CP8 Determining the wavelength of light

Question 1

What are the 2 methods?

Answer 1

Double slit method (DS)
Diffraction grating method (DG)

Question 2

How (DS)?

Answer 2

Shine monochromatic (single wave length) light at 2 slits to produce 2 coherent sources
Measure the distance from the slit to the screen / D
Measure the distance each slit is away from the each other / a
Measure the fringe separation / x

λ = ax/D

Question 3

Reducing uncertainty? (DS)

Answer 3

Being in a dark room
Dont use polychromatic (white) light as when diffracted colours will change
Measure across multiple stuff then divide
Increasing distances to measure

Question 4

How (DG)?

Answer 4

Find the slit width (1/slits per metre)
Find the distance between the grating and the screen
Measure the distance between orders and use nλ = dsin(x)
Repeat for different numbers of slits

Question 5

Reducing uncertainty?

Answer 5

Measure for multiple order lines and make an average
More slits per mm so that there is a greater distance between the orders
Increase distance from grating to screen

Question 6

Safety

Answer 6

Don’t look at the lasers
Make sure no reflective surfaces
The screen has a matte finish so no reflection and a decrease in intensity

Question 7

What are the benefits of using a laser?

Answer 7

They are monochromatic so coherent and don’t need to be focused, bright so can go further distances without dimming