Lecture 17. Image formation Ray tracing Calculation. Lenses Convex Concave. Mirrors Convex Concave. Optical instruments

Transcription

1 Lecture 17. Image formation Ray tracing Calculation Lenses Convex Concave Mirrors Convex Concave Optical instruments

2 Image formation Laws of refraction and reflection can be used to explain how lenses and mirrors operate Lenses Convex (converging) lens Parallel rays (e.g. from the Sun) passing through a convex lens : focal point f: focal length f Sunlight focused by magnifying glass may burn hole in paper placed at focal point.

3 Image formation Convex lens is the focal point. f is the focal length, an important characteristic f Strength in diopters (D) = 1 f ( f is in metres) Example A farsighted person requires an eyeglass of strength 2.5 diopters. What is the focal length of the eyeglass lens? D =2.5 f = 1 D =1/2.5 = 0.4 m = 40cm.

4 Image ormation (ray tracing) Object at a distance greater than the focal 1 length from the lens h 2 3 Optic axis h s s Ray 1 entering lens parallel to optic axis will exit and pass through the focal point Ray 2 passing through the focal point will exit the lens and travel parallel to optic axis Ray 3 will undergo only a small deviation (not shown) (thin lens) Real inverted image formed Real image (may be projected and displayed on a screen) Rays are reversible

5 Image ormation (ray tracing) Convex lens Object at a distance less than the focal length from the lens h h s s Simple magnifier Image virtual, upright magnified.

6 Concave (diverging) lens Optic axis f Rays entering lens parallel to axis appear to originate at focal point Dashed lines indicate the direction from which the rays appear to come

7 Concave (diverging) lens Object outside the focal length h h s Optic axis s Virtual image always produced by concave lens Cannot be viewed on screen since rays are diverging on the right of the lens However can be viewed with the eye since the eye converges the rays onto the retina.

8 h Image ormation---calculation Ray diagrams are useful in sketching the relationship between object and image Relationship may also be calculated A E B O C h D s s Triangles AOB and DOC are similar ' h h s s' ' h' s h s h Triangles EO and DC are similar h ' f s f ' ' s s f s ' f ' ' h s f h f s ' s f

9 Thin lens formula Image ---calculation s ' s f h B h s s Object distance s positive if object is in front of lens negative if object is behind lens Image distance s positive if image is formed behind the lens (real) negative if is formed in front of the lens (virtual) ocal length f positive -- convex lens negative --concave lens

10 Magnification h B h s s Magnification is defined as M s ' s or M h' h M Negative : inverted image M Positive : upright image

11 Simple magnifier h h s s Object placed inside focal length of converging lens; image viewed virtual, magnified.

12 Example An object 0.5 cm in height is placed 8 cm from a convex lens of focal length 10 cm. Determine the position, magnification, orientation and height of the image. M M s ' s f s ' 10 s = 10 x s ' 40cm 5 s 8cm h' h = - 40cm. s ' f s M +ve image upright h = M x h = 5 x 0.5cm = 2.5cm h h s s

13 Example An object is placed 45 cm from a lens of focal length -25 cm. Determine the position, magnification, and orientation of the image s ' s f 1 S s ' f s 1 1 = - S = cm -25 cm 45 cm M = - S S M +ve image upright M = cm 45 cm = 0.36 h s h Optic s axis

14 Combining Lenses Effective focal length (f eff ) of combination of a number of thin lenses close together f f f eff 1 2 Effective strength (S eff ) of combination of a number of thin lenses close together S S S... eff 1 2 Determine the combined strength of a thin convex lens and a thin concave lens placed close together if their respective focal lengths are 10cm and -20cm. Strength S, in diopters (D) = Seff f f f eff 1 2 S eff 1 f ( f is in metres) m.2m Seff diopters

15 Mirrors lat Mirror Concave Mirror Convex Mirror Curved mirrors are analogous to lenses Ray tracing and thin lens equation also valid. real and virtual images are also formed lat Mirror object d d image Object and image distance equal Object and image same size Image, upright, virtual

16 Spherical Mirrors Hollow sphere Spherical mirror C R Principal or optic axis Spherical mirror is a section hollow sphere Radius of curvature R = 2f

17 Curved Mirrors (Spherical) Concave mirror (converging) Positive focal length f Convex mirror (diverging) Negative focal length f Thin lens formula may be used to determine object and image distances and focal lengths etc Lenses and mirrors Real image : inverted (h negative), positive image distance s Virtual image: upright (h positive), negative image distance s

18 Mirrors Concave shaving/makeup mirrors C C is the centre of curvature Object placed at distance < f from mirror Image is virtual, upright and enlarged. Question: if object is placed at the focal point, where is image located? Application: searchlight

19 Mirrors Example An object is positioned 5 cm in front of a concave mirror of focal length 10 cm. Determine the location of its image and its characteristics s ' s f s ' f s s ' 10cm 5cm s = 5 cm f = 10 cm S = -10cm s ' 10cm 10cm 10cm Characteristics. Image virtual Located behind mirror M M s ' M s 10cm 5cm 2

20 Optical instruments Microscopes, telescopes, cameras etc System may have many optical elements (example, lenses and mirrors) Thin lens formula or ray tracing may be used to analyse behaviour of such systems Simple compound microscope two convex lenses Objective lens Object (height h) o e h eyepiece e inal image o h image formed by objective lens is inside focal length of eyepiece lens.

21 Optical instruments Dental loupes Important Characteristics Resolution* ield width * ield depth magnification Resolution ability to see fine detail Multiple lenses ield width Size of operating site when viewed through loupe unction of lens system diameter and magnification ield depth Depth or range of focus Depends on, available light, optical design, magnification and accommodation Magnification, important but not at the expense of resolution. Large fuzzy image of little use.

22 Optical instruments Dental loupes objective Galilean Design Operating site eyepiece Typical working distance cm Typically Magnification m Optical design allows observer focus at infinity thereby relieving eyestrain

23 Optical instruments Simple refracting telescope Objective lens forms image (real, inverted) at focal point 0 which is also the focal point e of the eyepiece; virtual image is then formed at infinity. Objective lens eyepiece f 0 f e o, e Virtual image at infinity, Magnified and inverted M f f 0 e

24 Optical instruments Simple reflecting telescope lat mirror Objective, concave mirror Eyepiece lens Example Effective focal length of the objective in the Hubble telescope is 57.8 m. What focal length eyepiece is required to give a magnification of -8.0 x M f f 0 e f e = -Mf 0 = -(-8.0 x 10 3 ) x 57.8 m =7.23 x10-3 m

25 Optical instruments Optical fibre: Total Internal Reflection Applications Image transport, Coherent fibre bundle Optical communications: Optical fibres used to transmit modulated laser beams; carrying information Telephone and internet communications Rate at which information can be transported proportional to frequencyof light Single fibre: many millions of phone conversations simultaneously. Cable has many fibres

26 Optical instruments Optical fibre: Total Internal Reflection Endoscope Endoscope for medical investigations inserted through small incision or orifice to inspect and facilitate operation on interior parts of the body flexible shaft includes: light source to illuminate area, image channel to view area under investigation, air or water conduit to clear debris, instrument conduit eyepiece Instrument entry Typical endoscope lexible shaft Transmits light, air, water

27 Camera Optical instruments aperature CCD array Lens translated to change image distance S to adjust for different object distances S. ocal length of lens is fixed. Real, inverted image formed on CCD array s ' s f

28 Example: An object of height 3 cm is positioned 40 cm from a concave lens with a focal length of -20 cm. Determine the position of the image, its magnification, height, and orientation.