Liceo Scientifico “Belfiore”
Mantova
Physics course
School year 2011-2012
Reflection and refraction of light
Light can
travel
through
a vacuum
Light has
different
colours
In the 17th century, some
properties of light were
already well known.
For example:
Light can be reflected and refracted.
These phenomena are described by
the
Laws of Reflection
and the
Laws of Refraction
Reflection
The phenomenon of reflection occurs when a beam of light
changes its direction because of a mirror
Specular reflection forms images
q1
q ’1
q1 = q’1
The laws of reflection
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
q1 = angle of incidence
q’1= angle of reflection
q1
q ’1
Refraction
• 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 (refracted) when
they cross from water back
into air before reaching the
eyes of an observer
Refraction
q1
medium 1
q ’1
medium 2
q2
A light ray passing from one
transparent medium to another is
partially reflected and partially
transmitted or refracted
A narrow beam of collimated and
monochromatic light changes in
direction due to its speed
Refraction of light is the
commonly observed phenomenon
of the deviation of a
monochromatic incident ray
Refraction
q1
medium 1
q ’1
medium 2
q2
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 is
described by Snell’s law
Snell’s law
q1
medium 1
q ’1
medium 2
q2
Snell’s law states that for a given
pair of media and a wave of a
single frequency (monochromatic),
the ratio of the sines of the angle
of incidence θ1 and the 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):
sin θ1 / sin θ2 = v1 / v2 = n2 / n1
Index of refraction
Dispersion of light and prism
A ray of white light contains seven different frequencies: it
contains the seven different colours of iris
It was Sir Isaac Newton who discovered that sunlight, falling
upon a prism, could split into its component colours
This process in known as dispersion
Dispersion of light
Dispersion also occurs when a ray of
white light is refracted, when it passes
from a medium 1 to a medium 2 trough
a transparent surface
With a prism, the incident ray is
dispersed when it is refracted passing
from air to glass
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
Dispersion of light
This is the spirit of the experimental
method of science: we reproduce the
phenomena that naturally occur in our
labs, through simple instruments
The dispersion of colours in a prism
occurs because of something called
the refractive index of the glass
When light enters a material, the
difference in the refractive index of air
and glass causes the light to bend
Dispersion of light
The angle of bending is different
for different wavelengths of light
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)
Dispersion and rainbow
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
Dispersion and rainbow
In the process, light is
broken into a spectrum
just like it is in a
triangular glass prism, like
this
Dispersion and rainbow
The angle between the ray
of light coming in and the
ray coming out of the drops
is 42°for red and 40°for
violet
The angles cause different
colours from different drops
to reach your eye, forming
a circular rim of colour in
the sky: a rainbow
Double 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
Rainbow
The next time you spot a rainbow, you will see it in a
whole new light
Rainbow and art
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
Rainbow and art
« My heart leaps up when I behold
A rainbow in the sky:
So was it when my life began;
So is it now I am a man;
So be it when I shall grow old,
Or let me die!… »
William Wordsworth, 1802
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
Rainbow and science
The science of rainbows is really very simple: it’s just basic optics
Light and colours
It is amazing that a simple ray of white light contains seven
different colours, for which seven different indexes of
refraction exist
Experience and Physics
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
Copyright 2012 © eni S.p.A.