Section 1 of 5Variables and Equipment
Variables and Equipment
Independent variable
- The shape of the block, such as a rectangular block, a semi-circular block or a triangular prism.
Dependent variable
- The direction in which the light is refracted, which is shown by the path of the ray through the block.
Control variables
- The width of the light beam and the frequency, or colour, of the light must be kept constant.
Equipment
- A ray box, Perspex blocks, a protractor, a ruler, a sharpened pencil and a sheet of paper.
Section 2 of 5Method
1
Place a block on a sheet of paper and trace around its outline carefully with a pencil.
2
Direct a narrow ray of light at a side face, and mark the incident ray, the point where it enters and the point where it exits.
3
Draw a dashed normal at each boundary point using a set square, so that the 90 degree line is accurate.
4
Remove the block, join the marked points with straight lines, then repeat for a new angle and for each block shape.
Section 3 of 5Evaluation and Safety
Systematic error
- Drawing the 90 degree normal by eye introduces a systematic error, so a set square should be used instead.
Random error
- Inaccurate marking of the ray points can be reduced by using a sharp pencil and marking the middle of the beam.
Protractor resolution
- A protractor reading to 1 degree limits how precisely the angles can be read, so a finer protractor is better.
Safety
- The ray box bulb can burn if touched, and looking directly into the light can damage the eyes.
Section 4 of 5Worked Example
Question Describe how to check that the angle of refraction readings for a rectangular block are reliable.
- Repeat the measurement at each angle of incidence with a fresh ray path, so that there are at least three readings for each angle.
- Check that the normal was drawn with a set square, so that every angle is measured from a correct line.
- Compare the repeat readings; if they differ widely, look for errors in marking the ray points.
Answer
- Consistent repeat readings drawn from an accurate normal show that the results are reliable;
- a wide spread points to random error in marking the ray.
Section 5 of 5Exam Checklist & Tips
3.2.3 – Key Points for the Exam
- Refraction The change in direction of light when it crosses a boundary between two transparent media.
- Normal A line drawn at 90 degrees to the boundary of a block, used to measure angles.
- Semi-circular block A Perspex block whose curved face lets light enter along a radius without bending.
- Angle of refraction The angle between the refracted ray inside the material and the normal.
- Resolution The smallest change in a quantity that a measuring instrument can detect.
Common Mistakes
- Using a different beam width for each block, which changes the path of the light and invalidates a fair comparison.
- Measuring the angles from the surface of the block rather than from the normal.
- Looking directly into the ray box light during the experiment.
- Drawing the normal by eye with a ruler instead of using a set square.