A student uses a lens of focal length 40cm and another of -20cm. Write the nature and power of lens each?

Question

Answer

Power of first lens of 40 cm ,therefore power is 100/f= 1/40=2.5 Dioptre .... since focal length is positive the lens is convex lens.

Power of second lens of -20 cm , therefore power = 100/f =1/-20 = -5 Dioptre ...... since focal length is negative the lens is concave lens

Answer

F1 = 40cm
=0.4 m
p = 1/f
=1/0.4
=2.5 D
so, this is a convex lens (p=+ve)
f2 = -20 cm
=-0. 2 m
p= 1/ f
=1/-0.2
=- 5 D
so, this is a concave lens ( p=-ve)
also, we will have to convert cm to m in case of focal length

Answer

Power of focal length 40 cm = 0.4m,,,
p = 1/fp = 1/0.4
p = 10/4
p = 2.5 D
It is a CONVEX lens...

Power of focal length -20 cm = -0.2m,,,
p = 1/f
p = 1/-0.2p = 10/-2
p = -5 D
It is a CONCAVE lens...

Answer

Nice Question

Answer

Introduction:
In optics, a lens is a transparent device that refracts, or bends, light rays to form an image. The properties of a lens are determined by its focal length, which is the distance between the lens and the point where parallel light rays converge or appear to diverge from. The focal length is usually denoted by the symbol 'f'.

Nature and Power of Lens:
The nature of a lens refers to whether it is converging or diverging, while the power of a lens quantifies its ability to converge or diverge light rays.

1. Lens with a focal length of 40cm:
The lens with a focal length of 40cm is a positive or converging lens. It is also known as a convex lens. This type of lens causes parallel light rays to converge after passing through it. The focal length of 40cm indicates that the lens has a moderate converging power.

2. Lens with a focal length of -20cm:
The lens with a focal length of -20cm is a negative or diverging lens. It is also known as a concave lens. This type of lens causes parallel light rays to diverge after passing through it. The negative focal length of -20cm indicates that the lens has a moderate diverging power.

Explanation:
A convex lens, such as the one with a focal length of 40cm, has a thicker center and thinner edges. When parallel rays of light pass through this lens, they converge and meet at a point called the focal point. This lens is commonly used to correct nearsightedness, as it helps to focus light onto the retina.

On the other hand, a concave lens, like the one with a focal length of -20cm, has a thinner center and thicker edges. When parallel rays of light pass through this lens, they diverge as if they originated from a point behind the lens. This lens is commonly used to correct farsightedness, as it helps to spread out the incoming light rays before they reach the eye's lens.

The power of a lens is defined as the reciprocal of its focal length and is measured in diopters (D). The formula for calculating the power of a lens is P = 1/f, where P represents the power and f represents the focal length.

For the lens with a focal length of 40cm, the power would be P = 1/0.4 = 2.5 D. This positive power indicates the ability of the lens to converge light rays.

For the lens with a focal length of -20cm, the power would be P = 1/(-0.2) = -5 D. This negative power indicates the ability of the lens to diverge light rays.

Conclusion:
In summary, the lens with a focal length of 40cm is a converging or convex lens, while the lens with a focal length of -20cm is a diverging or concave lens. The lens with a focal length of 40cm has a moderate converging power of 2.5 D, while the lens with a focal length of -20cm has a moderate diverging power of -5 D. These lenses have different optical properties and are used for different purposes in correcting vision or manipulating light rays in various optical systems.

Similar Class 10 Doubts

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