If \(f=0.5~\text m\) for a glass lens, what is the power of the lens?
1. \(+0.4~\text D\) 
2. \(+4.0~\text D\) 
3. \(+0.2~\text D\) 
4. \(+2.0~\text D\) 

The radii of curvature of the faces of a double convex lens are \(10\) cm and \(15\) cm. Its focal length is \(12\) cm. What is the refractive index of glass?
1. \(1.3\)
2. \(1.2\)
3. \(1.5\)
4. \(1.4\)

A convex lens has a \(20~\text{cm}\) focal length in air. What is the focal length in water?
(the refractive index of air-water \(=1.33,\) refractive index for air-glass \(=1.5\))
1. \(+72.72~\text{cm}\)
2. \(+78.2~\text{cm}\)
3. \(-72.72~\text{cm}\)
4. \(+70.2~\text{cm}\)

Find the position of the image formed by the lens combination in the given figure.

 

The final image is:

What focal length should the reading spectacles have for a person for whom the least distance of distinct vision is \(50~\text{cm}\)?
1. \(-50~\text{cm}\)
2. \(+50~\text{cm}\)
3. \(-60~\text{cm}\)
4. \(+60~\text{cm}\)