phet Simulation for the bending of light or also called as 'Refraction.'

"A Year 9 student was curious about the behavior of light. With a high expectation to enhance her knowledge, she investigated the critical angle with her favorite Physics teacher. She had a light source and water. She projected the source, in water, at different angle of incidence (see the gif file) and found the following:

• She observed that when the light ray was directed through the midpoint that is perpendicular to the flat surface of the water, the light ray passed straight through.

• When the light ray was directed at that midpoint at a certain angle of incidence, it bent away from the normal upon emerging from the water.

She thought of this as caused by light slowed down when traveling from an optically denser medium (water) to an optically less dense medium (air). She also observed that a small amount of light is reflected off the surface of the water. At the last, when she increases the angle of incidence, the refracted ray bends further away from the normal until the angle of refraction becomes 90˚.

Her Physics teacher told her that the angle of incidence that makes the angle of refraction equal to 90˚is given a special name, critical angle. The teacher, on the side of him, continued, “I will teach you more about critical angle and the consequence of having an incidence angle greater than the critical angle.”

The Year 9 student stared at him with a lot of curiosity, "Note down the angles and determine the value of the critical angle. When done, take the reciprocal of the sine of the critical angle c. The value you'd get is called refractive index, with symbol n. This n indicates the optical density of a medium."

sin c = 1 / n

After all of the time, in this experiment, she got perfect idea about the Total Internal Reflection, that is, the scenario in which light would not pass through a denser medium. Instead, it is trapped inside that medium, bouncing, ,and following the Law of Reflection. With that, she wrote on his blue notebook a summary of what she learned.

Here's what was written on the notebook.

“In summary, when light travels from medium with high value of n to a medium with lower value of n, the angle of incidence, in most cases, is lesser than the angle of refraction. When the angle of refraction reaches 90˚, then the angle of incidence measured is called critical angle. If the angle of incidence is increased even more than critical angle, there will be no more refraction or bending of light as it goes out from one medium to another. It is going to be a TOTAL INTERNAL REFLECTION."