RP2 Interference Effects

Question 1

Safety precautions when operating a laser

Answer 1

• never look directly at beam
• ensure no reflective surfaces are near lasers vicinity
• display warning notice

Question 2

why should the screen, where the interference patterns are desplayed have a matt finish?

Answer 2

reduces likelihood of laser beam reflecting from the screen into someones eye

Question 3

Describe how fringe spacing of interference pattern is measured

Answer 3

measure across a large number of fringes and divide by the number of spaces measured across

Question 4

Instrument used to measure slit seperation

Answer 4

vernier calliper

Question 5

instrument used to measure the distance from the slit to the sceen

Answer 5

metre ruler

Question 6

how is wavelength determined from a graph of fringe width against distance?

Answer 6

gradient = w/D —> multiplying gradient by slit seperation

Question 7

monochromatic light

Answer 7

ligh of a single wavelength (or frequency)

Question 8

Coherent light sources

Answer 8

same wavelength and constant phase difference

Question 9

what happens to light as it passes through a single slit

Answer 9

• light diffracts
• maximum diffracttion when slit spacing matches lights wavelength

Question 10

why do bright fringes form

Answer 10

waves meet in phase, constructive interference occures

Question 11

why do dark fringes form

Answer 11

waves meet in antiphase, destructive interference occurs

Question 12

why is light passed through a single slit before a double slit

Answer 12

• single slit acts as a point source and diffracts lights widely so it can pass double slits
  -both slits are illuminated by same souce
  -have same wavelength and constance phase difference

Question 13

describe interference pattern when white light is passed through double slits

Answer 13

• central bright white fringe
• all other bright fringes would be a spectra of light with violet light closest to the centre and red the furthest

Question 14

how is diffraction spacing calculated?

Answer 14

the reciprocal of the number of lines per metre

Question 15

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

Answer 15

• as number of slits increase, pattern becomes charper and brightness of fringes increases
• this is because more rays are reinforcing the pattern

Question 16

use for diffraction grating

Answer 16

x-ray crystallography

Question 17

Independent variable?

Answer 17

Slit-to-screen distance D.

Question 18

Dependent variable?

Answer 18

Fringe width w.

Question 19

Control variables?

Answer 19

Wavelength λ

Slit separation s

Question 20

Why darken the room?

Answer 20

To make fringes more visible → reduces uncertainty in w.

Question 21

What graph should be plotted?

Answer 21

w on y-axis vs D on x-axis.

Question 22

What is the gradient equal to?

Answer 22

gradient=λ​/s

Question 23

How do you calculate the wavelength λ?

Answer 23

λ=(gradient)×s

Question 24

How can systematic errors be reduced?

Answer 24

• Use set square to ensure perpendicular alignment.
• Avoid parallax by measuring at eye level.
• Use stable laser with known wavelength.

Question 25

Why use a diffraction grating instead of double slits?

Answer 25

More slits → sharper maxima → easier to measure → reduced uncertainty.