Reflection and Refraction. Geometrical Optics

Transcription

1 Reflection and Refraction Geometrical Optics

2 Reflection Angle of incidence = Angle of reflection The angle of incidence,i, is always equal to the angle of reflection, r. The incident ray, reflected ray and the normal all lie on the same plane. Normal Incident ray Angle of incidence - i Angle of reflection - r Reflected ray Mirror

3 Virtual Image An image that is formed by the eye Can not appear on a screen d d

4 All ray diagrams in curved mirrors and lens are drawn using the same set of rays. Concave Mirror Principal Axis Object Pole

5 Ray Diagrams- Object outside 2 1/. Inverted 2/. Smaller 2 3/. Real The images can be formed on a screen so they are real.

6 Object at 2 2 1/. Inverted 2/. Same Size 3/. Real The image is at 2

7 Object between 2 and 2 The image is outside 2 1. Inverted 2. Magnified 3. Real

8 Object at 2 The image is at infinity

9 Object inside 1/. Upright 2/. Magnified 3/. Virtual The image is behind the mirror

10 Convex Mirror The image is behind the mirror 1. Upright 2. Smaller 3. Virtual

11 Convex Mirror The image is behind the mirror

12 Calculations f=focal length u=object distance f u v v=image distance u v

13 ocal length of the concave mirror Concave mirror Cross Light source u Screen v

14 Description 1. Approximate focal length by focusing image of window onto sheet of paper. 2. Place the light source outside the approximate focal length 3. Move the screen until a clear inverted image of the cross is obtained. 4. Measure the distance u from the cross to the mirror 5. Measure the distance v from the screen to the mirror. 6. Calculate f using the equation f u v

15 Refraction The fisherman sees the fish and tries to spear it isherman use a trident as light is bent at the surface

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17 Refraction into glass or water AIR WATER Light bends towards the normal due to entering a more dense medium

18 Refraction out of glass or water Light bends away from the normal due to entering a less dense medium

19 Refraction through a glass block Light bends towards the normal due to entering a more dense medium Light bends away from the normal due to entering a less dense medium Light slows down but is not bent, due to entering along the normal

20 Laws of RERACTION The incident ray, refracted ray and normal all lie on the same plane Snell s law says that the ratio of the sine of the angle of incidence sin(i) to the sine of the angle of refraction sin(r) is constant for 2 given media, and is called the refractive index.

21 Critical Angle 1) Ray gets refracted 2) Ray still gets refracted refraction Brewster s angle Total Internal Reflection

22 Critical Angle Brewster s angle Varies according to refractive index sin C 1 n or example: sin 45 1 n 1 n n

23 Optical fibres: Uses of Total Internal Reflection An optical fibre is a long, thin, transparent rod made of glass or plastic. Light is internally reflected from one end to the other, making it possible to send large chunks of information Optical fibres can be used for communications by sending e-m signals through the cable. The main advantage of this is a reduced signal loss. Also no magnetic interference.

24 1) Endoscopes (a medical device used to see inside the body): 2) Binoculars and periscopes (using reflecting prisms )

25 Lenses Two types of lenses ocal Point ocal Point Converging Lens Diverging Lens

26 Converging Lens- Object outside Image is 1/. Real 2/. Inverted 3/. Smaller

27 Object at Image is 1/. Real 2/. Inverted 3/. Same size

28 Object between 2 and 2 2 Image is 1/. Real 2/. Inverted 3/. Magnified

29 Object at Image is at infinity

30 Object inside Image is 1/. Virtual 2/. Erect 3/. Magnified

31 Calculations f=focal length u=object distance f u v v=image distance u 2 2 v

32 MEASUREMENT O THE OCAL LENGTH O A CONVERGING LENS Show on OPTICAL BENCH Lamp-box with crosswire Lens Screen u v

33 1. Place the lamp-box well outside the approximate focal length 2. Move the screen until a clear inverted image of the crosswire is obtained. 3. Measure the distance u from the crosswire to the lens, using the metre stick. 4. Measure the distance v from the screen to the lens. 5. Calculate the focal length of the lens using f u v 6. Repeat this procedure for different values of u. 7. Calculate f each time and then find the average value.

34 Diverging Lens Image is 1/. Virtual 2/. Upright 3/. Smaller

35 Example An object is placed 30cm from a diverging lens of focal length 20cm find the position of the image formed. What is the nature of the image? Collect info f=-20 (because it is a diverging lens) and u=30 1 f 1 u 1 v v=60/5cm =12cm Virtual v v

36 Power of Lens Opticians use power to describe lenses expressen in dioptias: P 1 f

37 Lens in Contact Most camera lens are made up of two lens joined to prevent dispersion of the light, also called chromatic dispersion. The power of the total lens is P total =P 1 + P 2