LigLighthtRefraction of

plane

normal normal

center

Normal :

Angle of incidence, i

Angle of refraction, r

incident ray

Ray emergesparallel toincident 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.

• 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.

i

r

i > rsince Dair < Dglass

a

g

a > 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:

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)

Refractive indexRefractive index

Medium Refractive Index

Diamond 2.4

Glass About 1.5

Perspex 1.5

Water 1.33

Ice 1.3

Air 1.0

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.

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).

Apparentdepth

Realdepth

Fish

image of fish

AirWater

Refractive index of water = (Real depth / Apparent depth)

Example:• Find the refractive index of glass when light

travels from air to glass at the angle of incidence of 45o and an angle of refraction of 280.

450

280

Solution:Using n= sin i/ sin r, = sin 450/ sin 280 =1.5The refractive index of glass is 1.5.

• 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.

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 depth1.33 = 1.5 / d d = 1.5 / 1.33 = 1.13 m

Reversibility of light in reflection

Two fig. seen alike,Why ? Explain !

speed v1

speed v2 speed v2

speed v11 1

2 2

n1n1

n2 n2

airwater

airwater

airwater

i

ii = c, critical angle

i < c, critical angle

i > ci

Case 1

Case 2

Case 3

ReflectionWhen 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)

airwater

airwater

airwater

i

ii = c, critical angle

i < c, critical angle

i > ci

Case 1

Case 2

Case 3

When light ray traveled from denser material to less dense material (say, from water to air), if the angle of refraction equal to 900, 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 = 900)

Critical Angle

airwater

airwater

airwater

i

ii = c, critical angle

i < c, critical angle

i > ci

Case 1

Case 2

Case 3

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)

Critical Angle• since

– refracted index

= (sin x) / (sin y)

therefore

n = (sin x)/ (sin y)

when x = 900 and

y = c, the critical angle

Hence sin c = 1 / n

normal

Denser medium

Less densemedium

c=y

x

high density

low density

Optical Fibre ( Group Discussion)

light ray

Mirages

GCE O-LevelGCE O-Level Past Examination PaperPast Examination Paper

Science (Physics)

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 0.71B 1.22C 1.41D 1.50

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 / rB sin i / sin rC sin ( i/r )D sin i x sin r B

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

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

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

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

GCE O Nov 1994

12. Which diagram shows the path of a ray of light through a glass block ?

D

November 1989 9. A prism is made from glass with a critical angle

less than 45o.Which one of the following diagrams, showing light passing through the prism, is incorrect?

C

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 anddispersion produced by the prism?

E Note:not in 1999

syllabus

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

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 refractiveindex of the glass is equal tothe ratio of sin iand sin r.Thus, n = sin i / sin r

(continue on next slide)

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.

P1

P2

•Choose angle of incidence i (=300) and draw incident ray as shown.•Put two pins P1 and P2 on the incident ray as shown in the fig.

(continue on next slide)

iP1

P2

(Cont. …) Q. 10 Nov 1995

r

•10.(b)• From the opposite side , observe the two images of pins P1 and P2 and then insert another two pins P3

and P4, so that the images and these two pins lie in same straight line as shown in the fig.

P3P4

• 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)

(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.

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.

150

250

A B

C Dn = sin i / sin r = sin 250 / sin 150

= 1.63

(continue on next slide)

3(b) light passing thorough the block is partially reflected at two places. Draw, on the diagram both the reflected rays. [3]

250150250

(Cont. …) Q. 3 November 1992

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 ? [1]

Since > 300, therefore median B is the glass

angle = 300

(continue in next slide)

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

Whitelight

Since white lighthas higher frequency thanred light, therefore, (i) PX remains unchanged. (ii) PY will deflect slightly away from normal.