Refraction & Refractive Index Noadswood Science, 2012

Refraction & Refractive Index To explain refraction of waves Sunday, August 23, 2015

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

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

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

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 Tyre 1 Tyre 2

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

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…

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)

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

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

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

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

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…

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

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

Refractive Index Refractive index n of the glass is given by n = sin Ɵ 1 sin Ɵ 2 Angle Ɵ 1 must always be the angle in the vacuum (or air)

Refractive Index – Question A ray of red light is incident on the surface water as shown – state weather the light travels faster in air or water (refractive index of water is 1.33 for red light). Calculate the angle of refraction

Refractive Index – Question The light travels faster in air as the angle of incidence (50 o ) is greater than the angle of refraction ( Ɵ ) Ɵ 1 = 50 o n water = 1.33 n = sin Ɵ 1 sin Ɵ = sin50 o sin Ɵ sin Ɵ = Ɵ = 35 o