3 Light waves

Question 1

What are luminous objects?

Answer 1

Objects that emit their own light.

Question 2

What kind of waves are light waves?

Answer 2

Transverse waves.

Question 3

Can light waves be reflected and refracted?

Answer 3

Yes, all waves can.

Question 4

What is reflection?

Answer 4

Wave reflection occurs when energy traveling in the form of waves through a medium hits an obstacle and then bounces back.

Question 5

What is the law of reflection?

Answer 5

The angle of incidence is equal to the law of reflection.

Question 6

How do you draw a diagram showing reflection?

Answer 6

Question 7

What is the normal line?

Answer 7

It is a line which is drawn at right-angles to a surface.

Question 8

What can you use reflection for?

Answer 8

Periscopes.

Question 9

How do you explain how reflection works in a periscope?

Answer 9

Rays from the object strike the first mirror at a 45* to the normal line, these is then reflected again at 45, creating 90, so they are then reflected onto the second mirror where they again strike at 45* each to create a straight line out.

Question 10

How can you change the direction of a ray of light?

Answer 10

A mirror.

Question 11

What kind of waves reflect and refract?

Answer 11

All kinds of waves.

Question 12

What is refraction?

Answer 12

The change in the direction of a wave passing from one medium to another

Question 13

How do you draw a diagram showing refraction?

Answer 13

Question 14

What happens during refraction?

Answer 14

As a ray enters a glass block, it slows down and is refracted towards the normal line. As the ray leaves the block it speeds up and is refracted away from the normal line.

Question 15

What happens if the incidence ray strikes the boundary between the two media at 90*?

Answer 15

The ray continues without a change of direction.

Question 16

What affects how much the incidence ray bends away from the normal line?

Answer 16

When it travels from a less dense medium to a denser medium.

Question 17

What is the refractive index?

Answer 17

A measure of the change in speed a wave experiences when it travels across the boundary between two media, the index also describes how much the direction changes.

Question 18

What is the equation for the refractive index?

Answer 18

n = sin i / sin r

Question 19

What are the steps you take to investigate the refractive index for a glass block?

Answer 19

• Shine a ray of light onto one of the sides of the glass block, so that the ray emerges on the opposite side of the block. Trace over the line that the ray follows.
• Draw around the glass block before removing it.
• Draw in the direction of the ray that travelled inside the glass block and link it up the the incidence and emergence ray you previously drew before.
• Draw the normal line.
• Measure the angles of incidence and refraction.
• Use the equation n = sin i/sin r to find the refractive index of the glass.
• Plot the results on a graph of sin i to sin r.

Question 20

What are the steps you take to investigate the refractive index for a semi-circular block?

Answer 20

• You are investigating total internal reflection.
• By carefully increasing and decreasing the angle at which the ray strikes the flat edge of the glass block, we can discover the smallest angle at which most of the light is refracted along the edge of the glass block.
• This angle is the critical angle.

Question 21

How do you draw total internal reflection in a semi circular block?

Answer 21

Question 22

What are the steps you take to investigate total internal reflection in a triangular prism?

Answer 22

• If you shine a ray of light into a prism it will strike the surface at an angle of 45. The critical angle for glass is about 42 so the ray will be internally reflected. The two 45* will show that the ray will be reflected through an angle of 90*.

Question 23

How do you draw total internal reflection in a right-angled triangle?

Answer 23

Question 24

How do you draw total internal reflection in a non-right angled triangle?

Answer 24

Question 25

What is total internal reflection?

Answer 25

It is the reflection of the total amount of incident light at the boundary between two media.

Question 26

When can total internal reflection take place?

Answer 26

- When rays of light are travelling towards a boundary with a less optically dense medium. - The angle of incidence has to be large than the critical angle.

Question 27

What does less optically dense mean?

Answer 27

It means that the medium has a lower refractive index.

Question 28

How can you draw total internal reflection?

Answer 28

Question 29

What is the critical angle?

Answer 29

It is the angle at which light is no longer able to pass through the boundary between two mediums and instead gets completely reflected back into the original medium.

Question 30

How can you draw the critical angle?

Answer 30

Question 31

How do you draw what happens when i is equal to the critical angle?

Answer 31

Question 32

What is an optical fibre?

Answer 32

Is a very thin piece of fibre composed of two different types of glass. The centre is made of a glass that has a higher refractive index surrounded by a glass with a lower refractive index. This is to ensure that the light doesn't escape.

Question 33

How do optical fibres work?

Answer 33

Due to the fact that the fibres are so narrow, light entering the inner core always strikes the boundary of the two glasses at an angle that is greater than the critical angle (meaning total internal reflection will always occur).

Question 34

Why does the optical fibre have glass with a lower refractive index around it?

Answer 34

To ensure that the light doesn't escape.

Question 35

What things use total internal reflection in everyday life?

Answer 35

- Binoculars - Endoscopes - Bicycle or car reflectors

Question 36

How does total internal reflection happen in prisms?

Answer 36

Light entering the prism is totally internally reflected twice. The light emerges from the prism travelling back in the direction from which it originally came.

Question 37

What is the equation used to find the critical angle?

Answer 37

sin c = 1/n

Question 38

What kind of waves are sound waves?

Answer 38

Longitudinal.

Question 39

What does the type of wave sound waves are tell us about it?

Answer 39

That they can be reflected and refracted.