Light - 07 - Lenses - Power & Drawing Images

University:
High School
Course:
High School Physics
Academic year:

2024
Views:

470

Pages:

20
Author:

1 1

Lenses Types, Power & Images 8.1 Lenses: ⚫ ⚫ ⚫ Please write this A transparent object with uniformly curved surfaces Control the direction of light rays passing through it using refraction Types: ⚫ ⚫ Converging – bring light rays together – thicker at center than edges/ends Diverging – spread light rays apart – thinner at center than edges/ends P 107 Converging Diverging double convex double concave plano-concave plano-convex concavo-convex (positive meniscus) convex-concave (negative meniscus) Please write this ⚫ When light rays pass through a lens, they refract differently due to the curved surface F’ = Secondary Principal Focus (equal distance to lens as F) Refracted rays appear to emerge from focus F’ F’ Refracted rays converge at focus 8.2 Drawing Images Converging: Please write this ⚫ Ray 1 – ⚫ ⚫ Ray 2 – ⚫ ⚫ parallel to axis -> refracted through F passing through F’ -> refracted parallel to axis Ray 3 – ⚫ passing through centre of lens, is not refracted Ex F’ F Please write this With converging lenses, the image can be real or virtual, upright or inverted, larger or smaller depending where the object is placed!!! (same as mirror) If object is placed at F’, then no image is formed!!! Ex F’ F 8.3 Drawing Images Diverging: ⚫ Ray 1 – ⚫ ⚫ Ray 2 – ⚫ ⚫ parallel to the axis -> appears to be refracted from F directed towards F’ -> refracted parallel to the axis Ray 3 – ⚫ passing through centre of lens, is not refracted Ex F F’ Please write this With diverging lenses, the image is always virtual, upright, smaller. Please write this 8.4 Power: ⚫ Opticians use the reciprocal of focal length to describe optical power (P) in diopters (d) of a lens diopters (d) 1 P= f focal length (m) Please write this ⚫ The greater the power of the lens, the shorter the focal length ⚫ By convention, power for: ⚫ ⚫ Converging lens is positive (ie 2.5d) Diverging lens is negative (ie -2.5d) Ex What is the optical power of a converging lens with a focal length of 35cm? 1 P= f 1 P= 0.35 P = 2 .9 d Ex A lens has an optical power of -5.50d. What is the focal length? What type of lens is it? 1 1 − 5 .5 = P= f f f = − 0.182m diverging Please write this ⚫ Lenses, and their powers, can be combined Pt = P1 + P2 + .... Do these problems please: 1. Two lenses with powers of 3.0d and -4.5d are combined. What is the focal length of this combination? 2. A system of three lenses have a power of 8.0 diopters. Lens A has a focal length of 0.10m & lens B has a focal length of 0.40m. What is the focal length of lens C? Ex Two lenses with powers of 3.0d and -4.5d are combined. What is the focal length of this combination? Pt = P1 + P2 1 P= f Pt = 3d + (−4.5)d Pt = −1.5d 1 − 1.5 = f f = −0.67m Ex A system of three lenses have a power of 8.0 diopters. Lens A has a focal length of 0.10m & lens B has a focal length of 0.40m. What is the focal length of lens C? Pt = PA + PB + PC 1 P= f 1 PA = 0 .1 1 PB = 0 .4 PA = 10d PB = 2.5d 8d = 10d + 2.5d + PC PC = −4.5d 1 − 4.5 = f f = −0.22m Quantum: HW: pg 112 #2, 3, 8, 10 &16 pg 122 #1, 2