### Question 2B

*Using the diagram above as a guide: *

(i) Trace the outline **ABC** of the equilateral triangular glass

prism provided.

(ii) Remove the prism. Draw a line **MN** such that it makes

an angle i = 5° with the normal at **N** on side **AB** of the

outline.

(iii) Fix **two** pins at **P**I and **P**2 on **MN**. Replace the prism

on its outline.

(iv) Looking through the face **BC** of the prism, fix one pin

at P3 and another at P4 such that they are in a straight

line with the images of the pins at **PI** and **P **2'

(v) Remove the prism and the pins. Draw a line to join **P**4

and **P3**. Produce line **P4P**3 to meet the line BC of the

outline at **Q** and line **MN** produced at **P**.

### Observation

Part (a). This question was not popular among the candidates. Candidates were challenged in making traces as requires. They also exhibited poor knowledge in measuring angles **θ, ** due to

poor handling of protractor but they were able to evaluate **θ** . Some candidates did not attach their traces while some had traces without pin marks indicating that the experiment was not actually performed. However, they were able to plot the graph and determine the slope.

Part (b). This part were not satisfactorily answered by majority of the respondent.

Candidates were expected to

- Draw five complete traces showing at least incident ray, emergent ray,

from face **BC**, the normal at **N** and **Q** and the intersection of the incident

and emergent rays)

- Measure and record five values of measured and recorded in degrees and in trend.

Trend: as **i **increases, **e** increases.

- Measure and record five values of **θ **to at least 1 d.p in degrees and in trend.

Trend: as **i **increases, ** ** decreases.

- Evaluated five values of **= i + e**

- List **i****. **in a** **composite table

- Distinguish both axes

- Plot graph using appropriate scales

- Draw line of best fit

- Evaluate slope and intercept

- State any two of the following precautions.

Ensured pins were vertical / erect

Sharp pencil/neat traces (seen from traces)

Reasonable spacing of pins (about 4cm apart)

Avoided parallax error in reading protractor

Repeated readings shown on the table

The expected answers for part (b) are:

b(i) Whenever light travels from air to glass, the ratio of the sine of the angle of incidence to

the sine of the angle of refraction is 1.5[2]

**OR**

** **Velocity of light in air is 1.5 times greater than the velocity of light in glass.

**OR**

n = = 1.5

(ii) n = refractive index

n =

** OR**

C = sin-1)

= sin-1

= 37.3o