Waves, sound and lightp. 167

Speed of light

Speed of light is constant in a medium.c = 3 x 108 m.s-1 in a vacuum.

Refraction

The change in direction of a light ray due to

a change in speed when light travels from

one medium into another of different optical

density.

RefractionWhen waves move at an angle from one medium to another: frequency remains constant, wavelength decreases and speed decreases.

A measure of the refracting ability of

a medium.

The higher the optical density, the more the

light will be refracted or slowed down as it

moves through the medium.

Optical density

The speed of light changes when it moves from one medium to another. We call the relative speed of light in different media,

the optical density of the medium.

Refractive index (n) is the ratio of the speed of light in a vacuum (c)

versus the speed of light in the medium (v).

n = 𝑐

𝑣

When v increases, n decreases.

A material with higher optical density,

will have a higher refractive index.

Optical density and refractive index

Incident ray Normal

Normal:

The line which is perpendicular to the plane of the surface.

Incident ray Normal

θi

Angle of incidence (θi):

The angle between the normal to the surface and the incident ray.

Incident ray

Emergent

ray

Normal

θi

Refracted ray

θr

θr

Angle of refraction(θᵣ):

The angle between the normal to the surface and the refracted light ray.

Refraction of monochromatic light through a triangular prism

Incident ray

Refracted ray

Emergent ray

Angle of deviation (d)

θi θrθr θi

A light ray that moves from a lower density toa higher density is refracted towards the normal.

A light ray that moves from a higher density toa lower density is refracted away from the normal.

Draw a light ray through a medium:

Power gym

Exercise 2, p. 1772, 3, 4

Please write today's date next to number 2

2. Draw a ray diagram with complete labels to illustrate the path of a light ray that enters the surface at an angle for a …

2.1 rectangular prism

Ray of

incidence

Emergent

ray

Normal

θi

Refracted

ray

θr

θr

2. Draw a ray diagram with complete labels to illustrate the path of a light ray that enters the surface at an angle for a …

2.2 triangular prism

Ray of incidence

Refracted ray

Ray of emergence

Deviation angle

θi θrθr θi

3. Five parallel light rays enter a glass sphere as indicated in the diagram. Draw the path the light rayswill follow through the sphere.

4. Complete each of the following diagrams to illustrate the path of the light ray:

4. Complete each of the following diagrams to illustrate the path of the light ray:

4. Complete each of the following diagrams to illustrate the path of the light ray:

4. Complete each of the following diagrams to illustrate the path of the light ray:

Snell's law

θi

θr

θi

θr

sinθi

sinθr

nisinθi = nrsinθr

𝑠𝑖𝑛𝜃𝑖𝑠𝑖𝑛𝜃𝑟

=𝑛𝑟𝑛𝑖

Snell's law - example

A ray of light shines onto a piece of glass (n = 1,5) at an angle of 48o. Calculate the angle of refraction of the light ray in glass.

nisinθi = nrsinθr

1sin48o = 1,5sinθr

sinθr = 0,495429883θr = 29,7o

Power gym

Exercise 2 (p. 177)12, 13, 14, 16, 17, 18, 19, 20

Write today's date next to number 12

12.1 The speed of light in ice is 2,29 x 108 m.s-1. Calculate the refractive index of ice.

12.2 Calculate the refractive index of a glass bead if the speed of light in it is 2 x 108 m.s-1.

13.1 Arrange the substances in this table in order of increasing optical density.

Air, plastic,Perspex, glass, diamond

13.2 Between glass or diamond:

13.2.1 Through which does light move faster?

Glass

13.2 Between glass or diamond:

13.2.2 Which refracts light the most for the same angle of incidence?

Diamond

13.2 Between glass or diamond:

13.2.3 Which is the optically more dense substance?

Diamond

13.3.1 Calculate the speed of light in glass.

v = 1,97 x 108 m.s-1

13.3.2 Calculate the speed of light in diamond.

v = 1,24 x 108 m.s-1

2,42

13.4 The refractive index of air is 1,003. Use the definition of refractive index to compare thespeed of light in a vacuum with its speed in air.

The greater the refractive index, the less the speed of light in the medium.

n = 𝑐

𝑣

RedOrangeYellowGreenBlueIndigoViolet

Visible light

Light can be reflected, absorbed or transmitted.

Reflection:

When an obstacle is placed in the way of moving waves, the waves are reflected.

Law of reflection:

When light is reflected, the angle of incidence is always equal to the angle of reflection.

Reflection Normal

Incident ray Reflected ray

Angle of incidence

θi

Angle of reflection

θr

Critical angle When light travels from a higher optical density

to a lower optical density, refraction takes place.

When the angle of refraction is 90o, we call

the angle of incidence the critical angle.

Critical angle:

The angle of incidence in the optically more dense medium where the angle of refraction in the optically less dense medium is 90°.

Normal

Critical angle

θi

θr

Total Internal Reflection

• The angle of incidence

must be greater than the

critical angle.

• Movement has to be from

an optically more dense to

an optically less dense

medium, and

Applications

Periscopes

Binoculars

Optical fibres

Endoscopes

Power work

Exercise 1 (p. 166)5 & 6

Please write today’s date next to number 5.

5. Calculate the magnitudes of the angle of incidence and the angle of reflection for each of the following:

5.1

40o

Angle of incidence = Angle of reflection =

50o

50o

5. Calculate the magnitudes of the angle of incidence and the angle of reflection for each of the following:

5.2

25o

Angle of incidence = Angle of reflection =

65o

65o

5. Calculate the magnitudes of the angle of incidence and the angle of reflection for each of the following:

5.3

75o

Angle of incidence = Angle of reflection =

15o

15o

6. Use diagrams to explain the difference between regular reflection and irregular reflection:

Regular Irregular

Power work

Exercise 3 (p. 187)4, 5, 8, 12, 14, 15, 16

Please write today’s date next to number 4.

4. A light ray originating from a watertight torch travels from the bottom of a bowl of water. Drawlight ray diagrams to indicate the path of the light ray with the angle of incidence …(critical angle of water = 49o)

4.1 less than 49o

i < 49o

4. A light ray originating from a watertight torch travels from the bottom of a bowl of water. Drawlight ray diagrams to indicate the path of the light ray with the angle of incidence …(critical angle of water = 49o)

4.2 equal to 49o

i = 49o

4. A light ray originating from a watertight torch travels from the bottom of a bowl of water. Drawlight ray diagrams to indicate the path of the light ray with the angle of incidence …(critical angle of water = 49o)

4.3 greater than 49o

i > 49o

5. The diagram shows three lightrays travelling from a point P at the bottom of a large glasscontainer of water, to the surfaceof the water.

5.1 Which phenomenon takes place when the light ray reaches theboundary surface?

Y

P

A

W

B G

T

Refraction

5. The diagram shows three lightrays travelling from a point P at the bottom of a large glass container of water, to the surfaceof the water.

5.2 What special name is given tothe angle of incidence at B?

Y

P

A

W

B G

TCritical angle

5. The diagram shows three light rays travelling from a point P at the bottom of a large glasscontainer of water, to the surface of the water.

5.3 Explain why the light ray PGT follows this path.

Y

P

A

W

B G

TThe angle of incidence is greater than the critical angle and the light travels from an optically more dense to a less dense medium, therefore total internal reflection takes place.

5. The diagram shows three lightrays travelling from a point P at the bottom of a large glasscontainer of water, to the surface of the water.

5.4 How would the bottom of thecontainer appear if viewed from W into the water?

Y

P

A

W

B G

T

Shallower

5. The diagram shows three light rays travelling from a point P at the bottom of a large glasscontainer of water, to the surfaceof the water.

5.5 Draw a diagram to explain theobservation in 5.4 above.

A

W

5. The diagram shows three lightrays travelling from a point P atthe bottom of a large glass container of water, to the surface of the water.

5.6 In which direction does thelight ray have to leave the bottom at P in order not to undergo any change indirection?

Y

P

A

W

B G

T

Y

5. The diagram shows three light rays travelling from a point P at the bottom of a large glass container of water, to the surface of the water.

5.7 Where will point P appear tobe if viewed from T?

Y

P

A

W

B G

TAbove the water

8.1

Air

Oil

8.2

8.3

Eye

8.4

Incident ray must move from optical more dense to optical less dense medium. The angle of incidence must be greater than the critical angle.

8.5

12.

14.1

14.2

Mirrors require a metal layer to reflect light and can be scratched. Prisms are more durable and are not scratched as easily, since they do not require any layer on the reflective surface.

15.1

Total internal reflection

15.2

15.3

Sharper image, more visible

15.4

Pluming, inspection of welded pipes, computer cables

15.5