Topic 16

Question 1

Describe the combination of thin lenses

Answer 1

The image of the first lens is the object for the next lens and so on.

If lenses are in contact, add their powers.

Question 2

What is lens abberation? Give 3 examples.

Answer 2

Limitations of sharpness

Spherical Aberration

Chrromatic Aberration

Astigmatism

Question 3

What is Spherical aberration?

Answer 3

Rays close to and away from the centre of the lens have different focuses.

Question 4

What is Chromatic Aberration?

Answer 4

The index of refraction of the lens splits the rays which have different focuses.

Question 5

What is Astigmatism?

Answer 5

Lens wont focus light to one point, but produces 2 images at different distances.

Question 6

Describe thick lenses.

Answer 6

Considered to have two refraction points

Images are made by dividing the lens into 2 principle planes where the ray is bending.

Question 7

What are the six cardinal points of a thick lens?

Answer 7

Focal points (F, F’)

Principal Points (P₁, P₂)

Nodal Points (N₁, N₂)

These characterise a thick lens

Question 8

How is ray-tracing achieved for thick lenses?

Answer 8

The real light path inside the lens is ignored

Reflections are assumed at the principle planes and nodal points

Question 9

Give the generalised form of the thin lens equation and its use.

Answer 9

Used in ray tracing in thick lenses