high school by ssl technologies physics ex-54 click the optical power of a lens is a measure of how...

High School

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Physics Ex-54

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The optical power of a lens is a measure of how much the lensbends light. The greater the optical power, the more the lensbends light.

Note that the optical power is the reciprocal of the focal lengthof the lens. The symbol for the optical power of a lens is P andthe unit for the optical power is dioptres and is designated by

the Greek symbol “”.

Low optical powerHigh optical power

More bending Less bending

Physics Ex-54

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The optical power of a lens is the reciprocal of its focal length.

Study these examples:

f = 1 cm P = 1/0.01 m = 100 f = 2 cm P = 1/0.02 m = 50 f = 4 cm P = 1/0.04 m = 25 f = 8 cm P = 1/0.08 m = 12.5 f = 10 cm P = 1/0.1 m = 10 f = 25 cm P = 1/0.25 m = 4 f = 50 cm P = 1/0.50 m = 2 f = 100 cm P = 1/1 m = 1

Note that the optical power of a converging lensis positive and for a diverging lens it is negative.

Physics Ex-54

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The optical power of a convex (or converging) lens is positive.

The optical power of a concave (or diverging) lens is negative.

Physics Ex-54

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Two or more lenses may be combined to form a compound lensor an optical system.

For such a compound lens, the totaltotal optical power is given bythe following formula:

Physics Ex-54Question-1

Define the optical power of a lens.

The ability of a lens to bend (converge or diverge) light rays.

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What type of a lens has positive optical power?

Converging lens.

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What type of a lens has a negative optical power?

Diverging lens.

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Listed below are the focal lengths of five lenses.

Determine their optical powers.

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a) f = 1 cm




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b) f = 5 cm




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c) f = 10 cm




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d) f = -20 cm

Note that the negative sign indicates a diverging lens.




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e) f = 15 cm


Listed below are the optical powers of four lenses.

Determine their focal lengths.

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a) P = 20




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b) P = 10




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c) P = -25 Note that the negative sign indicates a diverging lens.




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d) P = 8


A converging lens has a focal length of 25 cm.

Determine its optical power.

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The optical power of a diverging lens is – 8.33 .

Determine its focal length.

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Note that diverging lens has a negative optical power.


Draw the rays emerging from a lens having the following opticalpowers:

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a) P = 20

5 cm



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b) P = 4

25 cm



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c) P = -20

F

5 cm

REMINDER

By convention in using lenses,distances are positive on theside where light comes out.



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d) P = -4

F

25 cm


Two thin lenses are placed together to form an effective lens system.The lenses have optical powers of 20.0 and –12.0 respectively.

Find the focal length of the system.

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A lens system consists of a converging lens and adiverging lens. The focal length of the converginglens is 60 cm.

If the optical power of the system is to be 1.25 ,what should be the focal length of the diverging lens?

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The negative sign indicatesthe system is acting as a diverging (concave) lens.

Physics Ex-54

A) 13.5 δ

B) 10.3 δ

C) 9.6 δ

D) 3.4 δ

E) -2.9 δ

Question-11

An optical system is made using two thin lenses placed close toeach other. The optical power of the system is 4 δ (dioptres).

Knowing that one of the lenses has a focal length of -16 cm,determine the optical power of the second lens.

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REMINDER

The unit for length in the optical power formula must

be metres.

Convert to metres

Physics Ex-54

A) 0.08 δ

B) 0.23 δ

C) 2.70 δ

D) 16 δ

E) 43 δ

Question-12

Three lenses have focal lengths of 10.5 cm, -7.5 cm and 5 cmrespectively. The lenses are combined to form an optical system.

Determine the optical power of this lens system.

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Convert to metres.


Two lenses are placed together to forman optical system. One lens has afocal length of 20.0 cm while the otherlens has a focal length of -35.0 cm.

If an object 4.0 cm tall is placed 120 cm in front of thesystem determine the characteristics of the image.

Object

120 cm

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GivenCalculation of fT

Calculation of hi

Negative sign indicates inversion

Calculation of di


The focal length of a lens system is 25 cm.

What is the optical power of this system?

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A) 4.0

B) -4.0

C) 0.04

D) -0.04

E) 0.25

Convert 25 cm to meters

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