8.04: Lenses: Power of a Lens

The power of a lens is positive for a ...

• Concave (Diverging) Lens

• Convex (Converging) Lens

The power of a lens is negative for a ...

• Convex (Diverging) Lens

• Concave (Converging) Lens

The formula for the power of a lens is...

• $P\ =\ 1\ +\ f$

• $P\ =\ 1\ -\ f$

• $P\ =\ \frac{1}{f}$

• $P\ =\ \frac{1}{1\ +\ f}$

Calculate the power of a convex (converging) lens of focal length 40 cm

• + 2.5 m^-1

• - 2.5 m^-1

• + 2.5 cm^-1

• - 2.5 cm^-1

Calculate the power of a concave lens of focal length 60 cm

• $-1.67\ m^{-1}$

• $+1.67\ m^{-1}$

• $-1.67\ cm^{-1}$

• $+\ 1.67\ cm^{-1}$

A lens has a power of 12 m^-1. Calculate its focal length

• 8.33 cm

• 12.45 cm

• 0.8 m

• 7.5 cm

A diverging lens has a power of

 $-\ 0.025\ m^{-1}$ . Calculate its focal length.

• 4 m

• 40 m 

• 0.4 m

• 40 cm

Two convex lenses of power  $6\ m^{-1}$   and  $10\ m^{-1}$   are placed in contact. Calculate the combined power and the focal length of the combination.

•  $P\ =16\ m^{-1}\ ,\ f\ =\ 6.25\ cm$  
  

•  $P\ =\ -\ 16\ m^{-1}\ ,\ f\ =\ 6.25\ cm$  

•  $P\ =\ 6\ m^{-1}\ ,\ f\ =\ 0.6\ cm$  

•  $P\ =\ -6\ m^{-1\ },\ f\ =\ 6\ cm$  

A concave (diverging) lens of power $-0.02\ m^{-1}$  and a convex (converging) lens of power  $0.05\ m^{-1}$  are placed in contact. Find the power of the combination of the lenses. Does the combination behave as a convex or concave lens?

•  $P\ =\ 0.03\ m^{-1}$  

•  $P\ =\ -0.03\ m^{-1}$  

•  $P\ =\ 3\ m^{-1}$  

•  $P\ =\ -\ 3\ m^{-1}$  

A combination of two convex lenses has a focal length of 20 cm. If the focal length of one of the lenses is 60 cm, what is the focal length of the other?

• 30 cm

• 40 cm

• 20 cm

• 60 cm