Refraction & Refractive Index

1. Refraction & Refractive Index Noadswood Science, 2013

2. Refraction & Refractive Index To explain refractive index

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. air glass 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 air = fastest glass = slower diamond = slowest

6. Refraction

7. Tyre 2 Tyre 1 Refraction • 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

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. image of prey prey location 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 • 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 sinr • Angle Ɵ1must 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 iis the refractive index of the substance sin r

20. Rearranged • The ratio sin iis 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 32O sin r sin r = sin 32 ÷ 1.55 = 0.34 so r = sin-1 (0.34) = 19.9O

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 40O sin angle in air = 1.55 x 0.64 sin angle in air = 1O