Grade 12 University Physics
Tutorial 5
1. Two point sources in phase are oscillating in water 20 times every 3 seconds. The water waves move at
a speed of 30 cm/s. The distance between source 1 and a point where destructive interference occurs is
45 cm. The point of destructive interference is on the second nodal line.
a) Calculate the frequency of the wave
b) Calculate the possible distance that is separating the point of destructive interference and source 2.
c) If the two sources are out of phase, how does your answer in b change?
2. A double slit experiment is conducted using a red laser and a blue laser. The wavelength of the blue
laser (440 nm) is incident on the double slits. An interference pattern occurs on a screen located 1 m
from the slit, and the distance between successive minimas is measured to be 8.8 mm. When the red
light is incident, the distance between successive minimas is measured to be 13.6 mm. Calculate the
wavelength of the red light.
3. A double slit experiment is conducted using a blue laser of unknown wavelength. The angle between
the right bisector and the third minima is 1.5 degrees +/- 0.2 degrees. The width of the slit is 4.0x10-5 m.
Calculate the range of wavelengths for the blue laser.
4. A layer of oil (n = 1.2) is floating on top of a layer of water (n = 1.33). A wavelength of blue light
(wavelength is 440 nm) is seen brightly reflecting off of the oil. The oil is at least 500 nm thick.
Calculate the minimum thickness (greater than 500 nm) of the oil drop. Why do you see different
colours coming from different parts of the oil drop?
Answers:
1. a) 6.67 Hz = 7 Hz b) 51.75 cm, 38.25 cm OR 5x102 cm, 4x102 cm
c) 54 cm, 36 cm OR 5x102 cm, 4x102 cm
2. 680 nm OR 6.8x10-7 m
3.
4. 550 nm OR 5.5x102 nm or 5.5x10-7 m