It covers the topics-refraction ,absolute and relative refractive index,laws of refraction ,direction of bending of light,No refraction cases,refraction through glass slab

1. LEARNING OBJECTIVES --- VIDEO 5
•REFRACTION OF LIGHT
•ABSOLUTE REFRACTIVE INDEX
•RELATIVE REFRACTIVE INDEX
•CAUSE OF REFRACTION
•LAWS OF REFRACTION
•DIRECTION OF BENDING OF LIGHT
•CONDITION FOR NO REFRACTION
•REFRACTION THROUGH GLASS SLAB

2. REFRACTION OF LIGHT
• The phenomenon of change in the path of light in going from one medium
to another is called refraction of light. OR
• It is the phenomenon of bending of light from its original path on entering
another medium.
• Refraction occur right at the boundary of the two media.

3. REFRACTION OF LIGHT
• The angle which the incident ray makes
with the normal is called angle of incidence.
It is represented by <i.
• The angle which the refracted ray makes
with the normal is called angle of refraction.
It is represented by <r.
• The angle formed between the extended
incident ray and refracted ray is called the
angle of deviation.
• In case of refraction ,the angle of refraction
is never equal to angle of incidence. One may
be smaller or larger than other.

4. VELOCITY OF LIGHT CONCEPT OF REFRACTIVE INDEX OF AN
OPTICAL MEDIUM
• The speed of light in vacuum is a fundamental constant of nature. It is
represented by c.
• Speed of light in air and vacuum is 3*10^8 m/s.
• A transparent substance in which light can travel is called an optical
medium. E.g. Air, Water, Alcohol ,Glass etc.
• A medium in which speed of light is more is called optically rarer medium.
• A medium in which speed of light is less is called optically denser medium.
• Light travels faster in an optically rarer medium and light travels slower in
an optically denser medium.
• Speed of light in air=3*10^8 m/s , in water=2.25*10^8 m/s , in
glass=2*10^8 m/s.
• So air is an optically rarer medium compared to water and glass and glass is
an optically denser medium compared to air and water.

5. ABSOLUTE REFRACTIVE INDEX OF A MEDIUM
• It is defined as the ratio of speed of light in vacuum to the speed of light in
the medium. It is represented by n. Simply called refractive index.
• As refractive index is a ratio of two velocities, it has no units.
• Refractive index of glass nglass =speed of light in air /speed of light in glass
• =3*10^8/2*10^8=3/2=1.5
• Refractive index is a characteristic property of the medium, whose value
depends only on nature of material of the medium and the colour or
wavelength of light.

6. RELATIVE REFRACTIVE INDEX
• When light passes from one medium 1 to another 2,the refractive index of
medium 2 with respect to medium 1 is written as n21 and is called relative
refractive index.
n21=n2/n1 =(c/v2 )/(c/v1 ) =v1/v2
n21=speed of light in medium 1/speed of light in medium 2
• Refractive index of medium 1 with respect to medium 2 is
n12 =n1/n2 =(c/v1)/(c/v2) =v2/v1
On multiplying—
n21 * n12 =v1/v2 * v2/v1 =1
• n21=1/n12

7. CAUSE OF REFRACTION
• Basic cause of refraction is the change in the speed of light in going from
one medium to the other.
• E.g. when a ray of light travelling in air enters into glass , the speed of light
decreases. Therefore , bending of light or refraction of light occurs at the
interface of air and glass.
• Similarly ,when a ray of light travelling in glass enters into air, the speed of
light increases. Therefore , bending of light occurs at the interface of glass
and air.
• The angle of bending of a ray would depend upon difference in speeds of
light in the two media .Larger the difference in speeds of light, greater will
be the angle of bending and vice-versa.

8. LAWS OS REFRACTION
• There are three laws of refraction:----
1. Whenever light goes from one medium to another, the frequency of light
does not change. However the velocity of light and the wavelength of light
change.
2. The incident ray, refracted ray and normal to the interface of two media at
the point of incidence , all lie in the same plane.
3. The product of refractive index and sine of angle of incidence at a point in
a medium is constant.
n sin i = constant
For two media in contact,
n1 sin i1=n2 sin i2 i1=i and i2=r
n1 sin i=n2 sin r
sin i/sin r =n2/n1 =n21 (refractive index of medium 2 w.r.t. medium 1)
i.e the ratio of sine of angle of incidence to the sine of angle of refraction is
constant for the pair of media in contact.
This is called Snell’s law of refraction .

9. THE DIRECTION OF BENDING OF LIGHT
• CASE 1: In going from a rarer to a denser medium.
Let n1=nR=refractive index of rarer medium.
n2=nD=refractive index of denser medium
Acc. to Snell’s law of refraction
sin i/sin r = n2/n1 = nD/nR >1
sin i > sin r or i > r
The angle of refraction is smaller than the
angle of incidence. Therefore , the refracted
ray bends towards normal .
So, When light travels from a rarer medium to a denser medium, it bends
towards normal at the interface of two media.

10. DIRECTION OF BENDING OF LIGHT
• CASE 2: In going from a denser to a rarer medium.
Let , n1=nD and n2=nR
Acc. to Snell’s law
sin i/sin r = n2/n1 =nR/nD <1
sin i < sin r or i < r
i.e. angle of refraction is larger than the angle of
incidence. Therefore the refracted ray bends
away from normal.
So, When light travels from a denser to a rarer medium , it bends away from
normal to the interface of two media.

11. CONDITION FOR NO REFRACTION
• CASE 1: When light is incident normally on a
boundary.
Acc. To Snell’s law
sin i /sin r = n2/n1
or sin r =n1/n2 *sin i
sin r=n1/n2 *sin 0 = 0
So, no refraction occurs when light is incident
normally on a boundary of two media.

12. CONDITION FOR NO REFRACTION
• CASE 2: When the refractive indices of two
media are equal.
• When refractive index of medium 1 is equal to
refractive index of medium 2 i.e n1=n2, then
sin i/sin r = n2/n1 = 1
sin i =sin r
or i = r with <i =<r ≠ 0
and n1 =n2 = n
• No refraction occurs at the boundary that
separates two media of equal refractive
indices.

13. TWO REFRACTIONS THROUGH A RECTANGULAR GLASS SLAB
• In the rectangular glass
slab refraction occurs at
two points , point O and
O’.
• At point O, Acc to Snell’s
law sin i/sin r1 =ng /na (1)
• At point O’, Acc to Snell’s
law sin r/sin e = na /ng (2)
• Or sin e/sin r2 = ng /na (3)
• From eq. (1) and (3)
sin e/sin r2 = sin i/sin r1
< r1 = < r2
sin e = sin i e=i

14. REFRACTION THROUGH A GLASS SLAB
• Angle of emergence e at secondary boundary of glass slab is equal to angle
of incidence i at first boundary of glass slab.
• Light emerges from rectangular glass slab in a direction parallel to that in
which it entered the glass slab .
• LATERAL DISPLACEMENT:
• The sideways shift of the emergent ray from the direction of original
incident ray is called Lateral Displacement. It is represented by x.

15. SUMMARY
• REFRACTION OF LIGHT
• ABSOLUTE REFRACTIVE INDEX
• RELATIVE REFRACTIVE INDEX
• CAUSE OF REFRACTION
• LAWS OF REFRACTION
• DIRECTION OF BENDING OF LIGHT
• CONDITION FOR NO REFRACTION
• REFRACTION THROUGH GLASS SLAB