Light is a form of energy Crooke’s Radiometer proves light has energy Turns in sunlight as the light heats the black side Can you think of another example to demonstrate that light is a form of energy?
Light travels in straight lines How are shadows formed?
Reflection • Reflection is the bouncing of light off an object. • When light bounces off objects it scatters in all directions – diffuse reflection. • Highly polished surfaces (mirror) behave in a more predictable way.
Reflection 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.
Reflection Laws of Reflection Animation 1 Laws of Reflection Animation 2
Virtual Image • An image that is formed by the apparent intersection of light rays • Can not appear on a screen d d
Curved Mirrors • Curved mirrors consist of a series of small mirrors combined together. • Each individual mirror must obey the laws of reflection.
2F F Real Image • An image that is formed by the actual intersection of light rays. • 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
Ray Diagram Example • An object 4 cm high is placed at right angles to the axis of a concave mirror and at a distance of 30 cm from the mirror. If the focal length of the mirror is 10 cm find the position, size and nature of the image. • This can be done using a diagram or by calculation.
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
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 i r Repeat for different angles of incidence
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
Refractive Index • Ratio of speeds
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
Refractive Index and Critical Angle • Refractive Index is defined in relation to light going from air into that medium (i.e. air to glass or air to water) • Ex 1: The critical angle for a certain medium is 500 . Find its refractive index • Ex 2: The refractive index of glass is 1.5. What is the critical angle for glass?
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.