Refraction & Refractive Index

Noadswood Science, 2013

Refraction & Refractive Index

To explain refractive index

Wednesday, April 19, 2023

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 = fastest diamond = slowestglass = slower

Refraction

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 i

sin r

• Angle Ɵ1 must always be the angle in the vacuum (or air)

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

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

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

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

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