RP02: Interference effects

Question 1

What safety precautions should be taken when operating a laser?

Answer 1

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

Question 2

Why should the screen have a matt finish?

Answer 2

To reduce likelihood of laser beam reflecting from the screen into someone’s eye and causing harm

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 the number of spaces measured across

Question 4

If you measure from first bright fringe through to 12th bright fringe, what number should you divide through to obtain fringe spacing?

Answer 4

You need to divide by number of fringe spaces (not fringes) so in this case divide by 11

Question 5

Suggest a measuring instrument you could use to measure the slit seperation

Answer 5

Vernier calliper

Question 6

Suggest measuring instrument that could be used to measure distance from slit to screen

Answer 6

Metre ruler

Question 7

What equation links λ, slit separation, distance from screen and fringe spacing?

Answer 7

λ = (ws)/D

Question 8

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

Answer 8

Gradient is w/D so multiply by slit separation and you get λ

Question 9

What is monochromatic light

Answer 9

Light of a single λ (or f)

Question 10

What does it mean if two light sources are coherant?

Answer 10

Same λ and constant phase difference

Question 11

What happens to light as it passes through a slit?

Answer 11

It diffracts. Max diffraction when slit spacing matches lights λ

Question 12

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

Answer 12

Light diffracts as it passes through slits and these waves meet and undergo superposition.
Where they meet in phase, constructive interference occurs and bright fringes formed.

Question 13

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

Answer 13

Light diffracts as it passes through slits and these waves meet and undergo superposition.
Where they meet in antiphase, destructive interference occurs and dark fringes 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 and diffracts light widely so can pass through double slits. Both slits illuminated by same source so have same wavelength and phase difference.

Question 15

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

Answer 15

A central bright white fringe will be produced. All other bright fringes would consist of a spectra of light, with blue light on the side closest to the centre and red on the far side of each fringe

Question 16

What equation is used to determine the wavelength of light when using a diffraction grating?

Answer 16

nλ=dsinθ

Question 17

How is a diffraction grating spacing calculated?

Answer 17

Reciprocal of the number of lines per metre

Question 18

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

Answer 18

The distance from the diffraction grating to the screen (D) and the distance from the centre of the pattern to the higher order line (h) can be measured. The angle can then be calculated using trig.

Question 19

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

Answer 19

As no. slits increases, the pattern becomes sharper and the brightness of fringes increases. More rays are reinforcing the pattern.

Question 20

Suggest a use for a diffraction grating.

Answer 20

Diffraction gratings can be used instead of prism to analyse spectra. Eg. X-ray crystallography