Chapter 2 : REFLECTION

Introductory Experiment boundary Incident ray Reflected ray Refracted ray S = light source

Boundary : surface separating two transparent media. Reflection : sudden change of direction experienced by light rays at the boundary while remaining in the same medium. Refraction : sudden change of direction experienced by light rays when they cross the boundary. reflection refraction

1. The laws of reflection 1.1. Experiments. Consider a light ray hitting a plane (flat) mirror (M) at point I (the point of incidence).

M S I O N r i S : light source O : observer I : point of incidence M : reflecting surface (mirror) SI : incident ray IO : reflected ray NI : normal at the point of incidence i : angle of incidence r : angle of reflection SI and NI define the plane of incidence  D D D Diagram and vocabulary

1.2. The laws of reflection By trying different values of i, we can demonstrate experimentally that : the incident ray, the normal at the point of incidence and the reflected ray are all situated in the same plane the angles of incidence and of reflection are equal i = r LLLL aaaa wwww ssss o o o o ffff r r r r eeee ffff llll eeee cccc tttt iiii oooo nnnn ( ( ( ( cccc llll iiii cccc kkkk m m m m eeee ))))

This is what happens. The incident ray, the normal at the point of incidence and the reflected ray are all situated in the same plane. This is what does NOT happen. The incident ray, the normal at the point of incidence and the reflected ray are NOT all situated in the same plane.

2. Images formed by a plane (flat) mirror

2.1. To find the image of a point, draw a ray diagram the mirror the observer’s eye is at point O the point object at A O A

To locate the image of the object use its symmetry with respect to the mirror. O AA’

I is located at the point where the line OA’ and the mirror intersect. Draw a solid line (representing the reflected ray of light) over the portion IO of the line. No real ray travels between A’ and I. O A I Draw a broken line from the image to the eye.

Draw a heavy line from A to I to represent the incident ray. Add arrows to indicate the direction of the light rays. Add arrows to indicate the direction of the light rays. O A I A’

A' is the image of A. The image is an ‘optical illusion’ : this image cannot be captured on a screen. This type of image is said to be a ‘virtual image’. O A I T The eye which receives the reflected rays ‘sees’ the image of the object at A'.

An image is ‘real’ when the light rays actually come together at the image. Real images can be formed on a screen e.g.: images formed on a screen when a film is projected. An image is ‘virtual’ if the light rays DO NOT actually come together at the image. A virtual image cannot be formed on a screen.

2.2. How to find the image of an extended object (as opposed to a simple point) A B A' B' You must find the image of each point in the object AB = object A'B' = image of the object

A B A' B' Observer

The image formed by a plane mirror is: virtual (it cannot be formed on a screen) symmetric with respect to the mirror (not superposable on the object) the same size as the object

2.3. Exercises a) Construct the image of the letter M in the following diagram:

mirror observer object virtual i mage

b) Among the letters of the alphabet, which ones would have an image in a vertical mirror that would be identical in shape to the letters themselves? A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

The writing on this ambulance is reversed because… … the image in a plane mirror is also reversed, so this is what you would see in the rear-view mirror:

3. Reflection in curved mirrors

An image formed behind a convex mirror is smaller than the object An image formed behind a concave mirror is bigger than the object

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