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Key concept PSL6.1.1: Explaining refraction

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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 rays

A ray of light shows the direction light moves.

When light moves from air into glass it can refract.

It can change direction at the boundary between air and glass.

Light can also refract when it moves from glass into air.

A lens can refract light to form an image on a screen.

Refracting rays

1. Which ray of light shows what happens at a boundary between air and glass?

Refracting rays

2. Which ray of light shows what happens at a boundary between air and glass?

Refracting rays

3. Which ray of light shows what happens at a boundary between glass and air?

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Bending bananas

Light can refract as it moves from one transparent medium into another …

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… because a transparent medium can change the speed of light.

Each transparent medium has a refractive index.

The bigger the refractive index of a medium, the slower light travels through it.

Bending bananas

When light moves from air into water …

1. What is the rule for the refraction of light moving into water?

To do: in each row pick one statement that you think is right.

1

2

3

4

5

It changes direction at the boundary.

It changes direction through the water.

It changes direction through the air.

It bends towards the normal line.

It bends away from the normal line.

… it slows down.

… it speeds up.

… it continues at the same speed.

Light continues in a straight line if the angle of incidence is 0^o.

Light continues in a straight line if the angle of incidence is 90^o.

Light always bends as it moves from air into water.

Refractive index of air = 1.0 Refractive index of water = 1.3 (water is optically more dense)

Bending bananas

When light moves from air into water …

1. The rule for the refraction of light moving into water.

Answers:

1

2

3

4

5

It changes direction at the boundary.

It changes direction through the water.

It changes direction through the air.

It bends towards the normal line.

It bends away from the normal line.

… it slows down.

… it speeds up.

… it continues at the same speed.

Light continues in a straight line if the angle of incidence is 0^o.

Light continues in a straight line if the angle of incidence is 90^o.

Light always bends as it moves from air into water.

Refractive index of air = 1.0 Refractive index of water = 1.3 (water is optically more dense)

Bending bananas

When light moves from water into air …

2. What is the rule for the refraction of light moving out of water?

To do: in each row pick one statement that you think is right.

1

2

3

4

5

It changes direction at the boundary.

It changes direction through the water.

It changes direction through the air.

It bends towards the normal line.

It bends away from the normal line.

… it slows down.

… it speeds up.

… it continues at the same speed.

Light continues in a straight line if the angle of incidence is 0^o.

Light continues in a straight line if the angle of incidence is 90^o.

Light always bends as it moves from air into water.

Refractive index of air = 1.0 Refractive index of water = 1.3 (water is optically more dense)

Bending bananas

When light moves from water into air …

2. The rule for the refraction of light moving out of water.

Answers:

1

2

3

4

5

It changes direction at the boundary.

It changes direction through the water.

It changes direction through the air.

It bends towards the normal line.

It bends away from the normal line.

… it slows down.

… it speeds up.

… it continues at the same speed.

Light continues in a straight line if the angle of incidence is 0^o.

Light continues in a straight line if the angle of incidence is 90^o.

Light always bends as it moves from air into water.

Refractive index of air = 1.0 Refractive index of water = 1.3 (water is optically more dense)

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Refracting water waves

A wavefront diagram shows the crests of a wave.

It shows the tops of a wave from above.

Refracting water waves

As they move into shallow water, water waves slow down and refract.

Which wavefront diagram shows how water waves refract?

A

B

C

D

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Representing light

Light can refract at a boundary between one transparent medium and another.

Representing light

There are different ways to represent light.

Which is the best way to represent light to explain how it refracts?

A Ray diagram

C Wavefront diagram

B Wave diagram

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Refracting light

Light can refract at a boundary between air and glass because it has the properties of a wave.

Refracting light

A light wave crosses the boundary between air and glass.

A

Its speed becomes slower.

B

Its wavelength becomes shorter.

C

Its frequency is reduced.

I am sure this is right

I think this is right

I think this is wrong

I am sure this is wrong

What happens to the light wave as it enters the glass?

What do you think about each statement?

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Liquid refraction

A clear plastic box is filled with a liquid.

It can be used to investigate how different liquids refract light.

Liquid refraction

A ray of light is shone through two different liquids.

a. As it leaves each liquid, from which one will it be refracted the most?

A

B

Liquid refraction

b. What is the best reason for your answer to part a?

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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.

Measuring refraction

Light refracts as it enters a glass block.

Light refracts again as it leaves the glass block.

A ray lamp can be used to work out rules for refraction.

Measuring refraction

Apparatus

• Rectangular glass block
• Ray lamp and slit
• Power supply
• Ruler
• Protractor
• Large sheet of paper

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What are the rules for the refraction of light at a boundary?

Mains electricity!

The bulb can get very hot.

Measuring refraction

1. Shine a ray of light into a long side of the glass block so it leaves out of the opposite side.
2. Measure the angle of incidence and the angle of refraction as the ray of light enters the block.
3. Measure the angle of incidence and the angle of refraction as the ray of light leaves the block.
4. Repeat the measurements for a good range of angles.

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Method

Instructions for measuring all the angles follow on the next slide.

Measuring refraction

• Place a glass block in the middle of a large sheet of paper.
• Draw round it with a pencil.

• Turn out the lights.
• Shine a ray of light through the block – out of the opposite side.

• Mark two crosses in the middle of each ray.

• Take away the glass block.
• Turn the lights on.

• Use a ruler to draw the rays of light on the paper.

• Use a ruler to draw in the missing ray of light.

• Put a protractor on the point where the first ray meets the block.
• Mark 90^o.

• Use a ruler to draw the normal line.

• Use the protractor to measure the angle of incidence.

• Use the protractor to measure the angle of refraction.

• Repeat the measurements for when light leaves the glass block.
• Measure the angle of incidence and the angle of refraction.

Measuring refraction

Results

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To answer:

1. When a ray of light enters glass, does it bend towards or away from the normal line?
2. What is the rule for the refraction of light leaving a glass block?
3. Is light refracted most when it enters or leaves the glass block?
4. What is the angle of incidence when there is no refraction?

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Modelling refraction

Albert is modelling refraction.

He wants to show why a wave can bend when it moves across a boundary.

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To answer

1. State three ways in which this is a good representation of refraction.
2. State three ways in which this is not an accurate representation of refraction.
3. Use the wave model to explain how light refracts at a boundary between air and glass.

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Explaining refraction

When light crosses a boundary between one transparent medium and another it can refract.

It refracts if it meets the boundary at the right sort of angle.

Light refracts because it has the properties of a wave.

Light can be represented as

a series of wavefronts moving forward.

Explaining refraction

The wavefront in glass moves more slowly.

One end of each wavefront reaches the boundary before the other.

The other end of the wavefront travels faster until it reaches the boundary.

The wavefront in glass moves more quickly.

The other end of the wavefront travels more slowly until it reaches the boundary.

The wavefront in air moves more slowly.

The light wave is refracted towards the normal line.

The light wave is refracted away from the normal line.

The wavefront in air moves more quickly.

This swings the light wave round and it moves forward in a different direction.

1. Use some of these statements to explain how light moving from

air into glass can refract.

Start with: Light can be represented as a series of wavefronts moving forward.

Explaining refraction

1. Answer:

Light moving from air into glass.

Light can be represented as a series of wavefronts moving forward.

• One end of each wavefront reaches the boundary 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 light wave is refracted towards the normal line.

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Explaining refraction

2. Use some of these statements to explain how light moving from

glass into air can refract.

The wavefront in glass moves more slowly.

One end of each wavefront reaches the boundary before the other.

The other end of the wavefront travels faster until it reaches the boundary.

The wavefront in glass moves more quickly.

The other end of the wavefront travels more slowly until it reaches the boundary.

The wavefront in air moves more slowly.

The light wave is refracted towards the normal line.

The light wave is refracted away from the normal line.

The wavefront in air moves more quickly.

This swings the light wave round and it moves forward in a different direction.

Start with: Light can be represented as a series of wavefronts moving forward.

Explaining refraction

2. Answer:

Light moving from glass into air.

Light can be represented as a series of wavefronts moving forward.

• One end of each wavefront reaches the boundary before the other.
• The wavefront in air moves more quickly.
• The other end of the wavefront travels more slowly until it reaches the boundary.
• This swings the light wave round and it moves forward in a different direction.
• The light wave is refracted away from the normal line.

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Turning expectations

A clear plastic semi-circular box is filled with a liquid.

It can be used to investigate how different liquids refract light.

Turning expectations

Predict

How do you think the second liquid will refract light differently than the first?

Explain

Explain why you think the light will refract in this way?

Two different liquids are compared.

Light travels slower in the second liquid, which is optically more dense.

Turning expectations

Observe

Measure the angles of refraction for the same incident ray refracted by each liquid in turn.

Explain

Were your prediction and explanation correct?

Try to improve your first explanation to explain what happened more clearly.

Observe how the second liquid refracts, compared to the first.

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