Interference and diffraction

Question 1

Diffraction

Answer 1

Diffraction is where a wave hits a gap or an obstacle and spreads out.

When the wavelength is much greater than the size of the gap, there is a LARGE amount of diffraction.
The smaller the wavelength hitting a gap, the LESS the amount of diffraction.
wavelength stays theSAME before and after diffraction.

Question 2

Monochromatic light

Answer 2

Monochromatic light all has the same wavelength.

Question 3

Coherent waves

Answer 3

Coherent waves have the same frequency and wavelength and have a constant phase difference between them.

Light that is not coherent i.e. from an LED or sunlight that has been passed through a filter can also be made coherent by passing through a single slit.

Question 4

path difference

Answer 4

Two source separately travel to a point or screen along different paths which can be different lengths, it’s this path difference that leads to the constructive or destructive interference of the wave

S2P- S1P

When the path difference is a whole number of wavelengths, the waves will arrive in phase with each other and will constructively interfere to form a wave with a larger amplitude.
When the path difference is λ/2, 3λ/2, 5λ/2 etc the waves will arrive in antiphase (PI radians out of phase) and will destructively interfere to cancel each other out.

Question 5

Youngs double slit experiment

Answer 5

1. If the source is moved nearer to the slits, the separation of the fringes is unaffected but their intensity increases.
2. If the distance apart of the double slits is doubled, then by using the equation w= λD/s the separation of the fringes can be seen to halve.
3. If the source slit C is widened, the fringes disappear. The source is no longer coherent.
4. The fringes disappear, if one slit is covered up and a single slit diffraction pattern is seen.
5. If white light is used :

Question 6

Diffraction Gratings

Answer 6

• A set of many evenly spaced slits, essentially lots of double slits, in which the slit separation is very small.
• They normally consist of a large number of parallel lines ruled on glass (typically 300 per mm)

Question 6

Diffraction Gratings

Answer 6

• A set of many evenly spaced slits, essentially lots of double slits, in which the slit separation is very small.
• They normally consist of a large number of parallel lines ruled on glass (typically 300 per mm)

Question 7

d: the slit spacing

Answer 7

d = 1/N

N’ the number of lines/mm or m