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Define focal length of a divergent lens.

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#### Solution

The focal length of a diverging lens is half the value of its radius of curvature. Conventionally, the sign of the focal length of the diverging lens is taken as negative.

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#### RELATED QUESTIONS

The distance between principal focus and optical centre of the lens is _________.

- diameter
- focal length
- principal axis
- optical centre

Suppose you have focused on a screen the image of candle flame placed at the farthest end of the laboratory table using a convex lens. If your teacher suggests you to focus the parallel rays of the sun, reaching your laboratory table, on the same screen, what you are expected to do is to move the:

(a) lens slightly towards the screen

(b) lens slightly away from the screen

(c) lens slightly towards the sun

(d) lens and screen both towards the sun

**Fill in the blanks:**

An image formed by a concave ______________cannot be obtained on a screen.__

Mark ‘T’ if the statement is true and ‘F’ if it is false:

A concave lens always forms a virtual image. (T/F)

Mark ‘T’ if the statement is true and ‘F’ if it is false:

A real image cannot be obtained on a screen. (T/F)

State two differences between a convex and a concave lens.

Which type of lens forms always a virtual image?

A diverging mirror of radius of curvature 40 cm forms an image which is half the height of the object. Find the object and image positions.

Explain why, a stick half immersed in water appears to be bent at the surface. Draw a labelled diagram to illustrate your answer.

Write the new Cartesian sign convention for spherical lenses.

Two lenses *A* and *B* have power of (*i*) +2D and (*ii*) −4D respectively. What is the nature and focal length of each lens?

If a spherical lens has a power of, −0.25 D, the focal length of this lens will be :

(a) −4 cm

(b) −400 mm

(c) −4 m

(d) −40 cm

If a spherical lens has a power of, −0.25 D, the focal length of this lens will be :

(a) −4 cm

(b) −400 mm

(c) −4 m

(d) −40 cm

My grandfather uses a bifocal lens in his spectacle. Explain why.

Complete the ray diagram to show how the rays pass into and out of the lens?

Which diagram shows image formation of an object on a screen by a converging lens?

**Complete the following table for convex lens:**

Sr. No. |
Position of the object |
Position of the image |
Nature of the image |

1. | Beyond 2F_{1} |
__________________ | ________________________ |

2. | __________________ | At infinity | ________________________ |

3. | __________________ | __________________ | Real, inverted and enlarged |