# MCQ help (1 Viewer)

#### Bob99

##### temporarylol hater

Can someone explain why the answer is A.
I thought the speed decreases and hence frequency increases c=fλ (B).

someone pls help

#### Run hard@thehsc

##### Well-Known Member
idk also shouldn't frequency remain the same? cos only wavelength decreases to account for the decreasing speed? idk correct me if I am wrong pls

#### tyrone97

##### Member
I think the answer should be C (no change in frequency, speed decreases)

The frequency of any wave never changes when it moves into a different medium. This is because the frequency is dependent only on the source of the wave. Another way of seeing this is that if the photon frequency changes (i'm switching to the "particle" view of light only to illustrate this point), then its energy has changed which would violate conservation of energy.

The only things that are allowed to change are the speed and the wavelength.

Since the wave is bending towards the normal, it must be moving into a more optically dense medium and therefore slowing down (Snell's Law). The wavelength decreases.

#### Bob99

##### temporarylol hater
I think the answer should be C (no change in frequency, speed decreases)

The frequency of any wave never changes when it moves into a different medium. This is because the frequency is dependent only on the source of the wave. Another way of seeing this is that if the photon frequency changes (i'm switching to the "particle" view of light only to illustrate this point), then its energy has changed which would violate conservation of energy.

The only things that are allowed to change are the speed and the wavelength.

Since the wave is bending towards the normal, it must be moving into a more optically dense medium and therefore slowing down (Snell's Law). The wavelength decreases.
And damn the QAT paper has so many mistakes in its solutions.

#### Run hard@thehsc

##### Well-Known Member
And damn the QAT paper has so many mistakes in its solutions.
so tru lmao

#### wizzkids

##### Active Member
Yes, that answer sheet must be wrong. Frequency never changes when a light wave passes between two different mediums.
Think of it this way: Crests and troughs are arriving at the interface.
(i) If the frequency is going to increase on the other side of the interface, where are these extra crests and troughs going to magically appear from???
(ii) If the frequency is going to decrease on the other side of the interface, that means crests and troughs are going to accumulate at the interface. That can't be right either.