L8 Abberations

Question 1

intro to aberrations

Answer 1

outside the paraxial region (region where rays focus at one point), an optical system does not form a point image of a point object.

normally the image is a blurred region because rays from the point object which pass through different parts of the lens are not refracted to a single image point

the discrepancies are called aberrations

Question 2

chromatic aberrations

Answer 2

refractive index of most optical materials show a significant change with the colour of the light

this means that light from a WHITE point source forms images of different colours at slightly different places

bc this is normally rather small it may be ignored completely in the paraxial region

eg. red/green bg

Question 3

spherical aberrations

Answer 3

consider monochromatic light

happens outside of paraxial region, the only type of aberration affecting AXIAL IMAGES and on+off-axis objects because spherical mirrors/refracting surfaces like lenses have a STRONGER PERIPHERY than centre

Question 4

coma

Answer 4

the image formed is UNSYMMETRICAL like the tail of a comet, object is off axis

a lens with considerable coma may produce a SHARP image in the CENTRE of the field, but become increasingly blurred toward the edges

because of the DIFFERENCE IN LM for rays passing through different zones

Question 5

curvature of field

Answer 5

• image is CURVED (consider all other aberrations eliminated)
• image points for an extended objects will form a curved surface (not flat)
• curved surface: petzval surface

Question 6

oblique astigmatism

Answer 6

TWO images formed perpendicular to eo by a spherical lens

Question 7

distortion

Answer 7

occurs if spherical aberration, coma, astigmatism or curvature of field is/are present

distortion changes with stop position (hole, diaphragm) because difference in LM for points on the object at different distances from the optical axis. shape does not change significantly