3. What is Refraction?
Refraction is the change in direction of propagation of
a wave due to a change in its transmission medium.
4. Laws of Refraction
When light travels from one medium to another, it generally
bends, or refracts. The law of refraction gives us a way of
predicting the amount of bend. This law is more
complicated than that for reflection, but an understanding
of refraction will be necessary for our future discussion of
lenses and their applications. The law of refraction is also
known as Snell's law, named for Willobrord Snell, who
discovered the law in 1621.
5. Snell's law gives the relationship
between angles of
incidence and refraction for a
wave impinging on an interface
between two media with
different index of refraction. The
law follows from the boundary
condition that a wave be
continuous across a boundary,
which requires that the phase of
the wave be constant on any given
plane, resulting in
Laws of Refraction
6. When we talk about the
speed of light, we're usually
talking about the speed of
light in a vacuum, which is
3.00 x 108 m/s. When light
travels through something
else, such as glass, diamond,
or plastic, it travels at a
different speed. The speed of
light in a given material is
Refraction
related to a quantity called the index of refraction, n, which is
defined as the ratio of the speed of light in vacuum to the speed of
light in the medium:
index of refraction : n = c / v , where the speed of light in a
medium is v and c is the speed of light in vacuum.
7. Rules of Refraction
Rule-1 : When a light
ray travels from a rarer
medium to a denser
medium, the light ray
bends towards the
normal.
Rule-2 : When a light
ray travels from a
denser medium to a
rarer medium, the
light ray bends away
from the normal.
8. Some examples of refraction
around us 1) If you place a pencil in a glass of
water, you will see that the pencil
appears to be bent. Now, you know for a
fact that the pencil is not bent at all, it
just appears to be so. This is the effect of
light. Light is actually a form of energy
called electromagnetic radiation. There
is a wide spectrum of radiation with X-
rays, ultraviolet rays, infrared rays etc.
and most of them are present in the
atmosphere with their own definite
wavelengths and properties, but the only
wavelengths that we are able to see is
that of Visible Light. (Fig 1)
Fig 1
9. Since light is in the form of aves, it has many characteristic properties
like reflection (this helps us to see objects, because only when light
reflects off an object, we can see it), travels at different speeds in
different objects and most importantly, undergoes refraction.
2) The term refraction is defined as the bending of light as it passes
from one type of material into another. Because light travels at
different speeds in the two materials, it changes its speed at the
boundary of the two materials. If a beam of light hits this boundary at
an angle, then light hitting the side first will be forced to slow down or
speed up before light on the other side hits the new material. This
causes the beam to bend, or refract, at the boundary. Suppose we were
to place a coin in a glass of water. The light bouncing off the coin
underwater, for instance, would have to first travel through the water
and then the air to reach an observer's eye. At the boundary, it gets
refracted and reaches the observer's eye, thus appearing to be slightly
raised. (Fig 2)
11. Refraction by practical
methodApparatus:
A drawing board, rectangular glass slab, office pins, sheet
of white paper, a protractor and sharply pointed pencil.
About the experiment:
Materials Req.:
A rectangular slab of glass, a laser pointer, a few sheets of
paper, a sharp pencil, a ruler, and a protractor.
12. PQRS represents a glass slab. Consider that a ray of light enters
the glass slab along AE. It means that light is travelling from a
rarer medium (i.e., air) to glass which is denser medium. Thus
the refracted ray bends towards the normal making r<i.
At the other face of the slab, the ray EF while travelling
through glass meets the surface SR of air which is a rarer
medium . It emerges out along FD ,bending away from the
normal.The ray FD is known as the emergent ray .
The angle which the emergent ray makes with the normal at
the point of emergence is called the angle of emergence and
is denoted by the letter E
13. Fix a sheet of white paper on a drawing board with drawing pins.
Place the given glass slab nearly in the middle of the sheet.
Mark the boundary of the glass slab with a sharp pencil and label it
as PQRS after removing the slab from its position.
On the line PQ mark a point E and draw a normal N1EN2 at it. Draw
a line AE making angle AEN1 with the normal.The angle should
neither too small nor too large (say about 40 degree).
Now place the glass slab again on its boundary PQRS and fix two
pins A and B vertically about 10 cm apart on the line AE (say points
A and B).
Look through the glass slab along the plane of the paper from the
side SR and move your head until the images of the two pins A and
B are seen clearly. Closing your one eye ,adjust the position of your
head in such a way that the images of the pins A and B lie in the
same straight line.
Procedure :
14. Fix two other pins C and D vertically in such a way that the images
of the pins A and B and pins C and D, all these four, lie in the same
straight line. Ensure that the feet of the pins ( not their heads ) lie
in the same straight line.
Remove the slab and also the pins from the board and encircle the
pin-pricks on the paper,with a sharp pencil.
Join the points D and C and produce the line DC towards the slab so
that it meets the boundary line RS at the point F. Join the points e
and F. Thus for the incident ray represented by line AE, the
refracted ray and the emergant ray are represented by EF and FD
respectively.
On the line RS draw a normal N1'FN2' at point F. Now, with a
protractor, measure angle AEN1, angle FEN2 and angle DFN2'
labelled as angle i, angle r and angle e respectively.
Now place the glass slab at some other position on the sheet of
paper fixed on the board and repeat all the above steps again taking
another angle of incidence.
15. Measure the angle of incidence i.e angle of refraction, angle of
emergence, again.
Make a record of your observations in the observation table as
shown below.
Observation Table :
16. History of the Refractive Index
Thomas Young
Thomas Young was presumably the person who first
used, and invented, the name "index of refraction", in
1807. At the same time he changed this value of
refractive power into a single number, instead of the
traditional ratio of two numbers. The ratio had the
disadvantage of different appearances. Newton, who
called it the "proportion of the sines of incidence and
refraction", wrote it as a ratio of two numbers, like "529
to 396" (or "nearly 4 to 3"; for water). Hauksbee, who
called it the "ratio of refraction", wrote it as a ratio with
a fixed numerator, like "10000 to 7451.9" (for urine).
Hutton wrote it as a ratio with a fixed denominator,
like 1.3358 to 1 (water).
Young did not use a symbol for the index of refraction, in 1807. In the next
years, others started using different symbols: n, m, and µ. The symbol n
gradually prevailed.
17. The Refractive Index
In optics the refractive index or index of refraction n of
an optical medium is adimensionless number that describes
how light, or any other radiation, propagates through that
medium. It is defined as
where c is the speed of light in vacuum and v is the phase
velocity of light in the medium.
18. A ray of light that travels obliquely from one transparent medium
into another will change its direction in the second medium. The
extent of the change in direction that takes place in a given pair of
media is expressed in terms of the refractive index, the “constant”
appearing on the right-hand scale of Eq. ,
sin i / sin r = constant .
The refractive index can be linked to an important physical
quantity, the relative speed of propagation of light in different media. It
turns out that light propagates with different speeds in different media.
Light travels the fastest in vacuum with the highest speed of 3.108ms-
1. In air, the speed of light is only marginally less, compared to that in
vacuum. It reduces considerably in glass or water. The value of the
refractive index for a given pair of media, as given below.
19. Consider a ray of light travelling from medium 1 into medium 2, as
shown in the fig 4. Let v1 be the speed of the light in medium v2 be the
speed of light in medium 2. The refractive index of medium 2 with respect
to medium 1 is given by the ratio of the speed of light in medium 1 and the
speed of light in medium 2. This is usually represented by the symbol n21.
This can be expressed in an equation form as
Fig 4
n21 = Speed of light in
medium 1 / Speed of light
in medium 2
= v1 / v2
By the same argument,
the refractive index of
medium 1 with respect to
medium 2 is represented as
n12.. It is given by,
20. n12 = Speed of light in medium 2 / Speed of light in medium 1
= v2 / v1
If medium 1 is vacuum or air, then the refractive index of
medium 2 is considered with respect to vacuum. This is called the
absolute refractive index of the medium. It is simply represented as n2. .
If c is the speed of light in air and v is the speed of light in the medium,
then, the refractive index of the medium nm is given by
nm = Speed of light in air / Speed of light in the medium
= c / v
The absolute refractive index of a medium is simply called its
refractive index. The refractive index of several media is given in Table
1. From the Table you can know that the refractive index of water,
nw = 1.33. This means that the ratio of the speed of light in air and the
speed of light in water is equal to 1.33. Similarly, the refractive index of
crown glass, ng =1.52. Such data are helpful in many places. However,
you need not memorise the data.
22. Note from Table 1 that an optically denser medium may not
possess greater mass density. For example, kerosene having higher
refractive index, is optically denser than water, although its mass
density is less than water.
