2005 Question 7
A student used a laser, as shown, to demonstrate that light is a wave motion.
(i) Name the two phenomena that occur when the light passes through the pair of
narrow slits.
Diffraction and Interference
(ii) A pattern is formed on the screen. Explain how the pattern is formed.
The slits act as sources of two coherent waves which overlap to give areas of
constructive interference (bright lines) and destructive interference (dark lines)
(iii)What is the effect on the pattern when the wavelength of the light is increased?
The pattern becomes more spread out.
(iv) What is the effect on the pattern when the distance between the slits is increased?
The pattern becomes less spread out.
(v) Describe an experiment to demonstrate that sound is also a wave
motion.
• Walking slowly from X to Y, you will notice the loudness of the
sound increasing and decreasing at regular intervals.
• This is because sound waves from the two speakers will interfere both
constructively and destructively, along the path XY.
(vi) Sound travels as longitudinal waves while light travels as transverse waves.
Explain the difference between longitudinal and transverse waves.
Longitudinal waves: the direction of the vibrations is parallel to the direction of propagation
of the wave.
Transverse wave: the direction of the vibrations is perpendicular to the direction of the wave.
(vii) Describe an experiment to demonstrate that light waves are transverse waves.
Light source and two pieces of polaroid as shown.
Rotate one polaroid relative to the other and note that the light intensity
increases and decreases
Only transverse waves can be polarised, so light is a transverse wave.
2005 Question 12 (c)
(i) The frequency of a stretched string depends on its length.
Give two other factors that affect the frequency of a stretched string. Tension and mass
per unit length
(ii) The diagram shows a guitar string stretched between supports 0.65 m apart.
The string is vibrating at its first harmonic. The speed of sound in the string is 500 m s–
1.
What is the frequency of vibration of the string?
λ = 2 × 0.65 = 1.3 m
v = fλ f = v/ λ = 500 / 1.3 f = 384.6 Hz
(iii)Draw a diagram of the string when it vibrates at its second harmonic.
(iv) What is the frequency of the second harmonic?
f2nd = 2f1st = 769.2 Hz
