Question

A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. Applying the formula of image formation by a single spherical surface successively at the two surfaces of a thin lens, a formula known as lens maker’s formula and hence the basic lens formula can be obtained. The focal length (or power) of a lens depends on the radii of its surfaces and the refractive index of its material with respect to the surrounding medium. The refractive index of a material depends on the wavelength of light used. Combination of lenses helps us to obtain diverging or converging lenses of desired power and magnification.

A thin converging lens of focal length 20 cm and a thin diverging lens of focal length 15 cm are placed coaxially in contact. The power of the combination is ______.

पर्याय

• `(-5)/6D`

• `(-5)/3D`

• `4/3D`

• `3/2D`

Answer 1

A thin converging lens of focal length 20 cm and a thin diverging lens of focal length 15 cm are placed coaxially in contact. The power of the combination is `bbunderline((-5)/3D)`.

Explanation:

`1/f = 1/f_1 + 1/f_2`

⇒ `1/f = 1/20 - 1/15`

= `(3 - 4)/60`

= `(-1)/60`

⇒ f = −60 cm

Power of lenses, `P = 100/f`

= `100/-60`

So, Power = `(-5)/3D`