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Physics Ex-54 Click The optical power of a lens is a measure of how much the lens bends light. The greater the optical power, the more the lens bends light. Note that the optical power is the reciprocal of the focal length of the lens. The symbol for the optical power of a lens is P and the unit for the optical power is dioptres and is designated by the Greek symbol “ ”. Low optical powerHigh optical power More bendingLess bending

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Physics Ex-54 Click The optical power of a lens is the reciprocal of its focal length. Study these examples: f = 1 cmP = 1/0.01 m= 100 f = 2 cmP = 1/0.02 m= 50 f = 4 cmP = 1/0.04 m= 25 f = 8 cmP = 1/0.08 m= 12.5 f = 10 cmP = 1/0.1 m= 10 f = 25 cmP = 1/0.25 m= 4 f = 50 cmP = 1/0.50 m= 2 f = 100 cmP = 1/1 m= 1 Note that the optical power of a converging lens is positive and for a diverging lens it is negative.

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Physics Ex-54 Click The optical power of a convex (or converging) lens is positive. The optical power of a concave (or diverging) lens is negative. delay

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Physics Ex-54 Click Two or more lenses may be combined to form a compound lens or an optical system. total For such a compound lens, the total optical power is given by the following formula:

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Physics Ex-54 Question-1 Define the optical power of a lens. The ability of a lens to bend (converge or diverge) light rays. Click

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Physics Ex-54 Question-2 What type of a lens has positive optical power? Converging lens. Click

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Physics Ex-54 Question-3 What type of a lens has a negative optical power? Diverging lens. Click

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Physics Ex-54 Question-4 Listed below are the focal lengths of five lenses. Determine their optical powers. Click a) f = 1 cm

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Physics Ex-54 Question-4 Listed below are the focal lengths of five lenses. Determine their optical powers. Click b) f = 5 cm

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Physics Ex-54 Question-4 Listed below are the focal lengths of five lenses. Determine their optical powers. Click c) f = 10 cm

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Physics Ex-54 Question-4 Listed below are the focal lengths of five lenses. Determine their optical powers. Click d) f = -20 cm Note that the negative sign indicates a diverging lens.

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Physics Ex-54 Question-4 Listed below are the focal lengths of five lenses. Determine their optical powers. Click e) f = 15 cm

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Physics Ex-54 Question-5 Listed below are the optical powers of four lenses. Determine their focal lengths. Click a) P = 20

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Physics Ex-54 Question-5 Listed below are the optical powers of four lenses. Determine their focal lengths. Click b) P = 10

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Physics Ex-54 Question-5 Listed below are the optical powers of four lenses. Determine their focal lengths. Click c) P = -25 Note that the negative sign indicates a diverging lens.

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Physics Ex-54 Question-5 Listed below are the optical powers of four lenses. Determine their focal lengths. Click d) P = 8

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Physics Ex-54 Question-6 A converging lens has a focal length of 25 cm. Determine its optical power. Click

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Physics Ex-54 Question-7 The optical power of a diverging lens is – 8.33 . Determine its focal length. Click Note that diverging lens has a negative optical power.

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Physics Ex-54 Question-8 Draw the rays emerging from a lens having the following optical powers: Click a) P = 20 5 cm

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Physics Ex-54 Question-8 Draw the rays emerging from a lens having the following optical powers: Click b) P = 4 25 cm

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Physics Ex-54 Question-8 Draw the rays emerging from a lens having the following optical powers: Click c) P = -20 F 5 cm REMINDER By convention in using lenses, distances are positive on the side where light comes out.

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Physics Ex-54 Question-8 Draw the rays emerging from a lens having the following optical powers: Click d) P = -4 F 25 cm

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Physics Ex-54 Question-9 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. Click

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Physics Ex-54 Question-10 A lens system consists of a converging lens and a diverging lens. The focal length of the converging lens 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? Click The negative sign indicates the system is acting as a diverging (concave) lens.

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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 to each 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. Click REMINDER The unit for length in the optical power formula must be metres. Convert to metres

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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 cm respectively. The lenses are combined to form an optical system. Determine the optical power of this lens system. Click Convert to metres.

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Physics Ex-54 Question-13 Two lenses are placed together to form an optical system. One lens has a focal length of 20.0 cm while the other lens has a focal length of -35.0 cm. If an object 4.0 cm tall is placed 120 cm in front of the system determine the characteristics of the image. Object 120 cm Click Given Calculation of f T Calculation of h i Negative sign indicates inversion Calculation of d i

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Physics Ex-54 Question-14 The focal length of a lens system is 25 cm. What is the optical power of this system? Click A) 4.0 B) -4.0 C) 0.04 D) -0.04 E) 0.25 Convert 25 cm to meters

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