Ans
(a) (v =) 20 (cm / s)
(v =) \(4(.0)\times 5(.0) \)
(v =) \(f \times \lambda \)
Detailed Solution:
Wave speed is calculated using \(v = f \lambda\). Substituting \(f = 4.0\ \mathrm{Hz}\) and \(\lambda = 5.0\ \mathrm{cm}\) gives \(v = 4.0 \times 5.0 = 20\ \mathrm{cm/s}\). This applies the core wave equation from Topic 3.1.
(b) any three from:
refraction
direction of waves / wavefronts changes
(due to) change in speed
wavelength changes
as depth of water changes
Detailed Solution:
As waves move into shallower water over the block, their speed decreases. This causes refraction: the wavelength shortens and the wavefronts change direction unless approaching along the normal. This demonstrates wave behavior from Topic 3.1.
(c)(i) radio waves OR microwaves OR infrared
Detailed Solution:
In the electromagnetic spectrum, regions with longer wavelengths than visible light are radio waves, microwaves, and infrared. Any of these is a correct response under Topic 3.3.
(ii) security marking OR detecting forged bank notes OR sterilising food / water
Detailed Solution:
Ultraviolet radiation is used to reveal security markings on banknotes or to sterilize surfaces and water by killing microorganisms. This is a practical application listed in Topic 3.3.
(iii) (both have) same speed owtte
Detailed Solution:
All electromagnetic waves travel at the same speed in a vacuum, \(3.0 \times 10^8\ \mathrm{m/s}\). Therefore, radio waves and gamma rays have identical speeds as per Topic 3.3.
