Geometrical and Visual Optics Week 3

What is a lens?

• optically transparent material

• Bound by two polished surfaces

• Atleast one curved

How do you find the power for the first refractive medium?

F1 = n’1-n1/r1

How do you find the refractive power for the second surface?

F2 = n2’-n2/r2

When do you consider the thickness of the lens and its effect on the Vergence?

• when the lens is thick, but if its thin then then it can be ignored

How do you calculate the total power of a thin lens?

F = F1 + F2

Does astigmatic surfaces produce a point image like a spherical surface?

• no

• Refracted pencil of rays is called an astigmatic pencil

Why does astigmatism happen?

• the surface has different curvatures/powers in different principal meridians

• Not shaped evenly

• Minimum curvature in one meridian and maximum curvature in perpendicular meridian

What effect does astigmatism have on the image?

• stretched or compressed due to different curvatures of the eye

How do you correct astigmatism?

• sphere-cylindrical lens (lens with spherical and cylindrical surface)

• Axis meridian = sphere

• Power meridian = sphere + cyl