Refraction – Learning Outcomes

1. Refraction – Learning Outcomes • Define refractive index. • Demonstrate refraction. • State the Laws of Refraction. • Solve problems about refraction. • HL: Solve problems about refractive index in terms of relative speeds. • Give examples of refraction in nature. • Define critical angle and total internal reflection. • Demonstrate total internal reflection.

2. Refraction – Learning Outcomes • Solve problems about total internal reflection. • Give uses and natural occurrences of refraction. • Discuss transmission of light in optical fibres. • Give uses of optical fibres.

3. Refraction • Refraction is the bending of light as it passes from one medium to another.

4. To Demonstrate Refraction • Aim a narrowed beam from a ray box at the side of a block of glass. • Vary the angle of incidence and note that the angle of refraction increases with angle of incidence. • Note that the ray exiting the block is parallel to the incident ray.

5. Laws of Refraction • The incident ray, the normal at the point of incidence, and the refracted ray all lie in the same plane. • The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. • The second law is also called “Snell’s Law” which we must verify experimentally. • The constant in the second law is the refractive index between the two media, xny, i.e. the ratio of the absolute refractive indices. • Formula:

6. Refractive Index • The refractive index of a medium is the ratio of the sine of the angle of incidence to the sine of the angle of refraction when light travels from a vacuum into that medium.

7. Snell’s Law • e.g. A ray of light enters glass from air. The angle of incidence is 30o and the angle of refraction is 19o. What is the refractive index of the glass? • e.g. A ray of light enters water from air. If the angle of incidence is 40o, find the angle of refraction if the refractive index of water is 1.33. • e.g. Light enters water from glass. If the angle of incidence is 40o and the angle of refraction is 46.3o, what is the refractive index between glass and water?

8. Depth • Due to refraction, objects immersed in a fluid will appear to be closer to the surface than they really are. • This is given by: • Formula:

9. Depth • e.g. Chloe draws a mark on a sheet of paper and places a glass block with thickness 8 cm over it. When viewed from above the glass, the mark appears to be 5.33 cm from the surface. What is the refractive index of the glass? • e.g. A pool of water is 12 m deep. If the bottom of the pool is viewed from the air, how deep does it appear? nwater= 1.33

10. Refraction in Nature – Bears • Bears have to recognise refraction when fishing – the fish appears to be higher up than it really is. Real fish by unknown artist – public domain Bear by Joseph Smit – public domain

11. HL: Speed of Light • Light travels at different speeds in different media. The ratio of speeds between two media is the refractive index between them. • Formula: • For any medium, it follows that: • Formula:

12. HL: Speed of Light • e.g. The refractive index of water is 1.33. If the speed of light in air is , what is the speed of light in water? • e.g. Light enters glass from air. The angle of incidence is 35o and the angle of refraction is 22o. If the speed of light in glass is , calculate the speed of light in air.

13. Total Internal Reflection • When light travels from a denser to a rarer medium, the critical angle, C is the angle of incidence which gives an angle of refraction of 90o. • Total internal reflection (TIR) occurs when light travelling from a denser to a rarer medium is incident at an angle greater than the critical angle.

14. To Demonstrate TIR • Aim a narrowed beam from a ray box at a semi-circular slab of glass • Aim the beam so that it is incident on the flat face of the slab internally. • Starting with a small angle of incidence, increase this angle. • When the critical angle is reached, the refracted ray skims along the flat face of the glass. • For higher angles of incidence, the refracted ray changes to a totally internally reflected ray.

15. To Demonstrate TIR

16. Total Internal Reflection • Given , we can set up the equation for TIR. • If the rarer medium is a vacuum, then:

17. Total Internal Reflection • e.g. The critical angle for a certain medium is 50o. Find its refractive index. • e.g. The refractive index of glass is 1.5. What is the critical angle of glass?

18. TIR in Nature – Mirages • The refractive index of air changes with temperature. • On hot days, light from the sky can bend away from a road towards your eye, creating a “puddle” image.

19. TIR in Nature – Mirages By Brocken Inaglory from Wikipedia – CC-BY-SA-3.0

20. TIR in Nature – Snell’s Window • Looking up while underwater, only light from within a certain radius will reach you – the rest is totally internally reflected from underwater. This is called Snell’s window.

21. TIR in Nature – Snell’s Window

22. Uses • Prism reflectors are used in road signs to ensure that light from headlights reflects back at the driver. • Safety reflectors on bikes and cars use the same effect. • Many modern devices use round reflectors, which work on the same principle.

23. Uses – Optical Fibres • Optical fibres are thin transparent glass rods that can transmit light via total internal reflection.

24. Uses – Optical Fibres

25. Optical Fibres • Optical fibres are used to transmit telephone, television, and internet signals as pulses of light. • It is better than the old copper cables in pretty much every way – lower loss, lower size, lower interference. • They are also used in medicine as flexible cameras, called endoscopes.