Light Waves

Question 1

How would you measure the critical angle experimentally?

Answer 1

1. Direct a lazer light at a glass box (a light bulbs light would disperse).
2. Measure the angle of the incidence from the normal using a protractor.
3. Measure the angle of refraction from the normal using a protractor.
4. Then use equation n= sini / sinr

Question 2

What is total internal reflection?

Answer 2

When light goes from one medium to a more optically dense medium (e.g. Air to glass) some light is reflected and some light is refracted.

2. When the angle of refraction is 90* to the normal, the angle of incidence is called the critical angle.
3. Therefore when the angle of incidence is greater than the critical angle all the light is reflected and this is called Total Internal Reflection.

Question 3

What is the critical angle?

Answer 3

An angle of incidence where the angle of reflection is 90* and is when total internal reflection occurs.

Question 4

What is Snell’s Law?

Answer 4

n= sini / sinr

n is Refractive Index

Question 5

How do you figure out the critical angle for a particular medium?

Answer 5

sinc = 1 / n

Question 6

What is a virtual image?

Answer 6

Images through which rays of light do not actually pass.

Question 7

What is a real image?

Answer 7

Images created when rays of light actually pass through them.
Image produced on screen is real.

Question 8

How is TIR used in every day life?

Answer 8

• optical fibres: modern telecommunications and an endoscope (medicine).
• light enters and coating of different (less optical dense glass) density to the inner core, light is TIR in optical fibre and the light stays in the fibre and can pass message/illuminates the object to be viewed, light reflected by object travels up second bundle of fibres and an image of the object is created by the eyepiece.
• large numbers of these fibres fixed together form a bundle.

Question 9

What are the properties of an image in a plane mirror?

Answer 9

The image is a far behind the mirror as the object is in front.
The image is the same size as the object.
The image is virtual (cannot be produced on a screen)
The image is laterally inverted (the left side and the right side of the image appear to be interchanged).

Question 10

What happens when white light passes through a prism?

Answer 10

1. A spectrum is formed because white light is a mixture of colours and each colour travels through the prism at a slightly different speed, so each colour is refracted by a different angle
2. The prism has a different refractive index for each colour. Because of this each of the colours emerges from the prism travelling in a slightly different direction. This process is called dispersion.
3. The speed of red light changes the least so it has the smallest deviation.
4. The speed of violet light is changed the most and so violet light has the largest deviation.
5. Can’t use a rectangular block as it has parallel boundaries so the rays bend by the same amount when they leave the block as when they entered so the rays emerge parallel, but in a triangular prism the boundaries are not parallel so the different wavelengths don’t emerge parallel.

Question 11

How can you experiment with refraction/ find the refractive index of glass using a glass box?

Answer 11

1. Shine a light ray at a glass block at an angle, and some of the light is reflected but some is also refracted.
   2.Trace the incident and emergent ray.
2. Draw in the refracted ray through the block by joining the end of the other two rays with a straight line.
   (Draw in the normal at 90 degrees to the edge of the block, at the point where the rays enter the block)
3. Use a protractor to measure the angles between the rays and the normal (angle of incidence and angle of refraction)
4. The work out the refractive index of the material in the block by using Snell’s Law

Question 12

Explain how optical fibres work

Answer 12

1. Optical fibres made out of a central core of an optically dense material plastic or glass consist (with a high refractive index) surrounded by cladding that has a lower refractive index
2. The core of the fibre is so narrow that light signals passing through it always hit the core-cladding boundary at angles higher than the critical angle, so the light is always totally internally reflected
3. It only stops working if the fibre is bent too sharply
   - E.G. Endoscopes, used by doctors to see inside of body and by engineers to see inaccessible parts of machinery
4. Large numbers of these fibres fixed together form a bindle and bundles can carry sufficient light for images of objects to be seen through them
5. If the fibres are tapered it is also possible to produce a magnified image

Question 13

How do you use semicircular blocks to show total internal reflection?

Answer 13

1. Aim the incident light ray at the curved edge of the box so that it always enters at right angles to the edge
2. This means it does not bend as it enters the block only when it leaves from the straight edge
3. To investigate the critical angle, C, mark the positions of the rays and the block on paper and use a protractor to measure i and r for different angles of incidence. Record your results in a table

Question 14

What happens if the angle of incidence is less than the critical angle?

Answer 14

Most of the light passes out but a little bit of it is internally reflected

Question 15

What happens if the angle of incidence is equal to the critical angle?

Answer 15

The emerging ray comes out along the surface. There is quite a bit of internal reflection

Question 16

What happens if the angle of incidence is greater than the critical angle?

Answer 16

No light comes out, it is all internally reflected (total internal reflection)

Question 17

What is the law of reflection?

Answer 17

The angle of incidence equals the angle of reflection

Question 18

How can optical fibres be used to see something?

Answer 18

1. Light travels down one bundle of fibres and illuminates the object to be viewed
2. Light is reflected by the object travels up a second bundle of fibres
3. An image of the object is created by the eyepiece

Question 19

Why do triangular prisms disperse White light?

Answer 19

1. White light is a mixture of colours
2. When white light passes through a prism it emerges as a band of colours called a spectrum
3. A spectrum is formed since the different wavelengths of light refract by different amounts, since each colour travels through the prism at a slightly different speed so each colour is refracted by a different angle, so the white light disperses into different colours as it enters a prism (A spectrum)
4. The prism has a different refractive index for each colour
5. Because of this, each of the colours emerges from the prism travelling in a slightly different direction (process called dispersion)

Question 20

Why does white light not disperse with a rectangular block?

Answer 20

1. A rectangular block has parallel boundaries, so the rays bend the same amount when they leave the block as when they entered, so the rays emerge parallel
2. But with a triangular prism the rays boundaries are not parallel which means the different wavelengths do not emerge parallel and so you get a nice rainbow effect

Question 21

How is total internal reflection used in prisms?

Answer 21

1. The prismatic periscope
2. Light enters and passes normally through the surface AB of the first prism, and enters the prism at 90 degrees and so is undeviated
3. The light then strikes the surface AC of the prism at 45 degrees
4. The critical angle of the glass is 42 degrees
5. Therefore the light is totally internally reflected and is turned through 90 degrees
6. Therefore the light continues to travel until the second prism where it strikes another barrier DE at 45 degrees and the refractive index of the glass is still 42 degrees
7. As a result the light is totally internally reflected again and so the ray emerges parallel to the direction it was originally travelling
   - The first image created by this type of periscope is likely to be sharper and brighter than that produced by the periscope that uses two mirrors, as no multiple images are created

Question 22

What is another way that total internal reflection is used in prisms?

Answer 22

1. Reflectors
2. Light entering the prism undergoes TIR twice
3. The light then emerges from the prism travelling back in the direction from which it originally came (After TIR the light travels back towards the source (e.g. car headlights))
4. This arrangement is used in bicycle reflectors and binoculars

Question 23

How can you experimentally investigate the refraction of light using a triangular prism?

Answer 23

1. Shine a source of white light at a glass prism

Question 24

How can you experimentally investigate the refraction of light using a rectangular block?

Answer 24

1. Place a rectangular block of a particular material e.g. glass on top of a piece of paper
2. Shine a light ray at an angle into the block and some of the light is reflected but a lot passes through the glass and gets refracted as it does so
3. Trace the incident and emergent rays onto the piece of paper and remove the block
4. You can draw in the refracted ray through the block by joining the ends of the other two rays with a straight line
5. You should see that as the light passes from the air into the block (a denser material) it bends towards the normal. This is because it slows down
6. When the light reaches the boundary on the other side of the block it is passing into a less dense medium and so it speeds up and bends away from the normal (some of the light is also reflected at this boundary)
7. The light ray that emerges on the other side of the block is now travelling in the same direction it was to begin with, it has been refracted towards the normal and the back again by the same amount
8. You can measure the angles between the rays and the normal to work out the refractive index of the material in the block

Question 25

Describe how waves can be refracted

Answer 25

1. Waves travel at different speeds in substances which have different densities 2. EM waves travel more slowly in denser media (usually) 3. Sound waves travel faster in denser substances 4. So when a wave crosses a boundary between two substances, from glass to air say it changes speed 5. If the wave hits the boundary 'face on' it slows down but carries on in the same direction 6. But if a wave meets a different medium at an angle, this part of the wave hits the denser layer first and slow down, while this part carries on at the first, faster speed 7. So the wave changes direction - it has been refracted