(A Level Phy) Young's double-slit experiment

In the 17^th century, there were two opposing views regarding light. Isaac Newton believed light was composed of particles he called corpuscles while Christian Huygens thought light was a wave.

Thomas Young conducted his double-slit experiment using a monochromatic, coherent light source, a thin card with two slits and a screen. He observed alternating bright and dark bands or fringes. The bright fringes are equally spaced. It's a symmetrical pattern. He couldn't explain this pattern using Newton's idea of light as corpuscles.

The light source must be monochromatic and coherent for the interference pattern to be produced...

In Young's time, sound was known to be a wave. He noticed that when two sound waves cross, interference occurs to produce beats at certain locations. He realised that the pattern of bright and dark fringes was an interference pattern. It was an evidence that light is composed of waves, not particles. Despite Young's strong argument, other scientists didn't want to believe Newton was wrong.

The interference pattern in the double-slit experiment shows the wave nature of light...

When light passes through the slits, they diffract and undergo interference in accordance to the principle of superposition. Bright fringes or maxima are formed when the waves at those points are in phase (path difference = nλ) and undergo constructive interference. Dark fringes or minima are formed when the waves at those points are out of phase (path difference = nλ/2) and undergo destructive interference.

Interference pattern is formed because of the superposition of waves...

Try these questions before you check with the answers below. You can refer to the Internet and print media to find the answers which cannot be found in the above passage.

1. Explain these terms: (i) the principle of superposition (ii) diffraction (iii) interference (iv) monochromatic (v) coherence
   
2. Distinguish between path difference and phase difference.
   
3. (a) What are the conditions needed to form an interference pattern? (b) Can interference pattern in a double split experiment be observed if an incandescent light bulb is used as the light source? Explain.
   
4. (a) What will happen to the distance between two successive maxima if the spacing between the two slits is doubled? (b) What happens to the distance between two successive maxima if the distance between the slits and the screen is twice?
   
5. Light of wavelength 650 nm is incident on a double slit arrangement. The interference fringes formed are viewed on a screen placed parallel to and 1.2 m from the plane of the double slit. The fringe separation is 0.70 mm. Calculate the separation of the slits.
   
6. The distance between two slits is 0.050 mm and the distance from the slits to a screen is 2.50 m. Find the spacing between the first and second order bright fringes for yellow light of 600 nm wavelength. 

Answers: 1. (i) The principle of superposition states that when two or more waves of the same type meet at a point, the resultant displacement of the waves is equal to the vector sum of their individual displacements at that point. (ii) Diffraction is the bending of waves around small obstacles and the spreading of waves past small openings. (iii) Interference is the interaction of waves to form resultant waves of greater or lower amplitude. (iv) Monochromatic light is light of a single wavelength. (v) Coherence is a property of waves that have a constant phase relation. 2. Path difference is the difference in metres between the lengths of the paths. If two waves don't exactly line up, then there is a phase difference which is measured in radians, degrees or fractions of a wavelength. 3. (a) The conditions for interference pattern: The sources must be coherent i.e. Maintain a constant phase relation. The sources are monochromatic i.e. Of a single wavelength. The waves have the same polarization state. (b) Light emitted from a light bulb is incoherent as the light consists of different wavelengths and they don't have a constant phase relationship. 4. (a) If d is doubled, ∆x (= λD/d) will be halved. (b) If D is twice, ∆x (= λD/d) will be doubled. (a) 5. d = λD/∆x = (650 x 10^-9 m)(1.2 m)/(0.70 x 10^-3 m) = 1.11 mm. 7. ∆x = λD/d = (600 x 10^-9 m)(2.50 m)/(0.050 x 10^-3 m) = 0.06 m = 6 cm