# Light as a Wave Flashcards

1
Q

What is Huygen’s principle?

A

That all waves consist of smaller wavelets.

They propagate a distance of the wavelength after one period, and create the next wave front.

2
Q

What is refraction?

A

When the direction of light changes due to a change in the velocity.

3
Q

When does refraction occur?

A

When light travels from one medium to another.

As light travels from a medium from a higher refractive index to a lower (water-air) it bends away from the normal, and vice-versa.

4
Q

What is the refractive index?

A

The ratio of the speed of light in a vacuum to the speed of light in a medium.

n = c/v

5
Q

What is dispersion and when does it occur?

A

It occurs due to refraction when white light passes through a prism.
When this happens, the light is split into all visible light ROYGBIV.

6
Q

What is diffraction? What is the ratio for diffraction?

A

The bending of waves/light when it travels through slits.
This creates interference patterns which can be constructive (bright spot) or destructive (dark spot).

Diffraction = λ/w

7
Q

Explain what happens with diffraction with the ratio of λ/w.

A

As the λ increases, diffraction increases.
As the λ decreases, diffraction decreases.

As the w increases, diffraction decreases.
As the w decreases, diffraction increases.

8
Q

What is polarisation? How does it support light being a wave?

A

Where a transverse wave can only vibrate in 1D.
Light waves are EM waves (electro-magnetic).
Since light can be polarised, it must mean that it’s a transverse wave.

9
Q

Which of the 2 competing models does Young’s Double Slit Experiment support?

A

The wave model.

10
Q

Why do the results of the experiment support Young’s model?

A

Because an interference is caused by

• Alternating constructive & destructive interference
• There’s diffraction through the two slits

Interference and diffraction are both wave behaviours.

11
Q

Why do the results of the Young’s experiment not support the competing model?

A

If light behaved as a particle, then we’d only expect to see 2 bright spots on the screen rather than an interference pattern.

12
Q

When is light produced?

A

When charged particles accelerate. They consist of an electric field which in turn creates a changing magnetic field. Hence, light is an EM wave.

13
Q

With reference to interference, explain why the central band observed in Young’s Double Slit experiment is bright and not dark.

A
• The light is diffracted as it comes through the slits, acting like 2 coherent sources of waves.
• Constructive interference occurs where crests and crests overlap, destructive where other.
• Since the central point is equidistant from both slits, it will be constructive and bright.