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Key concept PSL6.1.2: Explaining dispersion

B

Section 1: Diagnostic questions

A zip file containing all the resources for this key concept can be downloaded from www.BestEvidenceScienceTeaching.org

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The zip file provides:

• Teacher guidance for this key concept, including a summary of the research evidence on relevant preconceptions and misunderstandings.
• A full set of editable and printable student sheets and teacher notes for each diagnostic question. The teacher notes include a summary of research evidence, guidance for using the activity, expected answers and suggestions of how to respond to students’ misunderstandings.

• Use diagnostic questions to identify quickly where your students are in their conceptual progression, and what preconceptions and misunderstandings they may have.
• Then decide how to best focus and sequence your teaching.
• Use further diagnostic questions and response activities to move students’ understanding forwards.

Refracting red

A red filter is fitted to a ray lamp.

A ray of red light is shone through a glass prism.

Refracting red

Which diagram best shows how the red light is refracted?

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Prism rules

Red light is shone through a glass prism.

The glass is optically more dense than air.

Prism rules

Why does red light refract as it passes through the prism?

To do: connect the statements to make three sentences that explain what happens.

Prism rules

Why does red light refract as it passes through the prism?

Answers:

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Refraction blues

Blue light has a higher frequency than red light.

It refracts more than red light when it enters a glass prism.

Refraction blues

Blue light has a higher frequency than red light.

It refracts more than red light when it enters a glass prism.

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A

Blue light moves slower than red light in glass.

B

Blue light moves slower than red light in air.

C

The wavelength of blue light is shorter than that of red light.

I am sure this is right

I think this is right

I think this is wrong

I am sure this is wrong

These statements are about the differences between red light and blue light.

What do you think about each one?

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Double refraction

Red light refracts twice as it passes through this prism.

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Double refraction

a. How does blue light refract through the same prism?

Double refraction

b. What is the best reason for your last answer?

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A B C D

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A

At each boundary blue light changes speed more than red light.

B

In air and glass, red light moves faster than blue light.

C

In glass, blue light moves more slowly than red light.

D

All colours of light travel at the same speed.

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The colour violet

Violet light refracts because it has wave properties.

Different colours within white light refract at different angles.

The colour violet

Violet light refracts because it has wave properties.

Violet light refracts more than blue light.

A

Violet light travels faster in glass.

B

Violet light travels faster in air.

C

Violet light has a higher frequency.

D

Violet light has a longer wavelength.

I am sure this is right

I think this is right

I think this is wrong

I am sure this is wrong

How does violet light compare to blue light?

What do you think about each statement?

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Section 2: Response activities

A zip file containing all the resources for this key concept can be downloaded from www.BestEvidenceScienceTeaching.org

​

The zip file provides:

• Teacher guidance for this key concept, including a summary of the research evidence on relevant preconceptions and misunderstandings.
• A full set of editable and printable student sheets and teacher notes for each response activity. The teacher notes include a summary of research evidence, guidance for using the activity and expected answers.

• Response activities encourage students to talk and think about what they’re thinking (metacognition).
• This challenges students’ misunderstandings, facilitates meaning-making, and develops and consolidates their scientific understanding.

Prism blues

Moving from air into glass, red light refracts towards the normal line.

Moving from glass to air, red light refracts away from the normal line.

Prism blues

A wavefront diagram shows how red light slows down in glass.

It shows how red light speeds up again in air.

It helps to explain how a prism refracts red light.

1. Draw construction lines.

2. Draw wavefronts in the glass.

3. Draw more construction lines.

4. Draw wavefronts in the air.

Prism blues

To do:

Complete this wavefront diagram to show blue light moving through a prism.

To answer:

1. Why does blue light refract more than red light as it enters the prism?
2. Why does blue light refract more than red light as it leaves the prism?

Prism blues

Answer: Wavefront diagram for blue light.

Prism blues

Wavefront diagrams for red light and blue light.

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Rainbow light

If white light passes through a prism correctly, it can form a rainbow.

Light refracts at each boundary of the prism.

Different colours within the white light refract at different angles.

Rainbow light

White light is made of many colours, including red and blue.

Red light has a longer wavelength and a lower frequency than blue.

Why can white light be split into the colours of the rainbow?

To do: pick one statement in each row to explain the reason.

Rainbow light

White light is made of many colours, including red and blue.

Red light has a longer wavelength and a lower frequency than blue.

Why can white light be split into the colours of the rainbow?

Answers:

1

2

3

In air, all colours of light travel at the same high speed.

In air, red light travels faster than blue light.

In air, blue light travels faster than red light.

White light contains a range of different colours.

White light contains all the colours we see.

In glass, all colours move at the same slower speed.

Light moves slower in glass, but red light moves faster than blue.

Light moves slower in glass, but blue light moves faster than red.

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The greater the frequency of light at a boundary, the bigger the angle it refracts at.

The greater the change of speed of light at a boundary, the bigger the angle it refracts at.

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Making rainbows

Red light refracts as it passes through a prism because it has wave properties.

Different colours within white light refract at different angles.

Making rainbows

The following passage explains why red light refracts as it passes through a prism.

Use your understanding of white light to add more sentences that explain how a rainbow is made.

Red light can be represented as a series of wavefronts moving forward.

• One end of each wavefront reaches the prism before the other.
• The wavefront in glass moves more slowly.
• The other end of the wavefront travels faster until it reaches the boundary.
• This swings the light wave round and it moves forward in a different direction.
• The red light is refracted towards the normal line.

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Light comparison

Light refracts as it passes through a prism because it has wave properties.

Different colours within white light refract at different angles.

Light comparison

What do yellow and green light have in common?

How are they different?

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