State the surface powers required to produce the following lenses with +6.25D front curves.

a. -0.50 DS
b. +0.50 DS
c. +1.75 DS

To determine the surface powers required to produce lenses with a +6.25D front curve, we need to calculate the back curve of the lenses.

The back curve is calculated by subtracting the desired lens power (front curve) from the desired lens prescription power:

a. -0.50 DS:
Back curve = Front curve - Lens prescription power
= +6.25D - (-0.50D)
= +6.25D + 0.50D
= +6.75D

b. +0.50 DS:
Back curve = Front curve - Lens prescription power
= +6.25D - (+0.50D)
= +6.25D - 0.50D
= +5.75D

c. +1.75 DS:
Back curve = Front curve - Lens prescription power
= +6.25D - (+1.75D)
= +6.25D - 1.75D
= +4.50D

Therefore, the surface powers required to produce lenses with +6.25D front curves are:
a. -0.50 DS: +6.25D front curve and +6.75D back curve.
b. +0.50 DS: +6.25D front curve and +5.75D back curve.
c. +1.75 DS: +6.25D front curve and +4.50D back curve.