Diffraction + Diffraction grating (8.2, 8.4)

What is diffraction?

Diffraction - when a wave passes through an aperture/gap or past an obstacle, it spreads out

Speed, frequency and wavelength stay the same

How does diffraction change depending on the size of the aperture?

Diffraction is the greatest when the size of the gap is approximately the same as the wavelength of the wave

When the size of the gap is much larger than the wavelength, the wave is transmitted with little spreading

When the size of the gap is much smaller than the wavelength, there is little transmission as most of the wave is reflected

How does diffraction change depending on the size of the obstacle?

The wider the obstacle compared to the wavelength, the less diffraction

How can diffraction be demonstrated in an experiment?

Ripple tanks

By placing different sized apertures in a ripple tank the different ‘types’ of diffraction can be shown

What is a diffraction grating?

Diffraction grating - a slide with many equally spaced slits, very close together

What happens when monochromatic light is incident on a diffraction grating?

Bright spots called maxima or orders are seen on a screen, which is usually semi-circular.

These maxima are much sharper than the bright spots in the double-slit because there are many more rays of light reinforcing the pattern

How are the orders formed?

The zeroth order forms as each ray from each slit has a zero path difference with another ray so constructive interference occurs

What is the relationship between the order, wavelength, slit separation and angle to each order from the central line?

nλ = dsinθ

Where n is the order number

Where λ is the wavelength (m)

When d is the slit separation (m)

Where θ is the angle between each order and the central line

How do you work out slit separation from slits per m?

d = 1/N

Where d is slit seperation (m)

Where N is slits/lines per m (m)

How do you work out the total number of maxima?

Since the max value of sinθ is 1, plug it into nλ = dsinθ. Then round down the value of n you get. Do 2n + 1 to get the maximum number of orders.