###### Advertisements

###### Advertisements

If the power of a lens is -4.0 D, then it means that the lens is a ______.

#### Options

concave lens of focal length -50m

convex lens of focal length +50cm

concave lens of focal length -25cm

convex lens of focal length -25m

###### Advertisements

#### Solution

If the power of a lens is -4.0 D, then it means that the lens is a **concave lens of focal length -25cm**.

**Explanation:-**

P = -4 D

`P=100/(f(cm))`

`f(cm)=100/P`

`100/-4=-25"cm"`

Negative focal length means concave lens. Concave lens of focal length -25cm.

#### RELATED QUESTIONS

Where must the object be placed for the image formed by a converging lens to be:

real, inverted and same size as the object?

Where must the object be placed for the image formed by a converging lens to be:

virtual, upright and larger than the object?

Which type of lenses are:

thinner in the middle than at the edges?

What type of image is always made by a concave lens?

Fill in the following blank with suitable word:

A convex lens .................. rays of light, whereas a concave lens .................. rays of light.

With the help of a diagram, explain why the image of an object viewed through a concave lens appears smaller and closer than the object.

When an object is placed 10 cm in front of lens* A*, the image is real, inverted, magnified and formed at a great distance. When the same object is placed 10 cm in front of lens *B,* the image formed is real, inverted and same size as the object.

What is the nature of lens B?

A concave lens of focal length 15 cm forms an image 10 cm from the lens. How far is the object placed from the lens? Draw the ray-diagram.

An object 60 cm from a lens gives a virtual image at a distance of 20 cm in front of the lens. What is the focal length of the lens? Is the lens converging or diverging? Give reasons for your answer.

(a) Find the position and size of the virtual image formed when an object 2 cm tall is placed 20 cm from:

(i) a diverging lens of focal length 40 cm.

(ii) a converging lens of focal length 40 cm.

(b) Draw labelled ray diagrams to show the formation of images in case (i) and (ii) above (The diagrams may not be according to scale).

Draw a diagram to represent the second focus of a concave lens.

A ray of light, after refraction through a concave lens emerges parallel to the principal axis. (a) draw a ray diagram to show the incident ray and its corresponding emergent ray. (b) The incident ray when produces meets the principal axis at a point. Name the point.

Study the following diagram :

Name the lens LL’

An object is placed on the axis of a lens. An image is formed by refraction in the lens. For all positions of the object on the axis of the lens, the positions of the image are always always between the lens and the object name the lens.

**Complete the following sentence:**

An object is placed at a distance of more than 40 cm from a convex lens of focal length 20 cm. The image formed is real, inverted and ……….

State two applications a concave lens .

How will you differentiate between a convex and a concave lens by looking at (i) a distant object, (ii) a printed page?

Explain the working of an astronomical telescope using refraction of light.

A student obtained clear image of window grills on the screen. But the teacher told him to get the image of a tree far away, instead of window. To get a clear image, the lens must be ............................

Express power of a concave lens of focal length 50 cm with its sign.

A concave lens is a focal length 30 cm. Find the position and magnification (m) of image for an object placed in front of it at distance 30 cm.State whether the image is real on virtual?

**Choose the correct alternative and rewrite the following: **

Inside water, an air bubble behaves ....................

State three characteristics of the image of an extended source, formed by a concave lens.

Convex lens : converging : : concave lens : _______

Convex magnifying glass is called divergent magnifying glass and concave magnifying glass is called converging magnifying glass.

Distinguish between Concave lens and Convex Lens.

**Distinguish between:**

Concave Lens and Convex Lens