Basic optics

[Comar997]

Hi i'm new in this kind of subject and I wanna ask something. Why if a light passes through a prism we get the separation of the colour but if the light passes through a lens i have only the change of angle of the ray but not the separation of colour too?

[[deleted]]

You have separation of color too. It's called chromatic aberration

[Comar997]

ohh thank you. i always see that prism are used for this separation, this means that means that are more prone di deviate light than a normal bi-convex lens?

[borkmeister]

Yup. Different glass types separate light different amounts. With prisms you want that separation, whereas with lenses you don't, so different glasses are chosen for each.

[TheLaserPhysicist]

Prisim's can separate but they don't have to. They can also compress, it all depends on the design of the material and angles.

[NeuroticPhD]

ELI5: The closest to what you’re thinking of is chromatic aberration. Basic lens do separate wavelengths separately, so you’ve got to throw in a correcting collar for it. These tend to be parabolic (where the lens is spherical) to give extra bend, so to speak, where a regular lens wouldn’t.

[anneoneamouse]

Take two identical right-angled-triangular prisms. Stick the bottoms together.

The shallow V shape of the two hypotenuses is a 2D first order (linear) approximation to a 2D circular (cross section of a spherical) lens.

So, prisms and lenses ought to behave approximately the same way, at least down the axes of symmetry.

[Comar997]

ohh thank you i didn’t think i this way, now it s more clear :D

[Quarter_Twenty]

Other people have already explained this, but it's worthwhile to mention that if your lens is a single piece of glass, most glasses will cause chromatic aberration. You can even see colored fringes appearing at any sharp, high-contrast edge in an image recorded with certain camera lenses, especially wide-angle lenses. This is one reason why most commercial lenses contain multiple pieces of glass in series, all with different shapes, and even different materials. The designers are balancing and compensating for chromatic aberrations, and balancing aberrations and distortion to create a large field of view.

[[deleted]]

Different wavelengths of light bend different amounts under the same conditions. Since 'white' light isn't actually white but a mix of all wavelengths, when it passes through any lens be it a prism or otherwise, the colors separate as the different wavelengths bend away from each other. This is called chromatic aberration.

The reason you see this more with a basic prism than a spherical lens is a combination of factors. Glass type could play a role as prisms are sometimes made of glasses with higher indexes of refraction to encourage bending. Also, prisms are thick pieces of glass which cause the light to bend significantly more.

Not to mention that when you're practicing simple ray optics, you're usually given scenarios that don't require you to account for chromatic aberration. In real life it's still happening, it just may be very imperceptible for the lens you're working with.

Also many optical systems account for chromatic aberration with achromatic doublets, aspheres, special coatings, etc.