We were doing a refraction lab where you passed a lazer beam through a water in a semi-circular dish. I was wondering why we used a semi-circular shape?
Because the school con not afford a full circle
The laser beam probably hit the middle of the plane surface and refracted and went on to hit the semicircular surface. It would then hit the semicircular surface normally (perpendicular), and not be refracted again as it passes out of the dish. In this manner, it is easier to measure the refraction angle at the plane surface. If it went through two plane and parallel surfaces, the beam would emerge displaced but parallel to the incoming beam.
i still don't really understand why you use it though
Total internal reflection can be demonstrated using a semi-circular glass block. A "ray box" shines a narrow beam of light (a "ray") onto the glass. The semi-circular shape ensures that a ray pointing towards the center of the flat face will hit the surface at right angles by preventing refraction at the air/glass boundary. At the glass/air boundary what happens will depend on the angle.
Thanks :)
It isn't important as such to use a semi-circular block .. it's a matter of convenience ..
When you shine your light-ray towards the center of curvature it passes through the curved side at 90º to the medium (because the ray is running along a radial line and meeting a circumference).
Now that's convenient because these rays aren't refracted .. so you can make them follow predetermined angles that won't change as they pass into the block .. and only have to measure the angle of refraction for the rays emerging out of the block (no internal angle within the block has to be found.
So that you can eliminate refraction by the semi-circular surface.
If the initial refraction is in the center of the semi-circle, it will exit normal to the semi-circular surfact, no matter what the initial angle of refraction was.