LENSES

A concave lens forms the image of an object such that the distance between the object and image is 10 cm and the magnification produced is 1/4. The focal length of the lens will be : __________

• 8.6 cm

• -6.2 cm

• -10 cm

• -4.4 cm

A convex lens of focal length 40 cm is in contact with a concave lens of focal length 25 cm. the power of the combination is : ______

• - 1.5 D

• -6.5 D

• + 6.5 D

• + 6,67 D

A plano–convex lens fits exactly into another plano-concave lens. Their plane surface are parallel to each other. If the lenses are made of different materials, what is the focal length of combination? Given that:

𝑅 = Radius of curvature of the curved portion

𝜇1 = refractive index of material of plano-concave lens

𝜇2= refractive index of material of plano-convex lens

• $\frac{R}{\left(\mu2-\mu1\right)}$

• $\frac{R}{\left(\mu1-\mu2\right)}$
  

• $\frac{R}{\left(\mu2-1\right)}$

• $\frac{R}{\left(\mu1-1\right)}$

The focal length of a biconvex lens of radii of each surface 50 cm and refractive index 1.5, is

• 40.4 cm

• 75 cm

• 50 cm

• 80 cm

A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will

• become zero

• become infinite

• become small, but non-zero

• remain unchanged

The radius of curvature of the curved surface of a plano-convex lens is 20 cm. If the refractive index of the material of the lens be 1.5, it will

• act as a convex lens only for the objects that lie on its curved side.

• act as a concave lens for the objects that lie on its curved side.

• act as a convex lens irrespective of the side on which the object lies.

• act as a concave lens irrespective of side on which the object lies.

In the figure given below there are two convex lenses L_A and L_B having focal lengths F_A and F_B respectively. The distance between L_A and L_B is

• F_A

• F_B

• F_A + F_B

• F_A ‒ F_B

The focal length in this scenario is

• -1.5 units

• 1.5 units

• 2.5 units

• 4.0 units