Chapter 3: REFRACTION

boundary incident ray reflected ray refracted ray S = light source

1. EXPERIMENTS 1.1. Light passing from air into glass Consider the behaviour of a ray of light passing from air into a semicircular block made of glass or Plexiglas.

boundary S I N i DDDDiagram and vocabulary r Medium 1 : air Medium 2 : glass i : angle of incidence r : angle of refraction IR : refracted ray R

R R eeee ssss uuuu llll tttt ssss o o o o ffff m m m m eeee aaaa ssss uuuu rrrr eeee mmmm eeee nnnn tttt ssss ( iiii nnnn tttt eeee rrrr aaaa cccc tttt iiii vvvv eeee s s s s iiii mmmm uuuu llll aaaa tttt iiii oooo nnnn) Conclusions: if i = 0° (incident ray perpendicular to the boundary), then the light passes from one medium to the other without changing direction. If i ≠ 0° and the light is entering a denser medium the refracted ray is deviated towards the normal to the boundary.

1.2. Light passing from glass into air Use the same semicircular block as before but rotate it 180° so that the incident ray hits the curved side of the block.

boundary S I N i  D D D Diagram and vocabulary r Medium 1 : glass Medium 2 : air R

R eeee ssss uuuu llll tttt ssss o o o o ffff m m m m eeee aaaa ssss uuuu rrrr eeee mmmm eeee nnnn tttt ssss ( iiii nnnn tttt eeee rrrr aaaa cccc tttt iiii vvvv eeee ssss iiii mmmm uuuu llll aaaa tttt iiii oooo nnnn) Conclusions: when the light enters a less dense medium, the refracted ray deviates away from the normal to the boundary. if 42°< i < 90°, there is no refracted ray: all the light is reflected at the boundary which behaves like a perfect mirror – there is said to be ‘total internal reflection’.

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

3. APPLICATIONS 3.1. Light passing through a glass block with parallel faces air: air: glass: i i' r the emergent ray is parallel to the incident ray incident ray refracted ray

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

air water r i object

If we place a straight ruler in an aquarium filled with water and if we then look at the ruler from above, we have the impression that the ruler is bent. The light rays coming from point A refract away from the normal when they pass from the water into the air. The eye, when it receives these rays, extrapolates them back to determine their origin, and wrongly concludes that the object is at A’ and not at A. This gives us the impression that the ruler is not straight.

aaaa nnnn dddd t t t t hhhh eeee f f f f iiii ssss hhhh,,,, w w w w hhhh aaaa tttt d d d d oooo eeee ssss h h h h eeee s s s s eeee eeee ????

3.3. The prism Properties a prism consists of a block of transparent material which has 2 flat faces at an angle to one another. If a beam of white light passes through the prism then two things happen: The light is refracted (see next slide) The light is dispersed (see next slide but one)

The light is deviated as it enters the prism and also as it leaves: there is ‘double refraction’. prism angle i1 i2 i3 i4 i1 : angle of incidence i4 : angle of emergence air glass air

The white light is broken up (dispersed) into 7 coloured bands (red, orange, yellow, green, blue, indigo, violet) : this is the visible spectrum of white light. DDDD iiii ssss pppp eeee rrrr ssss iiii oooo nnnn

Applications of the prism Periscope There is total internal reflection; the ray does not exit the prism when it hits the glass-air boundary because the angle of incidence is 45° (recall that total internal reflection occurs if the angle of incidence at the glass-air boundary exceeds 42°). By using two prisms an upright image is obtained.

Rainbow

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