Question 2
The outline of a triangular prism ABC is traced on a drawing sheet. The prism is removed and a point of incidence N is marked on AC. A straight line, MN, making a glancing angle α with AC, is produced to the point O. Two pins are fixed at points P1 and P2 on MN. The prismis replaced and looking through the side AB, two other pins are fixed at points P3 and P4such that they appear to be in a straight line with the images of the pins at P1 and P2. The prism and the pins are removed. A straight line VT is drawn to joint P3 and P4 and further produced to meet MO at W. The angles α, βand γ are measured.
Fig. 2 (a,b,c,d,e )show various positions P1, P2 and their corresponding P3 and P4 with respect to the prism.
- Join P1and, P2with the straight line MO. also joinP3 and P4 with a straight line to meet MO at W.
- Measure the angles α, β and γ
- .Evaluate 0 – (90-α), ϕ - (90 –β) and U – (0 - ϕ).
- Tabulate the results
- Plot a graph with γ on the vertical axis and U on the horizontal axis.
- Determine the slope, s of the graph.
- State two precautions that are necessary when performing this experiment.
(b) (i) Draw a diagram to show how a right –angles isosceles triangular prism is used to change
the direction of a ray of light by 1800.
(ii) A prism with a refractive index of 1.75.
Calculate the angle of minimum deviation the prism can produce.
Observation
Part (a) Candidates’ responded poorly in this part as many candidates could not measure
correctly the angle
Part (b) Candidates’ responded poorly in this part as many candidates could not give the correct diagram.
The expected response is as stated below:
- Five complete traces showing at least MNO, VTW, angles b, a and γ.
- Five values of b correctly measured and recorded within tolerance of ±1 degree.
- Five values of a correctly measured and recorded within tolerance of ±1 degree.
- Five values of g correctly measured and recorded within tolerance of ±1 degree.
- Five values of q = (90 - a) correctly evaluated and recorded.
- Five values of f = (90 - b) correctly evaluated and recorded.
- Five values of U = (q - f) correctly evaluated and recorded.
- Composite table showing at least b, a, g, q, f and U.
- Plot a graph using reasonable scales.
- Draw line of best it.
- Determine the slope of the graph.
- State two of the following precautions.
e.g.
- Clean refracting surfaces of prism
- Avoid parallax error in reading protractor
- Sharp pencil / Neat traces
- Separate pins about 4 cm
- Fix pins vertically/erect
- Repeat readings stated
