1.Ans:

In general, a lens with higher power is thinner than a lens with lower power. Therefore, the +2D lens is thicker than the +4D lens, as the +4D lens has a shorter focal length and is designed to be more compact.

2.Ans:

To calculate the focal length of each lens we use the formula:

\(\rm P \) = $\frac{1}{f}$

where \(\rm P\) = Power of lens

and,   \(\rm f\) = focal length of lens

For +2D:

\(\rm P \) = $\frac{1}{f}$

\(\rm or,\) \(\rm +2 \) = $\frac{1}{f}$

\(\rm or,\) \(\rm f \) = $\frac{1}{2}$

\(\rm or,\) \(\rm P \) = \(\rm 0.5\)

Hence, for lens with power +2D focal length is 0.5 m or 50 cm.

For +4D:

\(\rm P \) = $\frac{1}{f}$

\(\rm or,\) \(\rm +4 \) = $\frac{1}{f}$

\(\rm or,\) \(\rm f \) = $\frac{1}{4}$

\(\rm or,\) \(\rm P \) = \(\rm 0.25\)

Hence, for lens with power +4D focal length is 0.25 m or 25 cm.