Power of a Lens

Two thin lenses of power $+4 \mathrm{D}$ and $-2 \mathrm{D}$ are in contact. The focal length of the combination?

Two lenses of powers $10 \mathrm{D}$ and $-5 \mathrm{D}$are placed in contact.

(i) Calculate the power of the new lens.

(ii) Where should an object be held from the lens, so as to obtain a virtual image of magnification $2$?

For a normal eye, the far point is infinity and the near point is $25 \mathrm{cm}$ in front of eyes. The cornea of the eyes provides a constant converging power of $+40 \mathrm{D}.$ To see the object at infinity, the least converging power (corresponding to max. focal length) of the eye-lens is $+20 \mathrm{D}.$ Estimate the distance between the eye-lens and retina.

If the power of a lens is $20 \mathrm{D}$, what is the focal length in centimetres?

A convex lens $A$ of focal length $20 \mathrm{cm}$ and a concave lens $B$ of focal length $5 \mathrm{cm}$ are kept along the same axis with distance $d$ between them. If a parallel beam of light falling on $A$ leaves $B$ as a parallel beam, then distance $d$ in $\mathrm{cm}$ will be,

A convex lens of focal length $40 \mathrm{cm}$ is in contact with concave lens of focal length $25 \mathrm{cm}.$ The power of combination is -

Find the optical power of a thin glass lens in a liquid medium with a refractive index $1.6$ if its optical power in air is $-8\mathrm{D}$. (Refrective index of glass $1.5$)

Optical power of a thin lens in a medium when $n_0$ is the refractive index of the medium and $n$ is the refractive index of the lens is

The near point and far point of a person are 40cm and 250cm respectively. Determine the power of the lens he/she should use while reading a book kept at distance 25cm from the eye.

A focal length of a lens is $10 \mathrm{cm}$. What is power of a lens in dioptre?

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

Assertion: Although the surfaces of a goggle lens are curved, it does not have any power.

Reason: In case of goggles, lenses are concavo convex and both the surface of lens have equal radii of curvature.

A hypermetropic person has to use a lens of power +5 D to normalize his vision. The near point of the hypermetropic eye is

A hypermetropic person having near point at a distance of 0.75 m puts on spectacles of power 2.5 D. The near point now is at

Two thin lenses of focal length 20 cm and 25 cm are placed in contact .The effective power of the combination is

The power of the combination of a convex lens of focal length 50 cm and concave lens of focal length 40 cm is

A thin convex lens of crown glass having refractive index $1.5$ has power $1 \mathrm{D}$. What will be the power of similar convex lens refractive index $1.6$?

Two thin lenses have a combined power of $+9 \mathrm{D}$. When they are separated by a distance of $20 \mathrm{cm}$, their equivalent power becomes $+\frac{27}{5}\mathrm{ }\mathrm{D}.$ Their individual powers (in $\mathrm{dioptre}$) are

Two lenses of power $-15 \mathrm{D}$ and $+5 \mathrm{D}$ are in contact with each other. The focal length of the combination is

If two $+5 \mathrm{D}$, lenses are mounted at some distance apart, the equivalent power will always be negative, if the distance is