Two identical thin plano-convex glass lenses (refractive index = \(1.5\)) each having radius of curvature of \(20\) cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of a refractive index of \(1.7\). The focal length of the combination is:
1. \(-20\) cm
2. \(-25\) cm
3. \(-50\) cm
4. \(50\) cm
1. | \(\dfrac{R}{2(\mu_1-\mu_2)}\) | 2. | \(\dfrac{R}{(\mu_1-\mu_2)}\) |
3. | \(\dfrac{2R}{(\mu_2-\mu_1)}\) | 4. | \(\dfrac{R}{2(\mu_1+\mu_2)}\) |
When a biconvex lens of glass having a refractive index of \(1.47\) is dipped in a liquid, it acts as a plane sheet of glass. The liquid must have a refractive index:
1. | equal to that of glass. |
2. | less than one. |
3. | greater than that of glass. |
4. | less than that of glass. |
1. | virtual, upright, height \(=0.5\) cm |
2. | real, inverted, height \(=4\) cm |
3. | real, inverted, height \(=1\) cm |
4. | virtual, upright, height \(=1\) cm |