Diffraction Gratings

Question 1

Diffraction (laser light)

Answer 1

• Laser light is monochromatic and coherent
• If the wavelength of the laser light is roughly the same as the same as the width of the single slit, we see a diffraction pattern.

Question 2

Laser diffraction pattern

Answer 2

• We see a bright centeral fringe with an alternating dark and bright fringes either side.
• The bright fringes are caused by constructive interference.
• The dark fringes are caused by destructive interference.

Question 3

White light diffraction pattern

Answer 3

• If we use white light instead of a laser, we see a different but similar diffractio pattern.
• White light is made up of all the visible colours of light. It is not monochromatic.
  Each colour is diffracted by different amounts.
• Red has the longest wavelength, so is diffracted the most and appears on the outside of the fringes.
• Blue has the shortest wavelength, so its diffracted the least and appears on the inside of the fringes.

Question 4

Width of centeral diffraction max

Answer 4

• If we increase the slit width, the width of the centeral maximum will decrease.
• This is because diffraction effects will decrease.
• If the width is very large the light will just pass straight through without being diffracted at all.
• The intensity of the centeral max will increase because the photons are less spread out.

Question 5

Wavelength

Answer 5

• If we increase the wavelength of the incident light, the width of the centeral maximum will increase.
• Longer wavelength light (red) being on the outside of white light fringes compared with shoter wavelength light (blue) being on the inside.
• The intensity of the centeral maximum will decrease because the photons are more spread out.

Question 6

Diffraction gratings

Answer 6

It is a device that can be used to split light into different wavelengths.
They contain tiny slits for light to pass through.

Question 7

Maxima and minima

Answer 7

• The bright and dark lines correspond to where constructive or destructive interference has taken place.
• The positions of maxima are called orders.
• This diffraction grating has many slits.

Question 8

Maxima and minima 2

Answer 8

• On a screen there will be a centeral point. This point is called the zero order
• The zero order line has the largest brightness out of all the other lines.
• Either side of the centeral line lies the first order lines. The zero order lines in the middle of the two.
• The further away from the centeral point, the dimmer the lines are the more orders that are visible.

Question 9

Number of slits

Answer 9

• The more slits in the grating, the sharper the pattern of lines on the screen.
• The pattern produced is the same as that of young’s double slit experiment, except that the lines are sharper and more easily measurable.

Question 10

Different colours, different wavelenghts

Answer 10

• The longer the wavelength of light, the larger the diffraction effect.
• Of the visible spectrum, red has the longest wavelength and blue has the shortest. They all travel the same speed.