**Question**

**Question**

(a) For a progressive wave, state what is meant by wavelength.

…………………………………………………………………………………………………………………………………

…………………………………………………………………………………………………………………………… [1]

(b) A light wave from a laser has a wavelength of 460nm in a vacuum.

Calculate the period of the wave.

period = ……………………………………………… s [3]

**(c) The light from the laser is incident normally on a diffraction grating.Describe the diffraction of the light waves at the grating.………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… [2](d) A diffraction grating is used with different wavelengths of visible light. The angle θ of thefourth-order maximum from the zero-order (central) maximum is measured for eachwavelength. The variation with wavelength λ of sinθ is shown in Fig. 4.1.**

**(i) The gradient of the graph is G.Determine an expression, in terms of G, for the distance d between the centres of two**

**adjacent slits in the diffraction grating.**

**d = ………………………………………………… (ii) On Fig. 4.1, sketch a graph to show the results that would be obtained for thesecond-order maxima.**

**Answer/Explanation**

**Ans:**

**(a)** distance moved by wavefront/energy during one cycle/oscillation/period (of source)

or

minimum distance between two wavefronts

or

distance between two adjacent wavefronts

**(b) **v =λ / T

or

v = fλ and f = 1 / T

\(T = 460 \times 10^{–9} / 3.00 \times 10^8 \)

\(= 1.5\times 10^{–15} s\)

**(c)** waves pass through/enter the slit(s)

waves spread (into geometric shadow) B1**(d)(i)** n λ = d sin θ

G = sin θ / λ

d = 4 / G

**(d)(ii)** straight line from 400 nm to 700 nm that is always below printed line

straight line has smaller gradient than printed line and is 5 small squares high at wavelength of 700 nm

*Question*

(a) For a progressive wave, state what is meant by:

(i) the wavelength

………………………………………………………………………………………………………………………….

………………………………………………………………………………………………………………………….

(ii) the amplitude.

………………………………………………………………………………………………………………………….

………………………………………………………………………………………………………………………….

**(b) A beam of red laser light is incident normally on a diffraction grating.**

**(i) Diffraction of the light waves occurs at each slit of the grating. The light waves emerging from the slits are coherent.Explain what is meant by:1. diffraction…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………2. coherent.…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………**

**(ii) The wavelength of the laser light is 650nm. The angle between the third order diffraction maxima is 68°, as illustrated in Fig. 4.1.**

**Calculate the separation d between the centres of adjacent slits of the grating.**

** d = …………………………………………….. m**

**(iii) The red laser light is replaced with blue laser light. State and explain the change, if any, to the angle between the third order diffraction maxima.………………………………………………………………………………………………………………………….………………………………………………………………………………………………………………………….………………………………………………………………………………………………………………………….**

**Answer/Explanation**

(a)(i)

distance moved by wavefront / energy during one cycle / vibration / oscillation / period (of source)

or

minimum distance between two wavefronts

or

distance between two adjacent wavefronts

(a)(ii)

maximum displacement (of particle / point on wave)

(b)(i)

1 light / waves spread (at each slit)

2 constant phase difference (between light / waves)

(b)(ii)

n λ = d sin θ

d = 3 × 650 × 10^{–9} / sin34°

d = 3.5 × 10^{–6} m

(b)(iii)

wavelength of blue light is shorter (than 650 nm / red light)

so angle (between third order diffraction maxima) decreases