3.3.2.2 - Diffraction

Question 1

What happens when white light is diffracted through a slit?

Answer 1

As white light is a mixture of colours, each with different wavelength, all the colours are diffracted by different amounts. Therefore you get a spectra of colours instead of fringes.

Question 2

Why is the central maximum the brightest part of the pattern?

Answer 2

The intensity of light is highest in the centre.

Question 3

What happens with monochromatic light and why?

Answer 3

All the photons have the same energy, so an increase in the intensity means an increase in the number of photons per second.

Question 4

What 2 things affect the width of the central maximum when light is shone through a slit?

Answer 4

• Increasing the slit width decrease the amount of diffraction. Therefore the central maximum is narrow and the intensity of the central maximum is higher.
• Increasing wavelength increases the amount of diffraction, this means the central maximum is wider, and the intensity of the central maximum is lower.

Question 5

When monochromatic light diffracts through a diffraction grating what is seen?

Answer 5

All the maxima are sharp lines. There is a line of maximum brightness at the centre called the zero order line. The lines just either side are called the 1st order lines. The next pair are called the second order and so on.

Question 6

What is the equation for a grating with slits d distance apart?

Answer 6

d sin θ = nλ

Question 7

Why is light transmitted by the grating in certain directions only?

Answer 7

• The light passing through each slit is diffracted.
• The diffracted light waves from adjacent slits reinforce each other in certain directions only, including light direction, and cancel out in all other directions.

Question 8

What equation do I need to remember how to derivate?

Answer 8

d sin θ = nλ

Question 9

What can diffraction gratings be used for?

Answer 9

Help to identify elements and atomic spacing, when you diffract white light through a grating. Astronomers and chemists often use this to see what elements are present in stars.

Question 10

What else can be diffracted and why is it useful?

Answer 10

X-rays in a process called X-ray crystallography which was used to discover the structure of DNA.