Core Practical 8: Diffraction grating interference patterns

Question 1

CP8 Aim

Answer 1

Find out wavelength of light using diffraction grating

Question 2

CP8 Equipment List

Answer 2

• Diffraction grating
• monochromatic/laser light
• ruler
• stand

Question 3

CP8 Method

Answer 3

1. Place laser on stand and diffraction grating in front of it
2. Use set square to ensure beam passes through grating at normal incidence (perpendicular)
3. Set distance D between grating and wall to be 1m using ruler
4. Darken room and turn on laser
5. Identify 0-order maximum
6. Measure distance to nearest 2-order maxima
7. Calculate mean of these 2 values
8. Repeat with a diffraction grating with diff no of slits per mm

Question 4

CP8 Analysis

Answer 4

nλ = dsinθ
n - order of diffraction pattern
λ - Wavelength of laser light (m)
d - distance between slits (m)
θ - angle between normal and maxima

If angle is not small, trig to calcu

Question 5

How to decrease uncertainty in CP8?

Answer 5

Use of set square
- Avoid parallax error in measurements
Grating of more lines per mm
- Greater values for d between slits; lower % uncertainty
Dark room
- Fringes clear

Increase distance from grating to screen
- Increases distance value measured reducing % uncertainty

Measure distance between central and 2nd order
- Increases distance value measured reducing % uncertainty

Question 6

CP8 Safety

Answer 6

Eyes
- Don’t shine or reflect laser in anyone’s eye

Question 7

What happens if laser isn’t perpendicular to screen?

Answer 7

Maxima on one side moves away from central maxima
Maxima on other side moves towards central maxima
Intensity of maxima different on each side of central maxima