[Unit 9.02] refraction of light

1. LigLightht
Refraction of

2. plane
p
normal
q
normal
center
Normal :

3. Angle of incidence, i
Angle of
refraction, r
incident ray
Ray emerges
parallel to
incident ray
Refracted ray
Refraction of light:
The change in the direction,
or bending of light when it
travels from one medium to
another is known as
refraction.

4. •When light travels from a less dense
medium to a denser medium the speed
of light decreases and the light ray is
refracted towards to the normal.

5. i
r
i > r
since D
air
< D
glass
l
a
l
g
l
a
> l
g
normal
Air
Glass
When light travels
from a less dense
medium to a denser
medium the speed
(or wavelength) of
light decreases and
the light ray is
refracted towards to
the normal.
Refraction
of light:

6. Laws of refraction:
•The incident ray, refracted ray and the
normal at the point of incidence all lie on
the same plane.
•Snell’s law:
When light travels from a first medium to
a second medium, the ratio of the sine of
the angle of incidence to the sine of the
angle of refraction is a constant called the
refractive index.
(continue in next slide)

7. Refractive indexRefractive index
1.0Air
1.3Ice
1.33Water
1.5Perspex
About 1.5Glass
2.4Diamond
Refractive IndexMedium

8. Refractive indexRefractive index
•The greater the refractive index
n, the slower is the speed of light
in that medium and the light
bends more in that medium
•The refractive index cannot be
less than 1.

9. Important Formulae:
n = sin i/ sin r


•n= (speed of light in the first
medium)/(speed of light in the
second medium)
•where n is the refractive index of
the second medium with respect to
the first medium (vacuum).

10. Apparent
depth
Real
depth
Fish
image of fish
Air
Water
Refractive index of water
= (Real depth / Apparent depth)

11. Example:
•Find the refractive index of glass when light
travels from air to glass at the angle of incidence
of 45
o
and an angle of refraction of 28
0
.
45
0
28
0
Solution:
Using n= sin i/ sin r,
= sin 45
0
/
sin 28
0
=1.5
The refractive index of glass is
1.5.

12. •When light travels from a less dense to a
denser medium, the speed of light
decreases and the light ray is refracted
towards the normal.

13. Example:
•The refractive index of water is 1.33. A girl
views the bottom of a pond of depth 1.5m
directly from above. What is the apparent
depth of the pond?
Solution:
refractive index of water
= real depth / apparent depth
1.33 = 1.5 / d
d = 1.5 / 1.33 = 1.13 m

14. Reversibility of light in
reflection
Two fig. seen alike,
Why ? Explain !
speed v
1
speed v
2
speed v
2
speed v
1
q
1
q
1
q
2
q
2
n
1
n
1
n
2
n
2

15. air
water
air
water
air
water
i
i
i = c, critical angle
i < c,
critical angle
i > c
i
Case 1
Case 2
Case 3
C
Reflection
When light ray traveled
from denser material to
less dense material (say,
from water to air), and
angle of incidence,
i < c, the critical angle,
most of the light ray will
be refracted to the less
dense medium and part
of the light will be
reflected.
(case 1)

16. air
water
air
water
air
water
i
i
i = c, critical angle
i < c,
critical angle
i > c
i
Case 1
Case 2
Case 3
C
When light ray traveled
from denser material to
less dense material (say,
from water to air), if the
angle of refraction equal
to 90
0
, then its angle of
incidence at this instant
is defined as critical
angle of the denser
material.
(case 2)
angle i = angle c
(if angle r = 90
0
)
Critical Angle

17. air
water
air
water
air
water
i
i
i = c, critical angle
i < c,
critical angle
i > c
i
Case 1
Case 2
Case 3
C
Total Internal Reflection
When light ray traveled
from denser material to
less dense material (say,
from water to air), and if
the angle of incidence,
i > c, the critical angle,
all the light ray will
reflected internally.
This is called total
internal reflection.
(case 3)

18. Critical Angle
•since
–refracted index
= (sin x) / (sin y)
therefore
n = (sin x)/ (sin y)
when x = 90
0
and
y = c, the critical angle
Hence sin c = 1 / n
normal
Denser medium
Less dense
medium
c=y
x

19. high density
low density
Optical Fibre ( Group Discussion)
light ray

20. Mirages

21. GCE O-LevelGCE O-Level
Past Examination PaperPast Examination Paper
Science (Physics)

22. C
O’ level Physics Nov 1995
12. The diagram shows a ray of light moving from
air into plastic.
What is the refractive index of the plastic ?
A 0.71
B 1.22
C 1.41
D 1.50

23. GCE O Nov 1996
10. A ray of light travels from air into glass. The
angle of incidence is i and the angle of
refraction is r.
How is the refractive index of glass calculated ?
A i / r
B sin i / sin r
C sin ( i/r )
D sin i x sin r B

24. Nov 1991
11. Which line shows the path a ray of light would
take through a glass block?
B
Hint:
Incident ray parallel
to ray B

25. GCE O Nov 1996
11. A ray of red light travelling in glass strike the
glass-air boundary. Some light is reflected and
some is refracted.
Which diagram shows the paths of the ray ?A

26. Nov 1990
7. The bottom surface of a glass block is silvered to
act as a mirror. Which diagram best represents the
path of a light ray that enters this block through the
top surface?
E

27. GCE O Nov 1997
10. The bottom surface of a glass black is silvered
to act as a mirror.
Which diagram could represent the path of a
light ray which enters this block through the
top surface ?
D

28. GCE O Nov 1994
12. Which diagram shows the path of a ray of
light through a glass block ?
D

29. November 1989
9.A prism is made from glass with a critical angle
less than 45
o
.
Which one of the following diagrams, showing
light passing through the prism, is incorrect?
C

30. November 1989
11.When a narrow beam of white light passes through a
prism, it produces a spectrum.
Which one of the
following diagrams
correctly shows the
refraction and
dispersion produced
by the prism?
E
Note:
not in 1999
syllabus

31. Nov 1990
8. Which diagram correctly shows how a prism
can be used to split white light into a spectrum?
(Only the colours at the ends of the spectrum are
marked on the diagrams.)
E

32. 10.(a) Draw a diagram to show a ray of light passing
through from air to glass. Label the angle of
incidence i and the angle of refraction r. Define
refractive index for the glass. [3]
GCE ‘O’ LEVEL Nov 1995
i
r
air
glass
i > r
The refractive
index of the
glass is equal to
the ratio of sin i
and sin r.
Thus,
n = sin i / sin r
(continue on next
slide)

33. 10.(b)Describe an experiment you would carry out to
determine the refractive index of glass, given a
rectangle glass block. State the measurements
you would make and show how you would use
them to calculate the result. [4]
(Cont. …) Q. 10 Nov 1995
i
• label the rectangular
glass block as shown in
the figure.
·
P
1
·
P
2
•Choose angle of incidence
i (=30
0
) and draw incident
ray as shown.
•Put two pins P
1
and P
2
on the incident ray as
shown in the fig.
(continue on next
slide)

34. i
·
P
1
·
P
2
(Cont. …) Q. 10 Nov 1995
r
·
•10.(b)
• From the opposite side , observe the two images of
pins P
1
and P
2
and then insert another two pins P
3
and P
4
, so that the images and these two pins lie in
same straight line as shown
in the fig.
·P
3
P
4
• Hence emergent ray is
obtained, and angle of
refraction also obtained.
• Repeat the above mentioned
steps with 4 different set of i and
then obtained another 4 sets of r.
(continue on next
slide)

35. (Cont. …) Q. 10 Nov 1995
10. (b)
• List the values of i and r in the following table.
angle of incidence, i
angle of refraction, r
• Hence, calculate 5 sets of the ratio sin i / sin r.
• Then the average of the ratios that determined is
the refractive index of the glass.

36. November 1992
3. The diagram shows a ray of light passing
thorough a glass block ABCD.
(a) Calculate the refractive
index of the glass,
showing your working.
15
0
25
0
A B
C Dn = sin i / sin r
= sin 25
0
/ sin 15
0
= 1.63
(continue on next slide)

37. 3(b) light passing thorough the block is partially
reflected at two places. Draw, on the diagram
both the reflected rays. [3]
25
0
15
025
0
(Cont. …) Q. 3 November 1992

38. Nov 1990
4. The diagram below shows a ray of red light
approaching a point P on the boundary between glass
and air.
a)Which medium, A or B, is the glass? [1]
b)What is the size of the angle a? [1]

Since b > 30
0
, therefore median B is the
glass
angle a = 30
0
(continue in next slide)

39. 4(c) The incident beam was then changed to white
light.
What effect would this have on the light rays
(i) PX,
(ii) PY? [2]
(Cont. …) Q. 43 Nov 1990
White
light
Since white light
has higher
frequency than
red light,
therefore,
(i) PX remains
unchanged.
(ii) PY will deflect slightly away from normal.