Diffraction

http://www.youtube.com/watch?v=a_qRKjeam5w

Diffraction

Diffraction is the spreading of waves when they pass through a gap.

The amount the waves spread out depends partly on the frequency of light

Spectra

The burning salt on the knife in the candle shows up orange on

the line spectra

Diffraction Grating Equation

d sin θ= nλd = distance between the slitsΘ = angle to maximan = integer number to next bright fringeλ = wavelength of light

Number of slits per metreN = 1/d

Sin θ can never be greater than 1, so there is a limit to the number of spectra that can be obtained.

Red light will be seen at

d = 1600nmλ = 600nmn = 1

sin θ= (1)*600nm/1600nm = 0.375 θ = sin-1(0.375) = 22°

d sin θ= nλ

If you used a DVD then θ would be 47.6°

Grating questionA grating is labelled '500 lines per mm'.1. Calculate the spacing of the slits in the grating.2. Monochromatic light is aimed straight at the grating and is found to give a first-

order maximum at 150. Calculate the wavelength of the light source. 3. Calculate the position of the first-order maximum when red light of wavelength

730 nm is shone directly at the grating. 4. The longest visible wavelength is that of red light with λ = 750 nm. The shortest

visible wavelength is violet where λ= 400nm. Use this information to calculate the width of the angle into which the first-order spectrum is spread out when white light is shone onto the grating.

A grating is illuminated with a parallel beam of light of wavelength 550 nm. The first-

order maximum is in a direction making an angle of 200 with the straight-through direction.

5. Calculate the spacing of the grating slits. 6. What would be the angle of the first-order maximum if a grating of slit spacing of

2.5 x 10–6 m were used with the same light source? 7. Calculate the wavelength of light that would give a second-order maximum at q =

320 with a grating of slit spacing 2.5 x 10–6 m

d sin θ= nλ

Grating answersd sin θ= nλ

3.

Grating answers4.

Diffraction through a single slitDiffraction at a single aperture

intensityacross

Single slit

distant screen

The middle fringe is twice the width of all the other fringes. The outer fringes are less intense than the first fringe

(W) Width of the central fringe

W =(λ \ a ) x D

λ = wavelengtha = width of single slit

D= distance to the screen

The distance between the fringes (w)

w