P5 - Diffraction patterns & Polarisation

Question 1

When do you get interference patterns?

Answer 1

• You get interference patters when waves of equal frequency or wavelength overlap.

Question 2

What happens when a wave-front passes through a gap?

Answer 2

When a wave-front passes through a gap, light from each point along the gap diffracts.

Question 3

What is needed for maximum diffraction from light?

Answer 3

• The gap must be about the same size as the wavelength of the light, otherwise the light won’t be diffracted very much.

Question 4

How do you get interference patterns from diffraction of light?

Answer 4

• Diffracted light from each of these points interferes with light diffracted from all the other points, so you get an interference pattern even from just one slit.
• The pattern has a bright central fringe with an alternating dark and bright fringes on either side of it.

Question 5

What are the two theories that explain light’s nature?

Answer 5

• Wave theory ( Christiaan Huygens )

* Particle theory ( Isaac Newton )

Question 6

What did the particle theory of light explain?

Answer 6

• Particle theory of light could explain reflection and refraction, but diffraction and interference are both unique to waves.

Question 7

What did Thomas Young’s double slit experiment show?

Answer 7

• It showed that light could both diffract ( through two narrow slits ) and interfere ) to form an interference pattern on a screen ).

Question 8

What was the double slit experiment?

Answer 8

• A coherent light source is shone through two slits.
• You get a pattern of light and dark fringes, showing constructive and destructive interference taking place,, which shows light behaves like a wave.

Question 9

What is the current accepted idea of the behaviour of light?

Answer 9

• It is now accepted that light shows properties of a wave ( diffraction, interference and polarisation ).

Question 10

What are electromagnetic waves?

Answer 10

Electro-magneticc waves are transverse waves - the vibrations are at 90 degrees to the direction of travel of the wave.

Question 11

How can you make a transverse wave?

Answer 11

• You can make a transverse wave by shaking a rope up and down or side to side, or in a mixture of directions.
• Whatever plane you are shaking it in, it is still a transverse wave.

Question 12

What are the only vibrations that will get through a fence?

Answer 12

• The only vibrations that will get through a fence are the vertical ones, the fence filters out vibrations in all other directions, this is called plane polarisation of the wave.

Question 13

What are ordinary light waves?

Answer 13

Ordinary light waves a mixture of vibrations in different directions.

Question 14

What does a polarising filter do?

Answer 14

• Passing the light through a polarising filter is like passing the rope through the fence, the filter only transmits vibrations in one particular direction.

Question 15

What is plane polarised light made of?

Answer 15

• Plane polarised light is made up of vibrations in one direction only.

Question 16

What happens when light is reflected from some surfaces ( like water )?

Answer 16

• When light is reflected some some surfaces, it is partly plane polarised.

Question 17

What do Polaroid sunglasses do?

Answer 17

Polaroid sunglasses act as polarising filters, they can filter out reflected glare from the sea or the snow.