Physics - Waves

What property of a wave doesnt change when moving from one medium to another?

Frequency

What is the purpose of an outer sheath in an optical fibre?

To prevent physical damage like scratching to the fibre.

What does it mean when two sources are coherent?

They have the same frequency and wavelength and are kept at a constant phase difference.

How are bright fringes formed on a screen from a double slit experiment?

Superposititon of the two waves which undergo constructive interference at the screen. They meet in phase, with a whole number of wavelengths path difference.

How can you obtain an accurate value for wavelength by changing the apparatus for the double slit experiment.

Increase the distance between the slits and the screen. Measure w across more than 2 maxima. This is because as you increase D, you increase W so it makes it easier to measure across multiple fringes. Percentage error will decrease.

What does a progressive wave transfer?

Energy without transferring material by utilising vibrating particles/fields.

How is a stationary wave produced?

Two progressive waves of the same frequency and amplitude travel in opposite directions and superposition of both waves happens. This produced reigons of no displacement called nodes.

What can two polarising filters kept a the same angle do to the light passing through?

Reduce the intensity.

Why is the central fringe always the brightest?

There is always no path difference so constructive interference occurs.

Why do the maxima get dimmer further away from the central maximum?

The intensity of light decreases with distance form the slits.

What happens to the intererence pattern when the opening of the slit is comparable to the wavelength?

The central maximum becomes wider but less intensity.

Why does refraction occur?

Light travels at different speeds in materials with different densities.

What is needed for TIR to occur?

Refractive index of first medium must be greater than that of the second medium at the boundary. The angle of incidence must be greater than the critical angle.

What happens if the light ray enters at an angle equal to the critical angle?

It will be refracted along the boundary, with an angle of 90 degrees.

Why are optical fibres transparent?

It minimises absorption.

Why are optical fibres narrow?

To increase the chance of TIR and it reduced multipath dispersion which causes the signal to weaken.

How can absorption be reduced as much as possible in an optical fibre?

Use an optical fibre repeater which regenerates the pulse before significant absorption happens.

What is material dispersion?

When light of multiple wavelengths enter the fibre meaning they travel at different speeds along the wave.

What is modal dispersion?

When monochromatic light enters the fibre at different angles meaning the wavelengths are reflected by different amounts so they take different amounts of time to travel along the fibre.

How do you find the number of positions of maximum light intensity?

Find the maximum possible order (when sin theta = 1) and multiply by 2 (to get maxima on either side) then add one for the central maximum

What are some applications of diffraction grating?

Analyse light from from stars, x-ray crystallography, monochromators