## Homework Statement

Here is a diagram of light passing through the curved edge of a planoconvex lens.

When a light ray strikes the planar edge, it is not refracted. Why?

2. The attempt at a solution
The website the diagram came from: http://www-optics.unine.ch/education/optics_tutorials/plano_convex_lens_aberration.html [Broken]

Light is refracted when it strikes the curved edge of the lens. I know that when light it is not refracted if it passes at 90 degrees to the border between the media. In the diagram, this isn't the case.

I've studied refraction with semicircular glass plates, but light is refracted when it passes out of the straight edge.

Any hints would be really useful.

Last edited by a moderator:

It's to do with light travelling slower when it enters a denser medium.
If it strikes the curved edge then one end of the light ray will travel slower through the glass while the other is travelling at the original velocity through the air. This causes the light to turn towards the normal of the curve.
If the light ray hits the planar edge, it still enters into a denser medium and will have a lower velocity, but it will travel in a straight line. Imagine a car being the light ray and the glass being sand. When the car drives into the sand at an angle, one wheel will get onto it first, because the sand has more friction, the wheel will travel slower than the other wheel still on the road. This creates a turning effect. If you imagine a car driving into the sand at 90 degress it will not change direction.
Hope this helped.

slakedlime, as you have correctly pointed out, the light is refracted by the planar interface. I'm not really sure what they are trying to say... perhaps someone else has seen this before and can enlighten us.
Unless they mean reversing the lens so that the normally incident rays arrive at the planar surface first?

Last edited:
Unless they mean reversing the lens so that the normally incident rays arrive at the planar surface first?

I know that incident rays arriving first at the planar surface aren't refracted because they strike the glass at 90 degrees (along the normal). I found that diagram as an applet on this site (which is about chromatic aberrations):

http://www-optics.unine.ch/education/optics_tutorials/plano_convex_lens_aberration.html [Broken]

I pasted one of the diagrams here. It seems that light isn't bent after coming out of the planar surface. Maybe I'm mistaken?

Last edited by a moderator:
In the diagram you give (first post) it is refracted, you just cannot see it clearly, because it's not by a lot.

Maybe if you ignore whatever is happening first on the right (assuming the light is going right->left), and imagine the light is just a 'horizontal' beam, like the one in the centre.

Are they not just asking you to talk about the 'normal', as you did above?

Maybe if you ignore whatever is happening first on the right (assuming the light is going right->left), and imagine the light is just a 'horizontal' beam, like the one in the centre.

Would it be logical to assume that all the light rays are horizontal?

In the diagram you give (first post) it is refracted, you just cannot see it clearly, because it's not by a lot.
That's what I first thought, but the website said otherwise. It said that light striking the planar surface wasn't refracted. Maybe the writer was simplifying because the difference is so little?

Doc Al
Mentor
When a light ray strikes the planar edge, it is not refracted. Why?
I'd say it was just a sloppy diagram. Light is refracted at both surfaces.

I'm a beginning secondary science teacher (admittedly with a biology specialism), and if I had to second guess a GCSE paper, I'd say it was simply asking why there is no refraction at A, whilst there is at B, in the diagram below:

http://img151.imageshack.us/img151/3686/flowroot5110jd8.png [Broken]

Which you can do, no problem I think! I'm sure you're on track for a good mark.

Obviously, it's hard to say without seeing the paper.. :-)

Last edited by a moderator:
Doc Al
Mentor
That's what I first thought, but the website said otherwise. It said that light striking the planar surface wasn't refracted. Maybe the writer was simplifying because the difference is so little?