RP 02 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, on which you use to display the interference patterns, have a matt finish?

Answer 2

A matt screen should be used to reduce the likelihood of the 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

When measuring the fringe spacing you should measure across a large number of fringes and then divide by the number of spaces measured across.

Question 4

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

Answer 4

You need to divide by the number of fringe spaces (not fringes), so in this case you should divide by 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 be used to measure the distance from the slit to the screen.

Answer 6

A Metre Ruler

Question 7

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

Answer 7

𝜆 = (w s) / D

Question 8

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

Answer 8

The gradient of the graph will be w/D, and so by multiplying the gradient by the slit separation you will obtain the wavelength.

Question 9

What is monochromatic light?

Answer 9

Monochromatic light, is light of a single wavelength (or frequency).

Question 10

What does it mean if two light sources are coherent?

Answer 10

Coherent sources have the same wavelength and a constant phase difference.

Question 11

What happens to light as it passes through a slit?

Answer 11

Light diffracts as it passes through a slit. The maximum diffraction occurs when
the slit spacing matches the light’s wavelength.

Question 12

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

Answer 12

The light diffracts as it passes through the slits and these diffracted waves meet and undergo superposition. At positions where the waves meet in phase, constructive interference occurs, and bright fringes are formed.

Question 13

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

Answer 13

The single slit acts as a point source, and diffracts the light widely so it can pass through the double slits. This means both slits are illuminated by the same source. They therefore have the same wavelength and a constant phase difference.

Question 14

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

Answer 14

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 15

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

Answer 15

n𝜆 = d sin𝜃
n: order of diffraction pattern
𝜆: wavelength
d: diffraction grating spacing
𝜃: angle from centre

Question 16

How is a diffraction grating spacing calculated?

Answer 16

The diffraction grating spacing is 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

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 trigonometry.

Question 18

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

Answer 18

As the number of slits increases, the pattern becomes sharper and the brightness of the fringes increases. This is because more rays are reinforcing the pattern.

Question 19

Suggest a use for a diffraction grating.

Answer 19

Diffraction gratings can be used instead of prisms to analyse spectra. An example use is in X-ray crystallography.

Question 20

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

Answer 20

The light diffracts as it passes through the slits and these diffracted waves meet and undergo superposition. At positions where the waves meet in antiphase, destructive interference occurs, and dark fringes are formed.