Is a lens attracted to the light?

[KentPhilip]

Okay. That’s the case when parallel rays are converged by a convex lens. But what about a convex lens that takes in diverging light rays and outputs parallel light rays? E.g. if the lens were close to a small light source, or the lens is really big compared to the source?

Hmm yes, I suppose you are right.

Make sure you don't set fire to the lens when it flies into the light

[expat]

Parallelism is an illusion. All lines converge at infinity.

Only with the one-point compactification.

Otherwise infinity is an illusion.

Anyway, parallelism is an option (postulate, or axiom).

[TimberWolf]

Parallelism is an illusion. All lines converge at infinity.

If that's true, would rays that are diverging by an infinitely small amount not cross at infinity? If not, would all divergent rays converge at infinity?

[Grinder]

Okay. That’s the case when parallel rays are converged by a convex lens. But what about a convex lens that takes in diverging light rays and outputs parallel light rays? E.g. if the lens were close to a small light source, or the lens is really big compared to the source?

Parallelism is an illusion. All lines converge at infinity.

[TimberWolf]

No it will be repelled, at an angle.
The photons of light have momentum in a direction directly away from the source of light. Then the lens changes it to an angle. Since the size of the momentum must remain the same, the vector part of it in the direction away from the original source of the light must reduce (by the cosine of the angle defracted by). Therefore to conserve momentum the lens must acquire momentum away from the light source to make up this difference.

Okay. That’s the case when parallel rays are converged by a convex lens. But what about a convex lens that takes in diverging light rays and outputs parallel light rays? E.g. if the lens were close to a small light source, or the lens is really big compared to the source?

[HairyArsedBloke]

Light has momentum? Fantastic...

[PRC1964]

Is a lens attracted to the light, like a moth?

Speaking as a member of the non BSc bunch, my days at university were spent by having my lenses distracted by norks and beer.

[scooterscot]

the light's momentum will change (or will it) and momentum must be conserved by the system as a whole.

Light has momentum? Fantastic...

[KentPhilip]

No it will be repelled, at an angle.
The photons of light have momentum in a direction directly away from the source of light. Then the lens changes it to an angle. Since the size of the momentum must remain the same, the vector part of it in the direction away from the original source of the light must reduce (by the cosine of the angle defracted by). Therefore to conserve momentum the lens must acquire momentum away from the light source to make up this difference.

[TimberWolf]

Is a lens attracted to the light, like a moth?

Hint: When light is refracted by a lens the light's momentum will change (or will it) and momentum must be conserved by the system as a whole.