# LIGHT. WHAT IS LIGHT? Light is a form of energy that travels away from the source producing it at a speed of 3 x 10 8 m s -1.

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LIGHT

WHAT IS LIGHT? Light is a form of energy that travels away from the source producing it at a speed of 3 x 10 8 m s -1

Transparent: allows light to pass through it, and can see clearly through it e.g. glass Translucent: allows light to pass through it, but cannot see clearly through it e.g. frosted glass Opaque: does not allow light to pass through it e.g. wood

Light Travels in Straight Lines This can be seen in the following examples: Laser Beam of light from a searchlight It can also be shown using pieces of cardboard with a small hole in the middle and a length of thread

ir Normal Angle of incidence Angle of reflection Plane Mirror Reflected rayIncident ray Plane Mirror

LAWS OF REFLECTION OF LIGHT 1. The incident ray, the normal and the reflected ray all lie in the same plane 2. The angle of incidence is equal to the angle of reflection (i = r)

HOW IS AN IMAGE FORMED IN A PLANE MIRROR

Properties of an image in a plane mirror Laterally inverted E.g. your right hand appears as a left hand The “ambulance” sign Erect (right way up) Virtual Same size as the object

Uses of Plane Mirrors Make up mirror The periscope

A virtual image cannot be formed on a screen A real image can be formed on a screen

Experiment to prove the angle of incidence equals the angle of reflection

i r Plane mirror Pins Sheet of paper Diagram

Experiment to prove the angle of incidence equals the angle of reflection Method 1. Set up the apparatus as in the diagram. 2. Mark the incident ray 3. Mark the reflected ray 4. Draw in the normal 5. Measure angles i and r 6. Repeat for different angles

Conclusion Angle i = angle r Precaution Make sure the mirror is perpendicular to the page Mark the back of the mirror on the paper Use a sharp pencil

Reflection of light is when light bounces off a surface

Experiment to find the position of an image in a plane mirror (Not a mandatory experiment) (Write up should be in homework copy)

Object pin Plane mirror Image (Object pin) OM I

Method 1. Set up the apparatus as in the diagram 2. Move the tall finder pin in and out behind the mirror until there is no parallax between the finder pin and the image of the object pin in the mirror

3. Measure the distance from the object pin to the mirror (OM), and the distance from the mirror to the finder pin (MI) Result OM and MI are equal Conclusion The image is as far behind the mirror as the object is in front of it

Spherical Mirrors Convex mirrors and concave mirrors CONVEX CONCAVE

Radius of curvature Focal length Centre of curvature Pole Focus point Diagram of concave mirror

The line from the centre of curvature to the pole is called the principal axis

Rules for Ray Diagrams for Concave Mirror 1. A ray travelling parallel to the principal axis is reflected through the focus 2. A ray travelling through the focus is reflected parallel to the principal axis 3. For a ray which strikes the pole, angle i will be equal to angle r

“In parallel, out through the focus” “In through the focus, out parallel”

Uses of concave mirrors Spotlights Reflectors in car headlights Shaving and make-up mirrors

Pole Diagram of convex mirror Centre of curvature Focus point SHINY SIDE

Ray Diagrams for Convex Mirrors

Uses of convex mirrors Shops (to deter shoplifters) Buses Dangerous bends in roads They give a wide field of view

The Mirror Formulae The focal length of a spherical mirror may be found using the formula: u = distance from object to mirror v = distance from image to mirror f = focal length

Example 1 An object is placed 15cm in front of a concave mirror, of focal length 12cm. Find the position and nature of the image

v = 60 cm

It is a real image since the object is outside f

Example 2 When an object is placed 16 cm in front of a concave mirror of focal length 8 cm, an image is formed. Find the distance of the image from the mirror and say whether it is real or virtual.

v = 16 cm

It is a real image since the object is outside f

(HL) Magnification m =

Example 3 (HL) An object is placed 20 cm from a concave mirror of focal length 25 cm. Find the position, magnification and nature of the image.

v = 100 cm

It is a virtual image since the object is inside f

m = m = 5

Example 4 (HL) A concave mirror of focal length 10 cm forms an erect image four times the size of the object. Calculate the object distance and its nature.

u = 7.5 cm

It is a virtual image since the object is inside f

Experiment to Measure the Focal Length of a Concave Mirror

RAY BOX CONCAVE MIRROR SCREEN CROSS THREADS

Method (An approximate value for the focal length can be found by focusing the image of a distant object on a sheet of paper (e.g. a tree or window))

Set up the apparatus as in the diagram Move the screen in and out until the image of the cross threads is in sharpest focus

Measure object distance (u) and image distance (v) Repeat and calculate an average value of f using

Precautions Measure v when the cross threads is in sharpest focus

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