In this experiment you will investigate reflection and refraction of light. You are provided with: - a transparent block - an illuminated slit or ray box - a ruler - a protractor. (a) Fig. 1.1 is on page 3 of your question paper. On Fig. 1.1: - switch on the lamp or ray box and position the slit at the top of the page so that a single ray of light lines up with the line AA - place the block on Fig. 1.1 and line up the corner of the block with the corner at B so that the ray…

(a) See working · (b) Candidate-dependent reading; e.g. 4.5 cm · (c) 67 · (d) As i increases, d decreases

Physics 5054/32 — May/June 2021

Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme

questions

marks

120

minutes

Topics Experimental Contexts · Observations and Measurements · Analysis, Conclusions and Evaluation · Use of Techniques, Apparatus and Materials

Q15MExperimental ContextsObservations and MeasurementsAnalysis, Conclusions and EvaluationFree sample

In this experiment you will investigate reflection and refraction of light.

You are provided with:

a transparent block
an illuminated slit or ray box
a ruler
a protractor.

(a)

Fig. 1.1 is on page 3 of your question paper. On Fig. 1.1:

switch on the lamp or ray box and position the slit at the top of the page so that a single ray of light lines up with the line AA
place the block on Fig. 1.1 and line up the corner of the block with the corner at B so that the ray of light passes through the long edge of the block at the point labelled C
mark with the letter D the point where the refracted ray emerges from the other side of the block
mark with the letter E the point where this refracted ray passes through line FF
draw a line from point D to point E
mark with the letter G the point where the reflected ray from point C passes through line FF
draw a line from point C to point G.

Fig. 1.1

DifficultyMedium-Easy

Worked solution

Answer

Draw a solid line from C through the block to the point D where the refracted ray emerges from the opposite long edge. Extend this line to intersect line FF at point E. Draw a solid line from C to point G, where the reflected ray intersects line FF. Ensure both C and G lie on line FF, and that the lines are drawn straight from the marked points.

Final answer

See working

Detailed explanation

Walkthrough

The experiment investigates how light behaves when it strikes a boundary between two media (air and the transparent block). At point C, the incident ray splits into two: a reflected ray that bounces back into the air, and a refracted ray that passes through the block and emerges from the opposite side. The candidate must trace both paths. The refracted ray is extended past the block to meet line FF at E. The reflected ray is extended to meet line FF at G. Drawing straight lines from C to G and D to E completes the ray diagram.

Key Takeaways

Light undergoes both reflection and refraction at a transparent boundary.
The reflected ray stays in the original medium, while the refracted ray enters the second medium.
Extending the rays to a reference line allows quantitative comparison of their positions.

Common Mistakes

Forgetting to extend the refracted ray past the block to point E.
Drawing the reflected ray on the wrong side of the normal (it must be in the same medium as the incident ray, i.e., above the block surface).
Not drawing straight lines from the marked points.

Things to Be Careful About

The diagram must be drawn 'by eye' to the precision of the ray box and block. Small deviations are acceptable as long as the geometry is physically correct (angles of reflection and refraction are reasonable).

Techniques used

draw the reflected and refracted rays from a transparent blockmark the intersection points with a reference line

(b)

Measure and record the distance $d$ between point E and point G.

$d$ = ______ $\text{cm}$

DifficultyEasy

Worked solution

Answer

Using a ruler, measure the straight-line distance between point E and point G along line FF. Record the value to 1 decimal place.

$d = 4.5 \text{ cm}$ (example value; any correct measurement from the candidate's diagram is accepted)

Final answer

Candidate-dependent reading; e.g. 4.5 cm

Detailed explanation

Walkthrough

The distance $d$ is the separation between the emergent refracted ray (point E) and the reflected ray (point G) where they cross the reference line FF. The candidate places the ruler along FF and reads the distance between the two marks. Because this is a practical experiment, the exact value depends on the candidate's drawing and the specific block used.

Key Takeaways

Physical quantities in practical experiments must be recorded to the precision of the measuring instrument.
A ruler should be read to the nearest millimetre, with a trailing zero if necessary (e.g. 4.5 cm, not 4.50 cm unless the scale supports it).

Common Mistakes

Reading the wrong end of the ruler or not aligning the zero mark with point E.
Recording the answer without a unit (the unit 'cm' is printed in the question, but the value must match the scale).

Things to Be Careful About

Ensure the measurement is taken along the line FF, not a perpendicular distance between the two rays. Read to the precision of the ruler (typically 1 mm or 0.1 cm).

Techniques used

measure the distance between two marked points on the reference line

(c)

Remove the block.

On Fig. 1.1, draw the normal to the surface of the block at point C and label with an $i$ the angle of incidence of the ray of light with the block.

Measure and record the angle of incidence $i$ .

$i$ = ______ $^\circ$

DifficultyMedium-Easy

Worked solution

Working

Place the centre of the protractor at point C and its baseline along the surface of the transparent block. Draw a dashed line perpendicular to the block surface at C, extending on both sides of the interface. This is the normal. Measure the angle between the incident ray (from A to C) and the normal on the air side. Label this angle $i$ .

Answer

$i = 67^\circ$ (accepted range $64^\circ - 70^\circ$ )

Final answer

Detailed explanation

Walkthrough

The angle of incidence is defined as the angle between the incident ray and the normal to the surface at the point of incidence. To find it, the candidate must first draw the normal: a line perpendicular to the block's surface at point C, extending into both the air and the block. The protractor is then placed with its centre at C and its $0^\circ$ line along the normal (or along the surface, reading the $90^\circ$ mark). The angle $i$ is read between the normal and the incident ray AA. In the mark scheme's model diagram, this angle is $67^\circ$ .

Key Takeaways

The normal is always perpendicular to the boundary surface at the point of incidence.
The angle of incidence is always measured from the normal, not from the surface.
Angles in optics are typically measured to the nearest degree.

Common Mistakes

Measuring the angle between the incident ray and the surface instead of the normal (this would give $23^\circ$ ).
Drawing the normal at an angle to the surface.
Forgetting to label the angle $i$ .

Things to Be Careful About

The normal must be drawn on both sides of the interface (air and block) to clearly show the reference line. Allow a tolerance of $\pm 3^\circ$ to account for drawing imprecision. Ensure the protractor is centred exactly on C.

Techniques used

draw a normal to the reflecting surface at the point of incidencemeasure the angle of incidence with a protractor

(d)

Place the block again on page 3 with its corner lined up at corner B. Carefully rotate the long edge of the block about the point C and observe how this changes the position of the refracted and reflected rays that pass through FF. Switch off the lamp.

Describe the relationship between $i$ and $d$ .

DifficultyMedium-Easy

Worked solution

Answer

As the angle of incidence $i$ increases, the distance $d$ between E and G decreases.
(Or equivalently: as $i$ decreases, $d$ increases.)

Final answer

As i increases, d decreases

Detailed explanation

Walkthrough

When the block is rotated about point C, the angle of incidence $i$ changes. As $i$ increases, the reflected ray moves further away from the normal, but the refracted ray also changes direction. In this specific apparatus setup, increasing $i$ causes the emergent refracted ray (point E) to move closer to the reflected ray (point G) on line FF, reducing the distance $d$ . The candidate must describe this trend based on their observations.

Key Takeaways

Changing the angle of incidence alters both the reflected and refracted ray paths.
Experimental observations must be summarised as a clear trend or relationship between the variables.

Common Mistakes

Stating 'they are equal' or 'there is no relationship' without justification.
Confusing which variable is independent ( $i$ , changed by rotating the block) and which is dependent ( $d$ , measured as a result).

Things to Be Careful About

The answer must be a directional relationship ('as X increases, Y decreases'). Simply saying 'they are related' is not sufficient. Ensure the direction of the relationship matches the actual physics of the setup (in this case, $i$ and $d$ are inversely related).

Techniques used

describe the relationship between two variables from experimental observations

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