Investigating Refraction Through a Glass Block

Aim

To investigate how light bends when it travels through a rectangular glass block and to compare the angle of incidence with the angle of refraction.

Learning Outcome

• Define refraction of light.
• Identify the incident ray, refracted ray and emergent ray.
• Draw and label ray diagrams.
• Measure angles correctly using a protractor.
• Explain how light bends when moving between different media.
• Explain optical density.
• Observe bending towards and away from the normal.

Time Allocation

Approximately 45 to 60 minutes.

How Does It Work?

Refraction occurs when light changes direction as it passes from one medium into another.

This bending happens because light travels at different speeds in different substances.

Scientific Background

Light travels at different speeds in different materials. Air is less optically dense than glass.

When light travels from air into glass, the speed of light decreases and the ray bends towards the normal.

When light travels from glass back into air, the speed of light increases and the ray bends away from the normal.

In a rectangular glass block, the emergent ray is parallel to the incident ray.

Important Definitions

Term	Definition
Refraction	The bending of light when it moves from one medium to another.
Incident Ray	The incoming light ray.
Refracted Ray	The light ray inside the glass block.
Emergent Ray	The light ray leaving the glass block.
Normal	An imaginary line drawn at 90° to the surface.
Optical Density	A measure of how much a material slows down light.

Hypothesis

If light travels from air into glass, then the light ray will bend towards the normal.

Variables

Independent Variable

Angle of incidence.

Dependent Variable

Angle of refraction.

Control Variables

• Same glass block.
• Same ray box.
• Same paper.
• Same light source.
• Same method of measurement.

Apparatus

• Ray box or light box
• Single slit ray plate
• Rectangular glass block
• White paper
• Pencil
• Ruler
• Protractor
• Foam board or cork board
• Four optical pins
• Adhesive tape

Safety Precautions

Important Notes

Experimental Setup

Preparing the Glass Block Diagram

1. Place a sheet of white paper on the foam board or cork board.
2. Put the rectangular glass block in the centre of the paper.
3. Trace around the glass block using a sharp pencil.
4. Remove the glass block carefully.
5. Choose a point on one side of the rectangle.
6. Draw a normal line at 90° to the surface using a ruler and protractor.

Setting the Incident Ray

1. Use a protractor to measure an angle of incidence of 10°.
2. Draw the incident ray towards the glass block.
3. Repeat the investigation later using 20°, 30°, 40° and 50°.
4. Draw arrows showing the direction of light travel.

Method

1. Place the glass block back onto the traced outline.
2. Set up the ray box using the single slit ray plate.
3. Direct a thin ray of light exactly along the incident ray.
4. Mark the light ray close to where it enters and where it leaves the glass block.
5. Push two optical pins upright into the paper along the incident ray.
6. The pins should be at least 5 cm apart.
7. Look through the opposite side of the glass block.
8. Observe the refracted images of the pins inside the glass block.
9. Move your eye position until the pin images appear in one straight line.
10. Place two additional optical pins so that they line up with the images seen through the glass block.
11. Remove the glass block and all four pins carefully.
12. Join the pin holes using a ruler to draw the incident ray, refracted ray and emergent ray.
13. Extend the emergent ray until it reaches the edge of the glass block outline.
14. Draw the refracted ray inside the glass block.
15. Use a protractor to measure the angle of incidence and the angle of refraction.
16. Record the results in the table.
17. Repeat the experiment for different angles of incidence.

Results Table

Trial	Angle of Incidence (°)	Angle of Refraction (°)
1	10
2	20
3	30
4	40
5	50

Sample Results

Trial	Angle of Incidence (°)	Angle of Refraction (°)
1	10	7
2	20	13
3	30	18
4	40	25
5	50	31

What Learners Should Observe

• The light bends when entering the glass block.
• The light bends towards the normal inside the glass.
• The light bends away from the normal when leaving the glass.
• The refracted angle is smaller than the incident angle.
• The emergent ray is parallel to the incident ray.

Ray Diagram Requirements

• Incident ray
• Refracted ray
• Emergent ray
• Normal
• Angle of incidence
• Angle of refraction
• Direction of light travel
• Glass block outline

Analysis of Results

Compare the angle of incidence and the angle of refraction.

Learners should find that:

• The refracted angle is smaller than the incident angle.
• The light bends towards the normal when entering glass.
• The light bends away from the normal when leaving glass.
• The emergent ray is parallel to the incident ray.

This confirms refraction of light.

Accuracy and Reliability

Improving Accuracy

• Use a thin light ray.
• Draw sharp pencil lines.
• Measure carefully with a protractor.
• Keep the glass block fixed.
• Ensure pins stand upright.
• Work in a slightly dim room.

Improving Reliability

• Repeat the experiment.
• Use several incident angles.
• Compare repeated measurements.

Sources of Experimental Error

• Thick light rays.
• Moving the glass block.
• Incorrect protractor placement.
• Unclear ray markings.
• Crooked optical pins.
• Poor line drawing.
• Parallax error during measurement.

Conclusion

The investigation shows that light bends when travelling between air and glass.

The experiment confirms that light bends towards the normal when entering a denser medium, bends away from the normal when leaving a denser medium, and the emergent ray remains parallel to the incident ray.

This phenomenon is called refraction.

Questions for Learners

1. What is refraction?
2. Why does light bend when entering glass?
3. What is the normal?
4. Why are angles measured from the normal?
5. What happened to the light ray when it entered the glass?
6. What happened when the light left the glass?
7. What is optical density?
8. Which medium is optically denser, air or glass?
9. Why is the refracted angle smaller than the incident angle?
10. Why is the emergent ray parallel to the incident ray?

Common Mistakes

• Measuring angles from the glass surface instead of the normal.
• Drawing incorrect normals.
• Moving the glass block during the experiment.
• Using thick light rays.
• Crooked optical pins.
• Incorrect protractor placement.
• Untidy ray diagrams.

Teacher Notes

• Learners often confuse reflection and refraction.
• Emphasise that refraction involves bending through a material, not bouncing off a surface.
• Encourage neat ray tracing and accurate measurements.
• Demonstrate proper normal construction before learners begin.
• Use a single slit ray for clearer results.

Teacher Tip

Darken the room slightly to make the light rays easier to observe.

Expected Results

• The ray bends towards the normal when entering glass.
• The ray bends away from the normal when leaving glass.
• The ray emerges parallel to the incident ray.

Extension Activity

Ask learners to investigate:

• Semicircular glass blocks
• Prisms
• Coloured filters
• Different transparent materials

Learners can compare how different materials affect refraction.

Real-World Application

Refraction is important in eyeglasses, cameras, microscopes, telescopes, magnifying glasses, swimming pools, prisms and fibre optics.