# Physics help

Can somebody explain to me how the angle of refraction is 21 degrees? I understand how they've concluded the angle of incidence but can't quite seem to understand how they've managed to get the angle of refraction. The answer in the mark scheme is A.
Original post by Y12EC
Can somebody explain to me how the angle of refraction is 21 degrees? I understand how they've concluded the angle of incidence but can't quite seem to understand how they've managed to get the angle of refraction. The answer in the mark scheme is A.

Hi,

I think the answer is wrong and it’s either C/D because the angle of incidence shown on the diagram is 57°.
Original post by subbhy
Hi,

I think the answer is wrong and it’s either C/D because the angle of incidence shown on the diagram is 57°.

Yeah, that's what I thought as well. So do you reckon the question just has missing or wrong information then?
Original post by Y12EC
Yeah, that's what I thought as well. So do you reckon the question just has missing or wrong information then?

I think the answer is wrong but there may be some sort of formula you need to use to get to the answer more definitively. What exam board do you do?
in optics questions we *always* work with the angle between the ray and the normal to the surface.

the diagram shows an angle of incidence that is not being measured from the normal to the surface - you need to convert.

this will eliminate 2 incorrect choices.

of the remaining 2 options one has an angle of refraction that is greater than the angle of incidence (measured as always from the normal) and the other has an angle of refraction that is less than the angle of incidence.

can you tell which way the refracted ray should go without having to use Snells law?
Original post by Joinedup
in optics questions we *always* work with the angle between the ray and the normal to the surface.

the diagram shows an angle of incidence that is not being measured from the normal to the surface - you need to convert.

this will eliminate 2 incorrect choices.

of the remaining 2 options one has an angle of refraction that is greater than the angle of incidence (measured as always from the normal) and the other has an angle of refraction that is less than the angle of incidence.

can you tell which way the refracted ray should go without having to use Snells law?

Unless I’m seeing wrong, both of the remaining answers C and D have an angle of refraction that is smaller than the angle of incidence so that doesn’t help with narrowing it down?
If there was no difference in the refractive index of the air and glasd the ray would go straight through the boundry and the angle if refraction would be 33 degrees.

You've got a choice of two angles of refraction, one is greater than 33 deg and one is less than 33 deg
Original post by Joinedup
If there was no difference in the refractive index of the air and glasd the ray would go straight through the boundry and the angle if refraction would be 33 degrees.

You've got a choice of two angles of refraction, one is greater than 33 deg and one is less than 33 deg

If there was no difference in the refractive index, the angle of refraction would be 0 because no refraction occurs, no?

A and B are discounted as options are they not as the diagram shows the angle of incidence as being 57°
Original post by subbhy
If there was no difference in the refractive index, the angle of refraction would be 0 because no refraction occurs, no?

A and B are discounted as options are they not as the diagram shows the angle of incidence as being 57°

1. no - because angle of refraction always means the angle from the normal to the surface - not the incoming ray or the surface or anything else, it's got to be the normal to the surface.

2. no - because angle of incidence also always means angle from the normal to the surface

if a question contains a diagram showing an angle measured from the surface (like this one) and asks you about angle of incidence the examiner is seeing if you are paying attention
Original post by Joinedup
1. no - because angle of refraction always means the angle from the normal to the surface - not the incoming ray or the surface or anything else, it's got to be the normal to the surface.

2. no - because angle of incidence also always means angle from the normal to the surface

if a question contains a diagram showing an angle measured from the surface (like this one) and asks you about angle of incidence the examiner is seeing if you are paying attention

Right! That makes much more sense, thanks!