reflection and refraction of light
speech transcript
Liceo Scientifico Belfiore – Mantova
reflection and refraction of light
in accordo con il
Ministero dell’Istruzione, Università, Ricerca
e sulla base delle
Politiche Linguistiche della Commissione Europea
percorso formativo a carattere
tematico-linguistico-didattico-metodologico
scuola secondaria di secondo grado
teachers
Claudio Borghi - Luigi Togliani
eniscuola / smartenglish – A cura di Linda Rossi Holden
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Label europeo 2010
reflection and refraction of light
Reflection and Refraction of Light
Teacher Borghi: Good Morning! In the 17th century, some properties of light were
already known. For example: light can travel through a vacuum; light has different
colours; light can be reflected and refracted. These phenomena are described by the
Laws of Reflection and the Laws of Refraction.
You know the phenomenon of reflection of light, when a beam of light changes its
direction because of a mirror. Specular reflection forms images.
The laws of reflection state:
1. the incident ray, the reflected ray and the normal to the reflection surface at the
point of the incidence lie in the same plane;
2. the angle which the incident ray makes with the normal is equal to the angle which
the reflected ray makes to the same normal;
3. the reflected ray and the incident ray are on the opposite sides of the normal.
Light rays may bend as they cross a boundary from one material to another, for
example from air to water.
This bending of light rays is known as refraction. The light rays from the straw are
bent (or refracted) when they cross from water back into air before reaching the eyes
of an observer.
A light ray passing from one transparent medium to another is partially reflected and
partially transmitted or refracted.
You learned that a narrow beam of collimated and monochromatic light, if it is
refracted when it passes from a medium 1 to a medium 2, changes in direction due to
a change in its speed.
Refraction of light is the commonly observed phenomenon of the deviation of a
monochromatic incidence ray.
You know the relationship between the angle of incidence (formed by the incidence
ray and the normal to the surface), the angle of refraction (formed by the refracted
ray and the normal to the surface) and the index of refraction of the two mediums.
The refraction is described by Snell's law, which states that for a given pair of media
and a wave with a single frequency (monochromatic), the ratio of the sines of the
angle of incidence θ1 and angle of refraction θ2 is equivalent to the ratio of phase
velocities (v1 / v2) in the two media, or equivalently, to the opposite ratio of the
indices of refraction (n2 / n1):
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reflection and refraction of light
What happens if the incident ray is not monochromatic, for example if it is a narrow
beam of white light?
Luca: It is refracted and we simply observe a white light that changes its direction,
approaching to the incidence surface normal if n2 > n1, because the index of refraction
of the second medium is greater than the index of the first one.
Teacher Borghi: Do you believe that a ray of white light is a simple entity?
Amedeo: What do you mean with the concept of “simple entity”?
Teacher Borghi: Your question is opportune. It is necessary to refer to an
experiment. I tell you, and I pray that you believe me, in this moment, that a ray of
white light contains seven different frequencies: it contains the seven different colours
of iris.
Vittoria: Who discovered this propriety of light?
Teacher Borghi: It was Sir Isaac Newton who discovered that sunlight, falling upon a
prism, could split into its component colours. This process is known as dispersion.
Newton named the component colours: red, orange, yellow, green, blue, indigo and
violet.
Most of these colours are easy to distinguish except for indigo. According to a legend
Newton included indigo because he felt that there should be seven rather than six
colours in a rainbow due to his strong religious beliefs.
Luca: Do you mean that dispersion needs a prism?
Teacher Borghi: Not necessarily. Dispersion also occurs when a ray of white light is
refracted, when it passes from a medium 1 to a medium 2 through a transparent
surface (for example, when it passes from air to water or from air to glass). In this
case, however, the dispersion of the ray is not so evident. With a prism, the incident
ray is dispersed when it is refracted passing from air to glass, then every single ray is
refracted a second time when it passes from glass to air and we can capture with a
screen the very beautiful image of the rainbow, staying in our lab of physics!
Alexandra: It is amazing! We can observe a natural phenomenon without waiting
that it occurs in nature!
Teacher Borghi: Yes, this is the spirit of the experimental method of science. We
reproduce the phenomena that naturally occur in our labs, through simple
instruments. In this case a prism, a simple solid made of glass.
Vittoria: Can you detail this phenomenon, please?
Teacher Borghi: The dispersion of colours in a prism occurs because of something
called the refractive index of the glass. Every material has a different refractive
index. When light enters a material -for example, when light traveling through the air
enters the glass of a prism - the difference in the refractive index of air and glass
causes the light to bend.
The angle of bending is different for different wavelengths of light. This means that
different wavelengths of light will travel at different speeds, and so the light will
disperse into the colours of the visible spectrum, with longer wavelengths - red,
yellow - being refracted less than shorter wavelengths - violet, blue.
In a rainbow, raindrops in the air act as tiny prisms. Light enters the raindrop,
reflects off of the side of the drop and exits.
In the process, it is broken into a spectrum just like it is in a triangular glass prism,
like this.
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reflection and refraction of light
Teacher Togliani: The angle between the ray of light coming in and the ray coming
out of the drops is 42 degrees for red and 40 degrees for violet. You can see in this
diagram that the angles cause different colours from different drops to reach your
eye, forming a circular rim of colour in the sky: a rainbow!
In a double rainbow, the second bow is produced because droplets can have two
reflections internally and get the same effect. The droplets have to be the right size to
get two reflections to work.
The next time you spot a rainbow, you will see it in a whole new light.
A rainbow is one of the most beautiful events nature has to offer - so beautiful, in
fact, that it has inspired countless fairy tales, songs and legends.
It's a good bet that most of the artists behind these tales were totally mystified by the
rainbow phenomenon, just like most people are today.
How you can see, the science of rainbows is really very simple. It's just basic optics.
Luca: It is true. When we observe a rainbow, we observe a million of microscopic
prisms in the sky!
Teacher Togliani: Yes, it’s right.
Alexandra: It is wonderful that we can physically understand a phenomenon that we
can consider even only in its imaginative aspect, and we can analyze it through simple
laws of physics…
Vittoria: I better understand the beauty of the world if I know the laws through which
I can explain it!
Teacher Togliani: I’m satisfied you understand that science can open your eyes to a
new world with its great variety of phenomena that occurs under your senses.
Vittoria: I understand that mathematics is very useful to calculate the deviations of
rays and to explain when a ray can be refracted or totally reflected. A rainbow can be
explained only from the knowledge of the two fundamental concepts of reflection and
refraction.
Teacher Togliani: Sure. It is fundamental even the concept of refraction index.
And it is amazing that a simple ray of white light contains seven different colours, for
which seven different indexes of refraction exist.
Amedeo: Light appears to be a simple entity but it is really a very complex
phenomenon.
Teacher Togliani: Yes. If you will more deeply analyze the phenomena that we have
explored in this lesson, you will discover other very important electromagnetic or
quantum laws that are implied in an apparently simple optical experience. Physics has
different degrees of complexity. Today we have discovered one of them.
Materiale sviluppato da eniscuola nell’ambito del protocollo d’intesa con il MIUR
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