3.2.3 Core Practical: Investigating Refraction · 3. Waves | Edexcel International GCSE Science Double Award 4SD0: Physics

Section 1 of 9Aim and apparatus

The practical investigates how the angle of refraction changes as the angle of incidence is varied for light passing through a rectangular glass or Perspex block.

Aim and apparatus

You need a ray box with a single slit or a laser, a power supply, a rectangular transparent block, a sheet of plain paper, a pencil, a ruler and a protractor.

If you use a laser, do not look into the beam and do not shine it at others.

Section 2 of 9Method with pins or a ray box

1

Place the block in the middle of the paper and draw round its outline with a sharp pencil.

2

Remove the block.

Draw a normal at 90° to the long side near the middle of the outline.
Mark and label a series of incident ray directions, with angles of 10°, 20°, 30°, 40°, 50°, 60° and 70° to the normal.

3

Replace the block. Direct a thin ray along the first incident line towards the point where the normal meets the block.

4

Mark with crosses the points where the ray enters the block and where it leaves it, and mark a point on the emergent ray.

5

Remove the block, join the entry and exit points with a straight line to show the path inside the block,

and measure the angle of refraction with the protractor from the normal inside the block.

6

Repeat for each angle of incidence and record the results in a table.

Section 3 of 9Results and graphs

A table should have columns for angle of incidence i, angle of refraction r, and, if needed, sin i and sin r.

1

Plot a graph of r against i; it is a curve that bends over because r increases more slowly than i.

2

Plotting sin i against sin r gives a straight line through the origin, so that sin i / sin r is a constant.

3

The gradient is the refractive index of the glass (see Snell's Law).

4

For a typical glass this is about 1.5, and for Perspex about 1.5 too.

The emergent ray is parallel to the incident ray, but displaced sideways, since the two faces of the block are parallel and the ray bends equal amounts in opposite directions. The thicker the block and the larger the angle of incidence, the greater the sideways shift.

Section 4 of 9Accuracy and safety

1

Use a thin, sharp ray so that the position of its edge is easy to mark.

2

Mark points as far apart as possible to draw the lines more accurately.

3

Read the protractor from eye level to avoid parallax, and measure to the nearest half degree.

4

Repeat each angle and take an average if the data are scattered.

5

The ray box lamp gets hot; do not touch it. Keep lasers away from eyes.

Section 5 of 9Common errors

Measuring angles from the surface rather than from the normal is the commonest error.

Common errors

Others include drawing the block outline too thickly,

which makes the entry and exit points uncertain, moving the block between marking and removing it,
- and measuring the angle at the wrong place inside the block.

The experiment confirms that light bends towards the normal on entering glass, and that the ray inside is a straight line.

Compare this with the more detailed analysis in the lesson on Investigating Snell's law.

Section 6 of 9Why the ray bends and what the pattern shows

The measurements can be interpreted using the idea that light slows down in glass.

Why the ray bends and what the pattern shows

At the air-glass surface, the ray bends towards the normal because the wavefronts slow down first on one side.

At the glass-air face on the way out, the ray speeds up and bends away from the normal by the same amount.

Since the two faces are parallel, the angle at which the ray leaves equals the angle at which it entered,

and the emergent ray is parallel to the incident ray.

You may also be asked to draw the path of a ray for a prism with non-parallel faces.

1

In that case the two bends add up rather than cancel, and the ray is deviated towards the thick end of the prism.

2

If white light is used, the red end is bent least and the violet end most, which makes a visible spectrum.

3

This is the basis of the colour dispersion seen with a glass prism.

4

Take care to describe the direction of every bend using "towards" or "away from the normal", never just "up" or "down".

Section 7 of 9Worked examples

In an experiment the following results are obtained for a glass block: i = 30°, r = 19°; i = 50°, r = 30°. Show that the refractive index is about 1.5 for both.
A student draws the outline of a block and the emergent ray is not parallel to the incident ray. Suggest two reasons.

Section 8 of 9Key terms

Term	Meaning
Ray box	Lamp producing a narrow ray of light
Normal	Reference line at 90° to the surface
Parallax	Apparent shift caused by viewing from the wrong position
Emergent ray	Ray leaving the block
Lateral shift	Sideways displacement of the ray through a block

Section 9 of 9Exam technique

1

Measuring angles from the block surface, not the normal.

2

Using one reading and drawing a conclusion – repeat the readings.

3

Saying the light bends away from the normal on entering the block.

4

Plotting r against i and calling the curve proportional.

5

Forgetting to mark the outline before moving the block.