Several students are conducting investigations with Young’s Double Slit Experiment.
In the first investigation, monochromatic light passes through a double-slit arrangement. The intensity of the fringes varies with distance from the central fringe. This is observed on a screen, as shown in the diagram below.
The intensity of the monochromatic light passing through one of the slits is reduced.
(a)
Explain the effect of this change on the appearance of the dark and bright fringes.
In investigation, two white light is incident on an orange filter, a single slit, and then a double-slit. An interference pattern of light and dark fringes is observed on the screen.
(b)
(i)
The orange filter is now replaced by a green filter. State and explain the change in appearance, other than the change in colour, of the fringes on the screen.
[1]
(ii)
The green filter is now removed. State and explain the change in appearance of the central maximum fringe, as well as the fringes away from this central position.
In a third experiment, the white light is replaced by orange light of wavelength 600 nm. The double-slit has a separation of 0.350 mm and the screen is 6.35 m away.
(c)
Calculate the distance between the central and first maximum as seen on the screen.
The diagram below shows an arrangement for observing the interference pattern produced by laser light passing through two narrow slits S1 and S2.
The distance S1S2 is d, and the distance between the double slit and the screen is D where D ≫d, so angles θ and ϕ are small. M is the midpoint of S1S2 and it is observed that there is a bright fringe at point A on the screen, a distance fn from point O on the screen. Light from S1 travels a distance S2Y further to point A than light from S1.
(a)
The wavelength of light from the laser is 650 nm and the angular separation of the bright fringes on the screen is 5.00 × 10−4 rad. Calculate the distance between the two slits.
The separation of the slits S1 and S2 is 1.30 mm. The distance MO is 1.40 m. The distance fn is the distance of the ninth bright fringe from O and the angle θ is 3.70 × 10−3 radians.
In the second investigation, a thin film of colourless oil floats on water as shown in the diagram below. The refractive index of the oil is 1.47 and the water is 1.52. The same red light is now incident on the oil.
(c)
Complete the diagram to show the two light rays reflected from the two surfaces of the oil and label them P and Q.
Monochromatic light is incident on a double-slit diffraction grating. After passing through the slits the light is brought to a focus on a screen. The intensity distribution of the light on the screen is shown in the diagram below.
The double-slit diffraction grating is now changed to a grating with many narrower slits, the same widths as the slits above.
(a)
Sketch the new intensity pattern for the light between points C and D on the screen.
Two sources of light now replace the light incident on the diffraction grating. One is the same as the wavelength of the previous source and the other has a slightly longer wavelength.
(d)
Compare and contrast the new intensity pattern with the original. Comment on the intensity of the central maxima and the width of all maxima.
