1. Refraction & Refractive Index Noadswood Science, 2013

2. Refraction & Refractive Index To explain refractive index Monday, August 24, 2015

3. Refraction Light travels in straight lines Light can bend at the boundary between two materials with different densities - this is called refraction The light ray bends towards the normal as it enters The light ray bends away from the normal as it leaves The ray entering the block is parallel to the ray leaving the block, if the block has parallel faces A ray entering the block at 90° is not refracted

4. Refraction Light can bend at the boundary between two materials with different densities - this is called refraction

5. Speed The speed of light waves depends on the material they are travelling through If light waves enter a different material (e.g. travel from glass into air) the speed changes, causing the light to bend or refract glass air air = fastestdiamond = slowestglass = slower

6. Refraction

7. Why does the light ray bend towards the normal when it enters the glass block, and then bend away from normal as it leaves? In the muddy field it slows down as there is more friction If it enters the field at an angle then the front tyres hit the mud at different times First to hit the mud is tyre 1, and will move more slowly than tyre 2. This causes the car to turn towards the normal When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal Tyre 1 Tyre 2

8. Refraction If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down

9. Water Water is denser than air, so light is refracted when it travels through the surface of the water. This is why sticks seem to be bent if they are partly underwater, and why swimming pools look shallower than they really are Refraction lets you see objects that are normally hidden…

10. Experiment Which of these would you expect to refract more: - –Light through Perspex –Light through glycerol –Light through water What is your prediction? Explain why you think this is so… Using the protractors measure and record the incident and refracted ray angles in a table Then work out the refractive index and plot your results for the three materials on a suitable graph (RI = sin i ÷ sin r)

11. Experiment Refractive index: - –Perspex = 1.48 –Glycerol = 1.47 –Water = 1.33

12. Archer Fish The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey image of prey prey location The fish does not aim at the refracted image it sees, but at a location where it knows the prey to be

13. Archer Fish The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey

14. Refraction The Sun also appears to have set below the horizon later than it actually has – due to refraction…

15. Using Refraction A forensic scientist may sometimes be asked to match pieces of glass, e.g. from a crime scene and from the clothes of a suspect…

16. Using Refraction To do this the forensic scientist will try to match the refractive index – a small piece of glass will be immersed in oil and then looked at under a microscope The oil is then slowly heated and cooled – this changes the oil’s refractive index… When the refractive index of the oil and glass match the glass will ‘disappear’ – a computer then converts the temperature of the oil to a refractive index value

17. Refractive Index When a ray of light is incident at normal incidence, (at right angles), to the surface between two optical materials, the ray travels in a straight line When the ray is incident at any other angle, the ray changes direction as it refracts The change in direction of a ray depends on the change in speed of the light and can be used to calculate refractive index

18. Refractive Index Refractive index n of the glass is given by n = sin i sin r Angle Ɵ 1 must always be the angle in the vacuum (or air)

19. Summary For a light ray travelling into glass from air you should find that: - The angle of refraction is always less than the angle of incidence The greater the angle of incidence (i) the greater the angle of refraction (r) Snell’s law states that sin i always has the same value sin r The ratio sin i is the refractive index of the substance sin r

20. Rearranged The ratio sin i is the refractive index of the substance sin r This can be rearranged to calculate the angle of incidence or refraction if you are given the refractive index of a substance

21. Moving Into More Dense Mediums E.g. a light ray travels from air into glass across a straight boundary. The angle of incidence of the ray in the air is 32°. The refractive index of the glass is 1.55 Calculate the angle of refraction of the light ray in the glass 1.55 = sin 32 O sin r sin r = sin 32 ÷ 1.55 = 0.34 so r = sin -1 (0.34) = 19.9 O

22. Moving Into Less Dense Mediums When a light ray travels from a transparent substance into air at a non-zero angle of incidence: - The light ray is refracted away from the normal The larger the angle of incidence the larger the angle of refraction The law of refraction can be adapted to cover both situations by using the formula: sin angle in air = n x sin angle in glass

23. Moving Into Less Dense Mediums E.g. a light ray travels from glass to air across a straight boundary. The angle of incidence of the light ray in the glass is 40°. The refractive index of the glass is 1.55 Calculate the angle of refraction of the light in the air sin angle in air = 1.55 x sin 40 O sin angle in air = 1.55 x 0.64 sin angle in air = 1 O