High School by SSL Technologies Physics Ex-54 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 “”. More bending High optical power Less bending Low optical power Click Physics Ex-54 The optical power of a lens is the reciprocal of its focal length. Study these examples: f f f f f f f f = = = = = = = = 1 cm 2 cm 4 cm 8 cm 10 cm 25 cm 50 cm 100 cm P P P P P P P P = = = = = = = = 1/0.01 m 1/0.02 m 1/0.04 m 1/0.08 m 1/0.1 m 1/0.25 m 1/0.50 m 1/1 m = = = = = = = = 100 50 25 12.5 10 4 2 1 Note that the optical power of a converging lens is positive and for a diverging lens it is negative. Click Physics Ex-54 The optical power of a convex (or converging) lens is positive. The optical power of a concave (or diverging) lens is negative. Click Physics Ex-54 Two or more lenses may be combined to form a compound lens or an optical system. For such a compound lens, the total optical power is given by the following formula: Click Question-1 Physics Ex-54 Define the optical power of a lens. The ability of a lens to bend (converge or diverge) light rays. Click Question-2 Physics Ex-54 What type of a lens has positive optical power? Converging lens. Click Question-3 Physics Ex-54 What type of a lens has a negative optical power? Diverging lens. Click Question-4 Physics Ex-54 Listed below are the focal lengths of five lenses. Determine their optical powers. a) f = 1 cm Click Question-4 Physics Ex-54 Listed below are the focal lengths of five lenses. Determine their optical powers. b) f = 5 cm Click Question-4 Physics Ex-54 Listed below are the focal lengths of five lenses. Determine their optical powers. c) f = 10 cm Click Physics Ex-54 Question-4 Listed below are the focal lengths of five lenses. Determine their optical powers. d) f = -20 cm Note that the negative sign indicates a diverging lens. Click Question-4 Physics Ex-54 Listed below are the focal lengths of five lenses. Determine their optical powers. e) f = 15 cm Click Question-5 Physics Ex-54 Listed below are the optical powers of four lenses. Determine their focal lengths. a) P = 20 Click Question-5 Physics Ex-54 Listed below are the optical powers of four lenses. Determine their focal lengths. b) P = 10 Click Physics Ex-54 Question-5 Listed below are the optical powers of four lenses. Determine their focal lengths. c) P = -25 Note that the negative sign indicates a diverging lens. Click Question-5 Physics Ex-54 Listed below are the optical powers of four lenses. Determine their focal lengths. d) P = 8 Click Question-6 Physics Ex-54 A converging lens has a focal length of 25 cm. Determine its optical power. Click Question-7 Physics Ex-54 Note that diverging lens has a negative optical power. The optical power of a diverging lens is – 8.33 . Determine its focal length. Click Question-8 Physics Ex-54 Draw the rays emerging from a lens having the following optical powers: 5 cm a) P = 20 Click Question-8 Physics Ex-54 Draw the rays emerging from a lens having the following optical powers: 25 cm b) P = 4 Click Physics Ex-54 Question-8 Draw the rays emerging from a lens having the following optical powers: 5 cm c) P = -20 REMINDER By convention in using lenses, distances are positive on the side where light comes out. F Click Physics Ex-54 Question-8 Draw the rays emerging from a lens having the following optical powers: 25 cm d) P = -4 F Click Question-9 Physics Ex-54 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 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? The negative sign indicates the system is acting as a diverging (concave) lens. Click Physics Ex-54 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. A) 13.5 δ B) 10.3 δ C) 9.6 δ Convert to metres REMINDER The unit for length in the E) -2.9 δ optical power formula must be metres. D) 3.4 δ Click Question-12 Physics Ex-54 Convert to metres. 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. A) 0.08 δ B) 0.23 δ C) 2.70 δ D) 16 δ E) 43 δ Click 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. Object 120 cm If an object 4.0 cm tall is placed 120 cm in front of the system determine the characteristics of the image. Given Calculation of fT Calculation of di Calculation of hi Negative sign indicates inversion Click Physics Ex-54 Question-14 Convert 25 cm to meters The focal length of a lens system is 25 cm. What is the optical power of this system? A) 4.0 B) -4.0 C) 0.04 D) -0.04 E) 0.25 Click SSLTechnologies.com/science