Physics Light
1 2
E B
3 Solar Panels
Crooke’s Radiometer 4 5
Pin­Hole Camera Object (Candle) Image (Inverted & Size)
6 7
8
• Regular e.g. mirrors • Diffuse e.g. page
9 Mirror Page
10 Regular Diffuse
11 Plane Concave Convex Spherical
12 IR Incident ray i Reflected ray Normal at … N r Angle of incidence Angle of reflection
RR 13 IR 1. Same plane … 2. Ð i = Ð r i N r RR
14 Plane Mirror
15 RB (or Laser) P PM i r H
16 O I Virtual image
17 18
Real ­ Rays do pass through the image Virtual ­ Rays do not pass through the image (… they only appear to pass through)
19 • Real or Virtual ?? • Size ?? • Position ?? • Erect or Inverted ?? • Laterally Inverted ...
20 Also ... Decorative Telescopes ….
21 22
No parallax between … S and image of O S PM
O v u u = v i.e. ... 23 24
C F P P F C
25 C F P C = centre of curvature F = focus P = pole P F C
Principle Axis PC = radius of curvature (r) PF = focal length (f) r = 2f 26 ???
27 Focus ­ Real or Virtual ?? Focus ­ Real or Virtual ??
28 Purpose = ?? ­ to locate an image using rays Overall: ­ take 2 rays from the object ­ image is where the 2 reflected rays meet
29 Rays that can be used F F
30 Rays that can be used P
F C F 31 Convex Mirror ­ Ray Diagrams Rays that can be used F F
32 P
F C
33 Java for the ray diagrams:
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• • • • • Shaving Car headlights Floodlights ... Dentists Projectors • • • • Cars ... rear­view Shops … Buses Dangerous corners Advantage & Disadvantages ...
36 u C F P f v u v P F f
37 u f
v 1 1 1 + =
u v f v height of image m = =
u height of object Sign Convention … to real = + … to virtual = ­
38 P1 An object is placed 30 cm in front of a concave mirror of focal length 20 cm. Find the position and nature of the image. 1/u + 1/v = 1/f 1/30 + 1/v = 1/20 1/v = 1/20 ­ 1/30 1/v = 3/60 ­ 2/60 1/v = 1/60 v = + 60 cm Real 60 cm from mirror … in front of mirror
39 P2 An object is placed 12 cm in front of a concave mirror of focal length 20 cm. Find the position, nature and magnification of the image.
1/u + 1/v = 1/f 1/12 + 1/v = 1/20 1/v = 1/20 ­ 1/12 1/v = 3/60 ­ 5/60 1/v = ­2/60 v = ­ 30 cm Virtual 30 cm behind the mirror m = v/u m = (­30) / (12) m = ­ 2.5 2.5 times higher (virtual) 40 P3 An object placed 15 cm in front of a concave mirror produces a real image which is 30 cm from the front of the mirror. (i) Calculate the focal length of the mirror. (ii) If the object is 5 cm high what is the height of the image? (ii)
(i) 1/u + 1/v = 1/f height of image v m = =
1/15 + 1/30 = 1/f height of object u 2/30 + 1/30 = 1/f height of image 30 = 3/30 = 1/f 5 15 f = + 10 cm height of image = 10 cm 41 P4 An image is formed in a concave mirror of focal length 20 cm. The image is three times the size of the object. Where must the object be placed if: (i) the image is real (ii) the image is virtual? (i) v/u = + 3 v = + 3u 1/u + 1/v = 1/f 1/u + 1/3u = 1/20 3/3u + 1/3u = 1/20 4/3u = 1/20 3u = 80 u = 26.67 cm … between F and C (ii) v/u = ­ 3 v = ­ 3u 1/u + 1/v = 1/f 1/u + 1/(­3u) = 1/20 3/3u ­ 1/3u = 1/20 2/3u = 1/20 3u = 40 u = 13.33 cm … inside F
42 P5 An object is placed 40 cm in front of a convex mirror of focal length 24 cm. (i) At what distance from the mirror will the image be formed? (ii) Find the nature and magnification of the image.
(i) 1/u + 1/v = 1/f 1/40 + 1/v = ­ 1/24 1/v = ­ 1/24 ­ 1/40 1/v = ­ 5/120 ­ 3/120 1/v = ­ 8/120 v = ­ 15 cm 15 cm behind the mirror (ii) Virtual m = v/u m = (­15)/40 m = ­ 3/8 Image is 3/8ths ... 43 P6 A convex mirror of focal length 15 cm forms an image that is one third the size of the object Find the positions of the object and the image. v 1 u m = = ­ v = ­ u 3 3 1 1 1 1
1 1 1 3 1 2 www.tryscience.org
www.tryscience.org + =
+ = ­ ­ = ­ = ­ u v f u (­u/3) 15 u u 15 u u = 30 cm u 30 v = ­ = ­
= ­ 10 cm 3 3 10 cm behind 44 LB u CW S v Method Sharp image by … Measure … Result 1/f = 1/u + 1/v Precautions Repeat, other … , to average f
45 LB u
S CM
v LB u CW S v 46 47
= 1/f 1/v Why?
1/u + 1/v = 1/f 0 + 1/v = 1/f 1/v = 1/f Get av
1/u = 1/f erage f 1/u + 1/v = 1/f 1/u + 0 = 1/f 1/u = 1/f 48 49
An object, 3 cm high, is placed (i) 60 cm, (ii) 25 cm, (iii) 10 cm, from a concave mirror of focal length 20 cm. Calculate the position, nature and size of the image in each case. Concave mirror … focus is real => f = +20 cm (i) 1/f = 1/u + 1/v 1/20 = 1/60 + 1/v 1/v = 1/20 ­ 1/60 = 3/60 ­ 1/60 = 2/60 = 1/30 v = + 30 cm m = v/u = (+30) ÷ (+60) = 0.5 height of image = 3 cm x 0.5 = 1.5 cm Ans. 30 cm in front of mirror, real, 1.5 cm high
50 An object, 3 cm high, is placed (i) 60 cm, (ii) 25 cm, (iii) 10 cm, from a concave mirror of focal length 20 cm. Calculate the position, nature and size of the image in each case. (ii) 1/f = 1/u + 1/v 1/20 = 1/25 + 1/v etc… v = + 100 cm m = v/u = 100 ÷ 25 = 4 height of image = 3 cm x 4 = 12 cm Ans. 100 cm in front of mirror, real, 12 cm high
51 An object, 3.0 cm high, is placed (i) 60 cm, (ii) 25 cm, (iii) 10 cm, from a concave mirror of focal length 20 cm. Calculate the position, nature and size of the image in each case. (iii) 1/f = 1/u + 1/v 1/20 = 1/10 + 1/v etc… v = ­ 20 cm m = v/u = (­20) ÷ (+10) = ­ 2 height of image = 3 cm x 2 = 6.0 cm Ans. 20 cm behind the mirror, virtual, 6.0 cm high
52 An object, 10 cm high, is place (i) 40 cm, (ii) 15 cm, from a convex mirror of focal length 30 cm. Calculate the position, nature and size of the image in each case. Convex mirror … focus is virtual => f = ­ 30 cm (i) 1/f = 1/u + 1/v ­ 1/30 = 1/40 + 1/v 1/v = ­ 1/30 ­ 1/40 etc… v = ­ 17.1 cm m = v/u = (­17.1) ÷ (40) = ­ 0.43 Size of image = 10 cm x 0.43 = 4.3 cm Ans. 17.1 cm behind the mirror, virtual, 4.3 cm high
53 An object, 10 cm high, is place (i) 40 cm, (ii) 15 cm, from a convex mirror of focal length 30 cm. Calculate the position, nature and size of the image in each case. Convex mirror … focus is virtual => f = ­ 30 cm (ii) 1/f = 1/u + 1/v ­ 1/30 = 1/15 + 1/v etc… v = ­ 10 cm m = v/u = (­10) ÷ (40) = ­ 0.25 Size of image = 10 cm x 0.25 = 2.5 cm Ans. 10 cm behind the mirror, virtual, 2.5 cm high
54 How far from a concave mirror of focal length 10 cm must an object be place so that the image is 3 times bigger and (i) real, (ii) virtual? (i) Real image => m = + 3 m = v/u = + 3 => v = 3u 1/f = 1/u + 1/v 1/10 = 1/u + 1/3u 1/10 = 3/3u + 1/3u = 4/3u 3u = 40 => u = 13.3 cm Note: … between F and C
55 How far from a concave mirror of focal length 10 cm must an object be place so that the image is 3 times bigger and (i) real, (ii) virtual? (ii) Virtual image => m = ­ 3 m = v/u = ­ 3 => v = ­ 3u 1/f = 1/u + 1/v 1/10 = 1/u + 1/(­3u) 1/10 = 3/3u ­ 1/3u = 2/3u 3u = 20 => u = 6.7 cm Note: … inside F
56 How far from a convex mirror must an object be placed so that its image is one­quarter the size of the object? Convex mirror => Image is virtual => m = ­ 1/4 m = v/u = ­ 1/4 => v = ­ (u/4) 1/f = 1/u + 1/v ­ 1/20 = 1/u + 1/(­u/4) = 1/u ­ 4/u = ­ 3/u u = 60 cm
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