Bmat 2012 Q16 physics/chemistry

Practice Question Solving
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  1. B

We have been given the speed and wavelength of the microwave in air. When the microwave enter a plastic food container, the speed reduces.

By using the formula, c = ƒ x λ, we know speed is directly proportional to wavelength. If the speed decreases, the wavelength will also decrease. The only options which have a lower wavelength than 12 cm are A, B and C. Therefore the wavelength would be 8cm.

By using the formula, ƒ = c/λ, we can calculate the new frequency. We need to convert 8 cm to metres. 8 cm = 0.08 m

ƒ = c/λ
ƒ = 2.0 x 108 / 0.08 = 2.5 x 109

hi cant really get how to solve it as the wave length is bigger and the frequency is supposed to be lower but it is higher here.

2 Likes
2

Hi
I guess frequency is still the same in both cases, if u calculate the frequency in air
c = f y(i can’t find lambda on my kb)
f = c/y
= 3* 10^8/12* 10^-2
=1/4* 10^8* 10^2
=0.25* 10^10
=2.5* 10^9
Mostly, the intrinsic frequency of EM waves remain constant even if they pass through another medium or object.

3

thanks that clarifys it for me :pray: