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

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 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

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

Answer 7

The gradient of the graph will be w/D. Multiplying the gradient by the slit separation gives the wavelength.

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

Light diffracts as it passes through a slit.

Question 11

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

Answer 11

Constructive interference occurs where waves meet in phase.

Question 12

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

Answer 12

Destructive interference occurs where waves meet in antiphase.

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 so both slits are illuminated by the same wave.

Question 14

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

Answer 14

A central white fringe with coloured fringes either side.

Question 15

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

Answer 15

nλ = d sinθ.

Question 16

How is a diffraction grating spacing calculated?

Answer 16

It 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

By measuring distances and using trigonometry.

Question 18

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

Answer 18

More slits produce sharper, brighter fringes.

Question 19

Suggest a use for a diffraction grating.

Answer 19

Analysing spectra, such as in X-ray crystallography.