Refraction and Reflection of Light E-practical
Introduction
When light passes from one medium to another, it is refracted – or bent – at the boundary between the two mediums. This can be easily observed by looking at a drinking straw in a glass of water. The straw looks to be bent at the contact surface between air and water. The amount by which the light is bent is different according to a property of the medium called the refractive index. The higher the refractive index, the more that light is bent. Diamond has a very high refractive index, and it is this property that allows many facets to be ground onto a diamond, making it sparkle much more than can be achieved with glass.
The exact law of refraction is called Snell’s law of refraction, after the Dutch astronomer Willebrord Snellius (1580–1626). The law gives the relationship as:
sin(i)/sin(r) = n1/n2
Where i is the incident angle, r is the refracted angle, n1 is the refractive index of the first medium, and n2 is the refractive index of the second medium.
Realistic experimental technique
E-practicals are 3d, highly realistic emulations based on a blend of theory and the actual practicals, giving an authentic experience of doing the actual practical in a laboratory. Students operate the apparatus, make measurements and obtain their own results. It is designed to reproduce the experience of performing the practical rather than simply demonstrating the underlying physics. Like a real experiment, it can be done badly. Careless setup or measurements can produce poor results, so students have to apply experimental technique and take care with their measurements.
Works directly in a browser
No installation is required. Windows, Mac and suitable mobile devices can run the WebGL e-practical.
See it in Action
This shows how the e-practical can be used. In particular it shows how the student or teacher interacts with the apparatus and how the interaction can be controlled using the mouse or the alternative on-screen controls.
What students do
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You can move by a restricted amount by using the cursor keys or the on-screen left joystick control.
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Each block can be selected by clicking on it, the block will then be moved to the graph paper immediately in front of the light box.
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The blocks, once in place, can be rotated either by using the mouse wheel, or by dragging when using the on-screen controls.
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You can position the protractor by dragging it; it can be rotated using the mouse wheel or the right-hand joystick when using the on-screen controls. You will probably find it much easier to measure the angles using an actual protractor held onto the screen and moved by hand.
For demonstrating that different substances bend light by different amounts, simply use the protractor to measure these angles.
To verify Snell’s law, proceed as follows.
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Set the incident angle for the light ray to 20 degrees. You can do this by having the glass block on graph paper and aligning the paper with the beam of light.
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Then, by putting the protractor up to the edge of the block as shown in the picture you can measure the incident angle by looking at the line (vertical in the picture) from the center of the protractor to its edge. The picture below shows this angle being set to 30 degrees; this is the incident angle, i.
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Use the protractor directly to measure the refracted angle.
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Repeat this for a range of angles noting the incident angle and the refracted angle (r) for each case.
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Create a table of your results and calculate the sin(i)/sin(r) value. Compare this with the known values for the refractive indices of the various transparent materials.
What you get
The application is supplied as a PowerPoint which provides background, a synopsis of what the student will learn, a description of the Physics involved, a description of the apparatus, the independent and dependent variables, the method, a link to the interactive online e-practical, analysis of the results and further discussion.
E-practical
Runs in a modern web browser. No software installation required. Recommended or desktops but will run successfully on tablets of medium to large size with a browser that supports WebGL.
School/Department licence £9.99
Valid for 1 year