Chapter 3: Chapter 3:

REFRACTIONREFRACTION

boundaryboundary

incident rayincident ray reflected rayreflected ray

refracted rayrefracted ray

S = light sourceS = light source

1. EXPERIMENTS1. EXPERIMENTS

1.1. 1.1. Light passing from air into glassLight passing from air into glass

Consider the behaviour of a ray of light Consider the behaviour of a ray of light passing from air into a semicircular block passing from air into a semicircular block made of glass or Plexiglas.made of glass or Plexiglas.

boundaryboundary

SS

II

NN

ii

Diagram and vocabularyDiagram and vocabulary

rr

Medium 1 : airMedium 1 : air

Medium 2 : glassMedium 2 : glassi : angle of incidencei : angle of incidencer : angle of refractionr : angle of refractionIR : refracted rayIR : refracted ray

RR

Results of measurements (interactive Results of measurements (interactive simulation)simulation)

Conclusions: Conclusions:

if i = 0° (incident ray perpendicular to the if i = 0° (incident ray perpendicular to the boundary), then the light passes from one boundary), then the light passes from one medium to the other without changing medium to the other without changing direction.direction.

If i If i ≠ 0° and ≠ 0° and the light is entering a the light is entering a denser denser mediummedium the refracted ray is deviated the refracted ray is deviated towards the normaltowards the normal to the boundary. to the boundary.

1.2. 1.2. Light passing from glass into airLight passing from glass into air

Use the same semicircular block as Use the same semicircular block as before but rotate it 180° so that the before but rotate it 180° so that the incident ray hits the incident ray hits the curvedcurved side of side of the block.the block.

boundaryboundary

S S

II

NN

ii

Diagram and vocabularyDiagram and vocabulary

rr

Medium 1 : glassMedium 1 : glass

Medium 2 : airMedium 2 : air

RR

Results of measurements (interactive Results of measurements (interactive simulation)simulation)

Conclusions:Conclusions: when the light enters a when the light enters a less dense less dense

medium, medium, the refracted ray deviates the refracted ray deviates away fromaway from the normal to the boundary. the normal to the boundary.

if 42°< i < 90°, there is if 42°< i < 90°, there is nono refracted refracted ray: ray: all the light is reflectedall the light is reflected at the at the boundary which behaves like a perfect boundary which behaves like a perfect mirror – there is said to be ‘mirror – there is said to be ‘total total internal reflectioninternal reflection’.’.

Refraction occurs because when light passes Refraction occurs because when light passes from one medium to another its speed from one medium to another its speed changes.changes.

3. APPLICATIONS3. APPLICATIONS3.1. Light passing through a glass block with parallel 3.1. Light passing through a glass block with parallel

facesfaces

air:air:

air: air:

glass: glass:

ii

i'i'

rr

the emergent the emergent ray is parallel to ray is parallel to the incident raythe incident ray

incident rayincident ray

refracted rayrefracted ray

3.2. Changes in apparent depth (the 3.2. Changes in apparent depth (the apparent ‘lifting’ of objects)apparent ‘lifting’ of objects)

airair

waterwater

rr

ii

objectobject

If we place a If we place a straight ruler in straight ruler in an aquarium an aquarium filled with water filled with water and if we then and if we then look at the ruler look at the ruler from above, we from above, we have the have the impression that impression that the ruler is bent.the ruler is bent.

The light rays coming from point A The light rays coming from point A refract away from the normal refract away from the normal when they pass from the water when they pass from the water into the air.into the air.

The eye, when it receives these The eye, when it receives these rays, extrapolates them back to rays, extrapolates them back to determine their origin, and determine their origin, and wrongly concludes that the object wrongly concludes that the object is at A’ and not at A. This gives us is at A’ and not at A. This gives us the impression that the ruler is the impression that the ruler is not straight.not straight.

and the fish, what does he see?and the fish, what does he see?

3.3. The prism3.3. The prism

3.3.1. Properties3.3.1. Properties

a prism consists of a block of transparent a prism consists of a block of transparent material which has 2 flat faces at an material which has 2 flat faces at an angle to one another.angle to one another.

If a beam of white light passes through If a beam of white light passes through the prism then two things happen:the prism then two things happen: The light is The light is refractedrefracted (see next slide) (see next slide) The light is The light is disperseddispersed (see next slide but (see next slide but

one)one)

The light is deviated as it enters the The light is deviated as it enters the prism and also as it leaves: there is prism and also as it leaves: there is ‘double refraction’.‘double refraction’.

prism angleprism angle

i1i1i2i2 i3i3

i4i4

i1 : angle of incidencei1 : angle of incidencei4 : angle of emergencei4 : angle of emergence

airair glassglass airair

The white light is broken up (dispersed) The white light is broken up (dispersed) into 7 coloured bands (red, orange, into 7 coloured bands (red, orange, yellow, green, blue, indigo, violet) : this yellow, green, blue, indigo, violet) : this is the visible is the visible spectrumspectrum of white light. of white light.

DispersionDispersion

3.3.2. Applications of the prism3.3.2. Applications of the prism

PeriscopePeriscopeThere is There is total internal total internal reflectionreflection; the ray does not ; the ray does not exit the prism when it hits the exit the prism when it hits the glass-air boundary because glass-air boundary because the angle of incidence is 45° the angle of incidence is 45° (recall that total internal (recall that total internal reflection occurs if the angle reflection occurs if the angle of incidence at the glass-air of incidence at the glass-air boundary exceeds 42°). boundary exceeds 42°).

By using two prisms an By using two prisms an upright image is obtained.upright image is obtained.

RainbowRainbow

When it rains and at the same time this sun is When it rains and at the same time this sun is shining, the raindrops refract and reflect the rays of shining, the raindrops refract and reflect the rays of sunlight as shown:sunlight as shown:

The ray, because it is passing from one medium into The ray, because it is passing from one medium into another, experiences refraction. As for the prism, the another, experiences refraction. As for the prism, the amount of deviation is different for each colour. The amount of deviation is different for each colour. The sunlight is thus sunlight is thus disperseddispersed, with red light deviated , with red light deviated least and violet deviated most. The light rays then least and violet deviated most. The light rays then experience total internal reflection within the drops experience total internal reflection within the drops then a second refraction which accentuates the then a second refraction which accentuates the dispersion even more. Since the light rays are dispersion even more. Since the light rays are reflected inside the drops it is necessary to have reflected inside the drops it is necessary to have your back to the sun in order to see a rainbow.your back to the sun in order to see a rainbow.