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PINHOLE CAMERA What do we need to know? • Light travels in straight lines • A pinhole is a tiny hole CAMERA OBSCURA • Camera Obscura (latin) > Dark Room • Projects a view of the outside onto the inside walls • The image is upside down The history… 5th Century BC: Collecting Place / Locked Treasure Room Mo-Ti 322 BC: 965 AC: Aristotle Alhazen 1500s: Leonardo Da Vinci Pinhole: Small Hole >> Sharper Image Bright Object Small Hole Light reflects off bright surface Pass through small holes Bigger Hole >>Brighter Image Light rays cross Upside down image on wall The history… Leonardo Da Vinci: "When images of illuminated objects ... penetrate through a small hole into a very dark room ... you will see [on the opposite wall] these objects in their proper form and color, reduced in size ... in a reversed position, owing to the intersection of the rays". Leonardo da Vinci • Used for observing eclipses 1558 > Lenses and mirrors 1558: Lens Based Camera Obscura, 1568 Correct for reversed image 1700: Hooke described ‘picture box’ in a paper to the Royal Society 1558 1568 1700 1728: Reversed the specimen to overcome upside down image 1728 The history… Translucent screen + 45° mirror: Override two inversions of image 1700 – 1900 1898 Silicon Image Detector, 1970 1970 The history… • Kodak handheld camera • 1888 Camera sent back to Kodak for processing • Digital Cameras/ SLR • Reflect IR beam off subject + focus lens J-Phone: • Cost: $ 500 • 110,000 pixels (0.1 MP) 1959 2000 Ray diagrams: Making a pinhole camera … • It can be very simple! • Dark box or tube + a tiny hole in the front • We’ll use a screen for viewing on the back instead of film STEP 1 • Roll a tube of black card OR • make a box with two open ends STEP 2 • Use the tinfoil to cover one end (Blocks light from entering) STEP 3 • Make a tiny pinhole in the front STEP 4 4 • Add a screen to the back • The image will appear here STEP 5 • The image will be dark so you’ll need to cover the screen • Make a cover that wraps around the screen. TEST IT! Problems with pinholes image plane r ( x, y , z ) y optical axis effective focal length, f’ z pinhole x r ' ( x' , y ' , f ' ) r' r f' z x' x f' z y' y f' z Problems with pinholes • Pinhole size (aperture) must be “very small” to obtain a clear image. • However, as pinhole size is made smaller, less light is received by image plane. • If pinhole is comparable to wavelength of incoming light, DIFFRACTION effects blur the image! • Sharpest image is obtained when: pinhole diameter Example: If f’ = 50mm, λ = 600nm (red), d = 0.36mm d 2 f ' Image Formation using (Thin) Lenses • Lenses are used to avoid problems with pinholes. • Ideal Lens: Same projection as pinhole but gathers more light! o i P P’ f Gaussian Lens Formula: 1 1 1 i o f • f is the focal length of the lens – determines the lens’s ability to bend (refract) light • f different from the effective focal length f’ discussed before! Two Lens System d final image object f2 i2 o2 i1 f1 o1 image plane lens 2 intermediate virtual image lens 1 • Rule : Image formed by first lens is the object for the second lens. • Ray passing through focus emerges parallel to optical axis. • Ray through optical center passes un-deviated. • Magnification: i2 i1 m o2 o1 The Eye • The human eye is a camera! • Iris - colored annulus with radial muscles • Pupil - the hole (aperture) whose size is controlled by the iris • Photoreceptor cells (rods and cones) in the retina PINHOLE CAMERA How to Build It