Light By Neil Bronks
Light is a form of energy Crooke’s Radiometer proves light has energy Turns in sunlight as the light heats the black side
Reflection-Light bouncing off object Angle of incidence = Angle of reflection Normal Reflected ray Incident ray Angle of reflection Angle of incidence Mirror
Laws 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.
Virtual Image • An image that is formed by the eye • Can not appear on a screen d d
2F F Real Image • Rays really meet • Can be formed on a screen
Pole All ray diagrams in curved mirrors and lens are drawn using the same set of rays. Concave Mirror Object F Principal Axis
You can draw any ray diagram by combining 2 of these rays The only difference is where the object is based. F
Ray Diagrams- Object outside 2F 1/. Inverted 2/. Smaller 3/. Real 2F F The images can be formed on a screen so they are real.
2F F Object at 2F 1/. Inverted 2/. Same Size 3/. Real The image is at 2F
2F F Object between 2F and F 1/. Inverted 2/. Magnified 3/. Real The image is outside 2F
2F F Object at F The image is at infinity
F Object inside F 1/. Upright 2/. Magnified 3/. Virtual The image is behind the mirror
Convex Mirror The image is behind the mirror 1/. Upright 2/. Smaller 3/. Virtual F
Convex Mirror – only one ray diagram F The image is behind the mirror
Uses of curved mirrors • Concave Mirrors • Dentists Mirrors • Make –up mirrors • Convex Mirror • Security Mirrors • Rear view mirrors
u v F Calculations f=focal length u=object distance v=image distance • Use the formula
10 20 Example An object is placed 20cm from a concave mirror of focal length 10cm find the position of the image formed. What is the nature of the image? Collect info f=10 and u=20 Using the formula V=20cm real
20 2 20 2 Magnification • What is the magnification in the last question? • Well u=20 and v=20 • As • m=1 • Image is same size
Example An object is placed 20cm from a concave mirror of focal length 30cm find the position of the image formed. What is the nature of the image? Collect info f=30 and u=20 Using the formula V=60cm Virtual
Example An object is placed 30cm from a convex mirror of focal length 20cm find the position of the image formed. What is the nature of the image? Collect info f=-20 and u=30 The minus is Because the Mirror is convex Using the formula V=60/5cm =12cm Virtual
Questions • An object 2cm high is placed 40cm in front of a concave mirror of focal length 10cm find the image position and height. • An image in a concave mirror focal length 25cm is 10cm high if the object is 2cm high find the distance the object is from the mirror.
MEASUREMENT OF THE FOCAL LENGTH OF A CONCAVE MIRROR Concave mirror Crosswire Lamp-box Screen u v
Approximate focal length by focusing image of window onto sheet of paper. Place the lamp-box well outside the approximate focal length Move the screen until a clear inverted image of the crosswire is obtained. Measure the distance u from the crosswire to the mirror, using the metre stick. Measure the distance v from the screen to the mirror. Repeat this procedure for different values of u. Calculate f each time and then find an average value. Precautions The largest errors are in measuring with the meter rule and finding the exact position of the sharpest image.
Refraction The fisherman sees the fish and tries to spear it Fisherman use a trident as light is bent at the surface
Light bends towards the normal due to entering a more dense medium Refraction into glass or water AIR WATER
Light bendsaway from the normal due to entering a less dense medium Refraction out of glass or water
Light bends towards the normal due to entering a more dense medium Light bendsaway from the normal due to entering a less dense medium Refraction through a glass block Light slows down but is not bent, due to entering along the normal
Laws of REFRACTION • The incident ray, refracted ray and normal all lie on the same plane • SNELLS LAW the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant for 2 given media. sin i = n (Refractive Index) sin r
Proving Snell’s Law Sin i i r Sin r A straight line though the origin proves Snell’s law. The slope is the refractive index.
Proving Snell’s Law Laser Sin i i Protractor Glass Block r Sin r A straight line though the origin proves Snell’s law. The slope is the refractive index.
H/W • LC Ord 2006 Q2
Refractive Index • Ratio of speeds
Real and Apparent Depth • A pool appears shallower
Cork Pin Apparent depth Mirror Real depth Water Image Pin MEASUREMENT OF THE REFRACTIVE INDEX OF A LIQUID
Finding No Parallax – Looking Down Pin at bottom Pin reflection in mirror No Parallax Parallax
Set up the apparatus as shown. Adjust the height of the pin in the cork above the mirror until there is no parallax between its image in the mirror and the image of the pin in the water. Measure the distance from the pin in the cork to the back of the mirror – this is the apparent depth. Measure the depth of the container – this is the real depth. Calculate the refractive index n= Real/Apparent Repeat using different size containers and get an average value for n.
Light stays in denser medium Reflected like a mirror Angle i = angle r Refraction out of glass or water
THE CRITICAL ANGLE Finding the Critical Angle… 1) Ray gets refracted 2) Ray still gets refracted 4) Total Internal Reflection 3) Ray still gets refracted (just!)
Critical Angle • Varies according to refractive index
Uses of Total Internal Reflection Optical fibres: 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.
Practical Fibre Optics It is important to coat the strand in a material of low n. This increases Total Internal Reflection The light can not leak into the next strand.