1. What Physics is involved here?

2. Define the term critical angle
Learning Objectives
By the end of this lesson you should be able to…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.

3. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refraction
Light is refracted (or changes direction) when it passes from one medium to another.

4. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refraction
Refraction occurs when a wave enters (or exits) a medium with a different density to that of the medium it is in. This speeds up / slows down the wave.

5. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refraction
Angle of refraction
Angle of incidence
Normal
Angles are measured relative to the ‘normal’ – a plane drawn at 90° to the surface of the medium.

6. Investigating Refraction
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Investigating Refraction
Determine the amount the light refracts upon entering and exiting a glass / Perspex block.
Follow the method carefully and get 2 tables of results – one entering the denser medium, one exiting the denser medium.

7. 2. Draw a normal at 90° to the top surface of the block.
1. Place a rectangular Perspex block on a sheet of paper and draw around it.
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
2. Draw a normal at 90° to the top surface of the block.
angle i
angle r
3. Shine light rays, with angles of incidence [i] of 20°, 40, 60° and 80°, into the block at the point where the normal meets the glass surface. Record the angle of refraction [r].
4. Repeat the investigation for rays leaving the glass block.

8. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Light bends _______ the normal when entering a _________ material, this is because it _______ ______ as it enters the medium.

9. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refraction
The speed of light depends on the material through which the light is travelling.
When light enters a different material (e.g. from air into glass), the speed of light changes.
This causes the light to bend or refract.
glass
air
The speed of light is affected by the density of the material through which it is travelling.

10. Results 10 20 30 40 50 60 70 80 Define the term critical angle
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Results
Angle of incidence, i ( o )
Angle of refraction, r ( o )
sin i
sin r 10 20 30 40 50 60 70 80

11. Conclusion What shape is the graph and what relationship does it show?
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Conclusion
What shape is the graph and what relationship does it show?
Why is the experiment not very accurate?
Find out from your text books what Snell’s Law is and include it in your conclusion.

12. Conclusion “Perfect results” Define the term critical angle
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Conclusion
“Perfect results”
Angle of Incidence (degrees)
Angle of Refraction (degrees)
0.00
10.0
7.5
20.0
14.9
30.0
22.1
40.0
28.9
50.0
35.2
60.0
40.6
70.0
45.0
80.0
47.8

13. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Conclusion
The relationship between the angles of incidence and refraction for two media is known as Snell's Law.
Snell's law applies to the refraction of light in any situation, regardless of what the two media are.

14. Snell's Law and Refractive Index
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Snell's Law and Refractive Index
n= sin i
sin r
The value ‘n’ is the refractive index.
The greater the refractive index the greater the change in direction seen in the light when entering the new medium.
Some refractive indices you should know;
Air – 1.00
Water – 1.33
Glass – 1.5 (depends on the type)

15. Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Applying Snell’s Law
Draw the two diagrams below. Using the information provided in the diagrams continue the rays into the new media.
*sin i = 45˚ on both diagrams

16. Refractive Index n = speed of light in vacuum speed of light in medium
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refractive Index
We can use the refractive index (n) to calculate the speed of light in a medium by using the formula:
n = speed of light in vacuum
speed of light in medium

17. Speed of light in material (m/s)
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refractive Index
Material
Speed of light in material (m/s)
Refractive index
air (or vacuum)
(3.0 × 108)
1.0
glass
(2.0 × 108)
Perspex
1.4
water
(2.25 × 108)
diamond
2.4
V light in vacuum
––––––––––––––
V light in medium
Which bends light more, glass or Perspex?
How does the refractive index of a material vary with the speed of light in it?

18. Speed of light in material (m/s)
Learning Objectives
By the end of this lesson you should…
Define the term critical angle
Distinguish between refraction, reflection and the critical angle.
Use the angle of incidence and angle of reflection to calculate the refractive index.
Explain how refractive index can be used to calculate the speed of light in a material.
Refractive Index - Answers
Material
Speed of light in material (m/s)
Refractive index
air (or vacuum)
(3.0 × 108)
1.0
glass
(2.0 × 108)
1.5
Perspex
(2.14 x 108)
1.4
water
(2.25 × 108)
1.33
diamond
(1.25 x 108)
2.4