Question

Read the following and answer any FOUR questions from 19 (i) to 19 (v)

A thick lens has shorter focal length because for a thick lens the optical path- length of the

light is more than for a thin lens, thus the bending of light will be more in case of a thicker

lens. Consequently, it has a shorter focal length. The focal length of a lens is determined

when the lens is focused at infinity.

The principal focal length of a lens can be calculated from the lens formula. Remember focal

length is the property of a lens . And the colour of light is dependent on wavelength. So focal

length of the lens is independent of the colour, wave length and frequency of the light which

is passing through the lens.

1x4

19 i A thick lens has

a. shorter focal length

b. longer focal length

c. neither short nor long

d. none of the above19.ii The graph between object distance ‘u’ and image distance ‘v ‘for a lens is given. The focal length

of the lens is

a. 5 (unit)

b. -5 (unit)

c. 0.5(unit)

d. -0.5(unit)

19-ii A student measures the focal length of a convex lens by putting an object (pin) at a distance

'u' from the lens and measuring the distance 'v' of the image. The graph between 'u' and 'v'

plotted by the student should look like:

a. b.

c. d.

19-iv So focal length of the lens is independent of

a. the colour, wave length and frequency of the light

b. the colour and wave length of the light

c. the wave length and frequency of the light

d. None of the above​

Answer 1

Explanation:

'u' from the lens and measuring the distance 'v' of the image. The graph between 'u' and 'v'

plotted by the student should look like:

a. b.

c. d.

19-iv So focal length of the lens is independent of

a. the colour, wave length and frequency of the light

b. the colour and wave length of the light

c. the wave length and frequency of the light

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