The ray through the optical centre
A ray through the middle of a lens goes straight on.
The middle of a lens
At its very middle a lens is as if a thin flat sheet of glass, with its two surfaces nearly parallel. A ray of light goes through the optical centre O.
Does the ray change direction when it passes through the optical centre?
What this lesson covers
The idea
A ray of light passing through the optical centre of a lens emerges without any deviation.
The middle of a lens
At its very middle a lens is as if a thin flat sheet of glass, with its two surfaces nearly parallel. A ray of light goes through the optical centre O.
Does the ray change direction when it passes through the optical centre?
- No, it goes straight on
- Yes, it bends towards the axis
- Yes, it bends away from the axis
Through O
Change the angle of the purple ray, which passes through O. Compare the angles before and after the lens. Then try the concave lens.
No deviation
The purple ray left the lens in the same direction as it came, for both lenses. The red ray, which did not pass through O, was bent.
A ray of light passing through the optical centre of a lens emerges without any deviation.
Notes
A ray passing through the optical centre of a lens emerges without any deviation.
Check yourself
A ray of light passes through the optical centre of a lens. After refraction it
Which of these rays is not bent by a lens?
A ray makes 20° with the principal axis and passes through the optical centre of a concave lens. At what angle to the axis does it leave the lens, in degrees?
Answer: 20 degrees
The ray emerges without deviation, so it keeps its direction: 20° with the axis.
In a ray diagram for a lens, the ray from the top of the object through O is easy to draw because
- bends towards the principal axis. Only rays that do not pass through O are bent by a convex lens.
- emerges parallel to the principal axis. That happens to a ray that passes through a principal focus, not to a ray through the optical centre.
- emerges without any deviation — correct. Yes!
- A ray through the optical centre — correct. Yes!
- A ray parallel to the principal axis. A ray parallel to the axis passes through (or seems to come from) the principal focus, so it is bent.
- A ray through the principal focus of a convex lens. That ray is bent so that it emerges parallel to the axis.
- it bends exactly like the ray parallel to the axis. The ray through O is not bent, unlike the ray parallel to the axis.
- it is simply a straight line — correct. Yes!
- it ends at the principal focus. The ray through O does not end at a focus. It goes straight on.