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E2.4 · Investigate refraction across media with different indices

Learn to investigate refraction across media with different indices through clear examples and targeted practice.

Ontario Grade 10 Science

Physics: Light and Geometric Optics

Use ray paths and angle observations to compare how light bends

A straight straw partly under water can look as if it changes direction at the water surface. The straw has not bent. Light from the underwater part changes direction as it passes from water into air. This change in direction is called refraction. In this lesson, you will use a simple ray model to investigate how refraction differs across media with different refractive indices. You will compare observations rather than calculate angles with a refraction equation.

What you will learn

Describe refraction using the path of a light ray.
Identify the boundary, normal, angle of incidence, and angle of refraction in a model.
Use refractive-index comparisons to predict whether a ray bends toward or away from the normal.
Plan a safe investigation and use observations as evidence.

1. From a familiar observation to a ray model

A medium is a material that light travels through. Air, water, and glass are examples. The boundary is the surface where two media meet. When light reaches a boundary, some light may pass into the second medium. Some may also bounce back. The change in direction of light that passes across the boundary is refraction.

This connects to earlier learning about light travelling in straight paths through a uniform material. A ray is a simple model for showing a light path. It is drawn as a line with an arrow that shows the direction of travel. A ray is not a thin solid object. It helps us describe and compare what we observe.

Consider a pencil partly covered by clear water. The underwater part can appear shifted from the part in air. Light from the pencil changes direction as it leaves the water and enters the air. Your eye traces the arriving light back in a straight line, so the underwater part seems to be somewhere other than its actual position.

Refraction and reflection are different. Reflection is light bouncing from a surface. Refraction concerns light that passes into another medium and changes direction. Both can happen at the same boundary.

A medium is a material through which light travels.
Refraction is a change in direction as light passes between media.
A ray is a model used to show the path and direction of light.

2. Describe and compare the bending

To make a ray diagram, draw the boundary between the two media. At the point where the ray reaches the boundary, draw a normal. The normal is a reference line at a right angle to the boundary. It is not a physical object.

The angle of incidence is the angle between the incoming ray and the normal. The angle of refraction is the angle between the ray that passes into the second medium and the normal. Measure both angles from the normal, not from the boundary. This lets different observations be compared fairly.

A refractive index is a number that describes how light behaves in a medium compared with how it behaves in a vacuum. For this investigation, use index values to compare the media. A larger index means light travels more slowly in that medium. When a ray crosses at an angle into a medium with a larger index, the ray bends toward the normal. Its angle to the normal becomes smaller. When it enters a medium with a smaller index, it bends away from the normal, so its angle to the normal becomes larger.

There is an important special case. If a ray reaches the boundary along the normal, it carries on in the same direction. It does not turn sideways at the boundary. The ray can still change speed when it enters the second medium.

A ray model helps you make a prediction, but an investigation checks the prediction against observations. Record the path and measure the angles. Do not assume every reading will be exact or identical.

The normal is perpendicular to the boundary.
Measure incidence and refraction angles from the normal.
Entering a higher-index medium at an angle: toward the normal.
Entering a lower-index medium at an angle: away from the normal.

3. Investigate with observations and evidence

A supervised classroom investigation can use a low-power ray box, paper, a clear block, a ruler, and a protractor. Use classroom equipment as directed by the teacher. Keep the beam below eye level. Never look directly into the beam or point it at anyone.

Mark the boundary and the point where the ray will meet it. Draw the normal at that point. Trace the incoming ray and the ray that passes through the second medium. Measure both angles from the normal. Write down which medium is on each side and which angle belongs to each ray.

Repeat the observation with a few different incoming directions. To compare media, keep the method consistent and change only what you are investigating. For example, use the same incoming angle when comparing two different second media. Take care not to mistake a change in the incoming direction for a change caused by the material.

Evidence is what you record: the traced paths, angle measurements, and the media used. A conclusion should match that evidence. If repeated observations show the transmitted ray closer to the normal in a higher-index medium, that supports the prediction that it bends toward the normal. If the marks are unclear or readings differ, say so rather than forcing a pattern.

Small differences can come from a thick pencil line, a wide light beam, or a protractor that is not lined up with the normal. Record what you observe. Label practice values as hypothetical; do not present them as laboratory results.

Use a consistent method and identify the media for each observation.
Trace the rays and measure from the normal.
Use repeated observations to judge whether a pattern is consistent.
Follow classroom safety directions and never look into a light beam.

4. Work from a prediction to a conclusion

The examples below use hypothetical situations. They show how to reason from the media and ray observations. They are not measurements from an actual experiment. A useful comparison asks what changed, what stayed the same, and what the ray did at the boundary.

Keep predictions, hypothetical practice examples, and real observations clearly separate.
Explain a conclusion by connecting the ray path to the media and the measured angles.

Worked example

Predict bending into a higher-index medium

A hypothetical ray travels from air, with refractive index 1.00, into a clear material with refractive index 1.40. It reaches the boundary at 35° to the normal. Predict the direction of bending and describe what an angle measurement should show.

Compare the media
The second material has a larger index than air. The ray is entering a higher-index medium.
Predict the path
At an angled crossing into a higher-index medium, the ray bends toward the normal. Therefore, the angle measured in the clear material should be smaller than the 35° incoming angle. This is a prediction, not a measured result.

Answer: The ray bends toward the normal. Its angle to the normal in the clear material should be less than 35°.

Check: The prediction follows the higher-index rule and describes an observable comparison.

Worked example

Interpret a hypothetical comparison

In a hypothetical classroom record, the same incoming direction is used for two trials. The incoming angle is 40° in each. In Trial A, the transmitted ray measures 28° to the normal. In Trial B, it measures 34°. The second medium in Trial A has a higher index than the second medium in Trial B. What does the comparison suggest?

Compare the measured angles
The transmitted angle is 28° in Trial A and 34° in Trial B. Trial A’s ray is closer to the normal because its angle is smaller.
Connect the evidence to the media
The incoming directions are the same, while the second media differ. The ray in the higher-index medium has the smaller angle. This hypothetical comparison supports the pattern that light bends more toward the normal when it enters a higher-index medium.
State the limit
These two hypothetical readings suggest a pattern; they do not prove that every material or measurement will behave exactly the same way. Repeated, careful observations would make the evidence stronger.

Answer: The comparison supports the idea that the ray bends closer to the normal in the higher-index medium.

Check: The reasoning uses the smaller measured angle and the stated difference between the second media.

Worked example

Spot a measurement and reasoning problem

A student traces a ray crossing from water into air. The student measures the incoming angle from the boundary and says the ray bends toward the normal because the ray looks less steep on the page. How should the student improve the observation and conclusion?

Correct the reference line
The angle must be measured from the normal, not from the boundary. The student should draw the normal at the crossing point and measure both the incoming and transmitted angles from it.
Compare the angles and indices
Water has a higher index than air, so a ray going from water into air enters a lower-index medium. At an angled crossing, the model predicts bending away from the normal. The corrected angle measurement should be checked against that prediction.
Use evidence before concluding
A ray can look steep or shallow depending on how the page is turned. A measured angle from the normal is a clearer basis for the conclusion. The student should report the traced path and actual measurement, even if it does not match the prediction neatly.

Answer: Draw the normal, measure both angles from it, and compare the measured path with the prediction that water-to-air bending is away from the normal.

Check: A valid conclusion uses the correct reference line and the observed path, rather than appearance alone.

Common mistakes and how to avoid them

Measuring an angle from the boundary.

Correction: Draw the normal first, then measure the angle between the ray and the normal.

Claiming that light always bends toward the normal.

Correction: Compare the indices. The ray bends toward the normal when entering a higher-index medium and away when entering a lower-index medium, for an angled crossing.

Treating a predicted or hypothetical angle as a real measurement.

Correction: Label predictions and hypothetical values clearly. Report only observations that were actually recorded.

Changing measurements to match a prediction.

Correction: Record what you observe. Check your setup and repeat the measurement if needed, but do not alter data.

Lesson summary

Refraction is a change in the direction of light as it passes from one medium into another.
A ray diagram uses a boundary and a normal to show the path of light.
Measure angles from the normal, not from the boundary.
At an angled crossing, light bends toward the normal when entering a higher-index medium and away when entering a lower-index medium.
Careful observations and repeated measurements provide evidence for a conclusion.

Check your understanding

Question 1

A ray crosses at an angle from air into a material with a higher refractive index. Which direction does it bend?

Toward the normal
Away from the normal
Along the boundary in every case
Back into the first medium in every case

Show answer and explanation

Toward the normal

At an angled crossing into a higher-index medium, the ray bends toward the normal.

Question 2

Where should the angle of incidence be measured?

Between the incoming ray and the boundary
Between the incoming ray and the normal
Between the two media
Between the transmitted ray and the normal

Show answer and explanation

Between the incoming ray and the normal

The angle of incidence is measured between the incoming ray and the normal.

Question 3

A ray enters a lower-index medium at an angle. Which observation would fit the model?

The transmitted ray has a smaller angle to the normal.
The transmitted ray has a larger angle to the normal.
The ray must travel along the boundary.
The ray must return completely into the first medium.

Show answer and explanation

The transmitted ray has a larger angle to the normal.

Entering a lower-index medium at an angle makes the ray bend away from the normal, giving a larger angle to it.

Key terms

Boundary: The surface where two media meet.
Medium: A material through which light travels.
Normal: A reference line drawn at a right angle to a boundary.
Angle of incidence: The angle between the incoming ray and the normal.
Angle of refraction: The angle between the transmitted ray and the normal.
Refractive index: A number describing how light behaves in a medium compared with how it behaves in a vacuum.
Refraction: A change in the direction of light as it passes from one medium into another.
Ray: A line-and-arrow model used to show the path and direction of light.

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Published by DoAssignment. This reviewed lesson follows Ontario Grade 10 Science (SNC2D), expectation E2.4. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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