KCL Section 2: Diffraction gratings (Waves and photons)

Question 1

Define wavefront

Answer 1

Wavefronts are lines connecting points on the wave that are at exactly the same phase position.

Question 2

Define Constructive interference

Answer 2

Constructive interference Is the superposition effect of two waves that are in phase producing a larger amplitude resultant wave.

Question 3

Define Destructive interference

Answer 3

Destructive interference is the superposition effect of two waves that are out of phase. producing a smaller amplitude resultant wave.

Question 4

Define coherent

Answer 4

Waves are said to be coherent if they have the same frequency and have a constant phase relationship. Coherent waves are needed to form a stable standing wave.

Question 5

Define A progressive wave

Answer 5

A progressive wave is a means for transferring energy via oscillations.

Question 6

What is a diffraction grating?

Answer 6

A diffraction grating is a solid with lots of slits cut into it, with constant spacing

Question 7

What do the symbols in nλ = dsinθ represent

Answer 7

d is the distance between slits - usually a very small number
Often you are given N, which is the number of slits/lines per metre (or millimetre). This is a very large number.
You must change N into d by dividing 1m by the number of lines (1/N)
θ is the angle between the normal ray and the ray you are considering
λ is the wavelength
n is the order of the ray you are considering

Question 8

A laser is shone through a diffraction grating in which the slits are 2 x 10-6 m apart. The first bright fringe is observed at an angle 10° from the normal. What is the wavelength of the radiation?

Answer 8

347 nm

Question 9

A laser is shone through a diffraction grating in which the slits are 3.5 x 10-6 m apart. The first bright fringe is observed at an angle 18° from the normal. What is the wavelength of the radiation?

Answer 9

0.15 nm

Question 10

A laser is shone through a diffraction grating in which the slits are 1.2 x 10-6 m apart. The first bright fringe is observed at an angle 24° from the normal. What is the wavelength of the radiation?

Answer 10

500 nm

Question 11

A laser is shone through a diffraction grating which has 500000 lines per m. The first bright fringe is observed at an angle 16° from the normal. What is the wavelength of the radiation?

Answer 11

551 nm

Question 12

A 2-slit experiment is set up in which the slits are 0.03 mm apart. The first bright fringe is observed at an angle 10° from the normal. What is the wavelength of the radiation? What sort of electromagnetic radiation was being used?

Answer 12

5.2 x 10-6 m, infrared

Question 13

Light, with a wavelength of 500 nm, is shone through 2 slits, which are 0.05 mm apart. What are the angles to the normal of the first three dark fringes?

Answer 13

n=0.5 2.86 x 10-4, n=1.5 8.59 x 10-4, n=2.5 1.43 x 10-3

Question 14

Some X-rays, with wavelength 1 nm, are shone through a diffraction grating in which the slits are 50 μm apart. A screen is placed 1.5m from the grating. How far are the first three light fringes from the point at which the normal intercepts the screen?

Answer 14

1 = 30 x 10-6, 2 = 60 x 10-6 , 3 = 90 x 10-6

Question 15

What will be seen if the spacing between the slits increases?

Answer 15

If d (the spacing between the slits) increases then sinθ and θ will decrease, so the bright spots will get closer together.

Question 16

What will be seen if the wavelength increases?

Answer 16

If λ (the wavelength) increases then sinθ and θ will increase, so the bright spots will get further apart.

Question 17

What will be seen if the distance between the grating and the screen increases?

Answer 17

If the distance between the grating and the screen increases then the bright spots will get further apart.