Towards or away from the normal
Into glass the ray leans towards the normal. Out of glass it leans away.
A ray meets a glass block
A ray of light goes at a slant from air into a glass block. The dashed line at right angles to the surface is called the normal.
Compared with its path in air, does the ray in glass lean towards the normal or away from it?
What this lesson covers
The idea
A ray going obliquely from a rarer to a denser medium bends towards the normal; going from a denser to a rarer medium it bends away from the normal.
A ray meets a glass block
A ray of light goes at a slant from air into a glass block. The dashed line at right angles to the surface is called the normal.
Compared with its path in air, does the ray in glass lean towards the normal or away from it?
- Towards the normal
- Away from the normal
- It does not lean at all
Both ways
Choose air to glass and then glass to air. Watch how r compares with i.
Rule of thumb
Going from air (rarer) into glass (denser) the ray moved closer to the normal. Coming out of glass into air it moved away from the normal.
A ray going obliquely from a rarer to a denser medium bends towards the normal. A ray going from a denser to a rarer medium bends away from the normal.
Notes
Rarer to denser: the ray bends towards the normal. Denser to rarer: it bends away from the normal.
Check yourself
A ray goes obliquely from air into glass. It bends
A ray goes obliquely from glass into air. It bends
In which case does the ray bend away from the normal?
A ray in air meets glass with angle of incidence 40°. The angle of refraction in the glass is
- away from the normal. Away from the normal is for a ray going from a denser medium to a rarer one, such as glass to air.
- towards the normal — correct. Yes!
- neither way, it carries straight on. A ray entering obliquely changes direction. Going into glass it bends towards the normal.
- towards the normal. Towards the normal is for a ray going from a rarer to a denser medium. Glass to air is denser to rarer.
- back into the glass, as if from a mirror. The ray goes out into the air. It is a refracted ray, not a reflected one, and it bends away from the normal.
- away from the normal — correct. Yes!
- From a denser medium to a rarer medium — correct. Yes!
- From a rarer medium to a denser medium. In that case the ray bends towards the normal.
- In both cases. The ray bends towards the normal in one case and away in the other.
- larger than 40°. Entering a denser medium the ray bends towards the normal, so the angle with the normal gets smaller.
- smaller than 40° — correct. Yes!
- exactly 40°. The ray changes direction when it goes obliquely into glass, so the angle changes.