Two point sources S₁ and S₂ emit monochromatic light of the same wavelength. The light is incident on a small aperture A and is then brought to focus on a screen.

The images of the two sources on the screen are just resolved according to the Rayleigh criterion. Sketch, using the axes below, how the relative intensity I of light on the screen varies with distance along the screen d.

A car is travelling at night along a straight road. Diane is walking towards the car. She sees the headlights of the car as one single light. Estimate, using the data below, the separation d between Diane and the car at which, according to the Rayleigh criterion, Diane will just be able to see the headlights as two separate sources.

Distance between the headlights = 1.4 m
Average wavelength of light emitted by the headlights = 500 nm
Diameter of the pupils of Diane’s eyes = 1.9 mm

The light from the car headlights in (b) is not polarized. State what is meant by polarized light.

correct shape of two diffraction patterns showing central maximum and at least one secondary maximum each side of central maximum;
intensity of secondary maxima no greater than one third intensity of central maxima; } (judge by eye)
first minimum of one pattern coincident with central maximum of other pattern;

or

Allow just the approximate dotted resultant intensity patterns:
correct pattern of two symmetrical principal maxima;
with local minimum between them;
at least one secondary maximum on each side which are no more than \(\frac{1}{3}\) of the intensity of the principal maxima;

angular separation for resolution=\(1.22\frac{\lambda }{b} = 1.22 \times \frac{{5.0 \times {{10}^{ - 7}}}}{{1.9 \times {{10}^{ - 3}}}} = \left( {3.21 \times {{10}^{ - 4}}} \right)\left( {{\rm{rad}}} \right)\);
=\(\frac{{1.4}}{d}\);
d=4.4(km);
Award [2 max] if 1.22 not used and answer is 5.3 km.
Award [3] for a bald correct answer.

light in which the electric/magnetic field (vector) vibrates only in one plane/direction;