Through a glass slab
The ray comes out parallel to the way it went in, but shifted aside.
A ray through a glass slab
A ray of light goes through a rectangular glass slab, whose two big faces are parallel. It enters through one face and leaves through the other.
How does the ray that comes out compare with the ray that went in?
What this lesson covers
The idea
In a rectangular glass slab the bending at the two opposite parallel faces is equal and opposite, so the emergent ray is parallel to the incident ray but shifted sideward slightly.
A ray through a glass slab
A ray of light goes through a rectangular glass slab, whose two big faces are parallel. It enters through one face and leaves through the other.
How does the ray that comes out compare with the ray that went in?
- It is parallel to it, but a little shifted to one side
- It is bent towards the normal for ever
- It is on the same line as before
Slide the angle
Change the angle of incidence. Compare the incident angle with the emergent angle. The grey dashed line shows the path without the slab.
Equal and opposite
At the first face the ray bent towards the normal. At the second face it bent away by the same amount. The two bends cancel in direction, but the ray is shifted aside.
In a rectangular glass slab the bending at the two opposite parallel faces is equal and opposite. So the emergent ray is parallel to the incident ray, but it is shifted sideward slightly.
Notes
In a rectangular glass slab the two bends are equal and opposite, so the emergent ray is parallel to the incident ray, but shifted sideward slightly.
Check yourself
A ray goes through a rectangular glass slab. The emergent ray is
Why is the emergent ray parallel to the incident ray?
At the first face (air to glass) the ray bends _; at the second face (glass to air) it bends _.
A ray makes 40° with the normal when it enters a glass slab with parallel faces. At what angle to the normal does it leave the other face, in degrees?
Answer: 40 degrees
The emergent ray is parallel to the incident ray, and the normals of the two parallel faces are parallel, so the angle is again 40°.
- on the same line as the incident ray. The ray is shifted sideward slightly, so it is not on the same line.
- not parallel to the incident ray. The bends at the two faces are equal and opposite, so the emergent ray is parallel to the incident ray.
- parallel to the incident ray, but shifted sideward slightly — correct. Yes!
- The bending at the two parallel faces is equal and opposite — correct. Yes!
- The ray does not bend inside the glass slab. The ray does bend, towards the normal at the first face and away from it at the second.
- The glass absorbs the bend. Glass does not absorb the bend. The two bends at the parallel faces are equal and opposite.
- away from the normal; towards the normal. It is the other way. Air to glass is rarer to denser, which bends the ray towards the normal.
- towards the normal; away from the normal — correct. Yes!
- towards the normal; towards the normal. Going back from glass into air the ray bends away from the normal.