RP2: Interference Effects

Question 1

What safety precautions should be taken when operating a laser?

Answer 1

● Never look directly at beam
● Ensure no reflective surfaces are in the vicinity
● Display a warning notice so others know laser is in use

Question 2

Why should the screen, on which you use to display the interference patterns, have a matte finish?

Answer 2

To reduce the likelihood of the beam reflecting off the screen into someone’s eye

Question 3

Describe how you should measure the fringe spacing of an interference pattern

Answer 3

Measure across a large number of fringes and divide by number of spaces measured across

Question 4

If you measure from the first bright fringe to the twelfth bright fringe, what number should you divide by to obtain the fringe spacing?

Answer 4

11

Question 5

Suggest a measuring instrument you could use to measure the slit separation?

Answer 5

A vernier calliper

Question 6

Suggest a measuring instrument that could he used to measure the distance from the slit to the screen

Answer 6

A metre ruler

Question 7

How can the wavelength of the laser be determined from a graph of fringe width against the distance from the screen?

Answer 7

By multiplying the gradient by the slit separation

Question 8

What is monochromatic light?

Answer 8

Light of a single wavelength (or frequency)

Question 9

What does it mean if two light sources are coherent?

Answer 9

They have the same wavelength and a constant phase difference

Question 10

What happens to light as it passes through a slit?

Answer 10

It diffracts

Question 11

When does the maximum diffraction occur?

Answer 11

When the slit spacing matches the light’s wavelength

Question 12

Why do bright fringes form on a screen when light passes through a double slit?

Answer 12

● light diffracts
● diffracted waves meet and superpose
● at positions where they are in phase, constructive interference occurs
● bright fringes are formed

Question 13

Why do dark fringes form on a screen when light passes through a double slit?

Answer 13

● light diffracts
● waves meet and superpose
● at positions where waves meet in antiphase, destructive interference occurs
● dark fringes are formed

Question 14

In a double slit experiment, why might the light be passed through a single slit before the double slit?

Answer 14

● single slit acts as point source
● diffracts the light widely so it can pass through both slits
● both slits are illuminated by same source
● they therefore have same wavelength and constant phase difference (coherent)

Question 15

Describe the interference pattern produced if white light is passed through the double slits

Answer 15

● central bright white fringe is produced
● other bright fringes consist of a spectra of light

Question 16

How is a diffraction grating spacing calculated?

Answer 16

It’s the reciprocal of the number of lines per metre

Question 17

How can the angle from the centre to a higher order line be calculated?

Answer 17

● distance from diffraction grating to the screen, and the distance from centre of pattern to the higher order line can be measured
● angle can then be calculated with trigonometry

Question 18

How does a diffraction pattern from a double slit experiment compare to one from a diffraction grating?

Answer 18

● as number of slits increases, pattern becomes sharper and brightness of fringes increases
● this is because more rays are reinforcing the pattern

Question 19

Suggest a use for a diffraction grating

Answer 19

Can be used instead of prisms to analyse spectra