Light Reflection

What do we see? All objects we see with our eyes are either luminous (give off light) eg the sun.

Or illuminated (reflect light) eg the moon.

Illumination The further from the light, the less the illumination. Illumination is inversely proportional to the distance squared.

How you see Your brain believes light to travel in straight lines. An illuminated object reflects light in many directions but only those rays coming towards your eye are “seen”.

For all plane mirrors, the distance from the mirror to the object is equal to the distance from the mirror to the image.

Law of reflection Angle of incidence = angle reflection.

Which is the angle of incidence and which the angle of reflection?

Draw the reflected rays

Ray Diagrams

Parallax – the apparent movement. the farther an object, the smaller is its parallax, and its converse, the smaller the parallax, the farther the object

Optical Illusions Link to tour

Curved mirrors

Concave (converging mirrors) C-centre curvature r- radius curvature F- principal focus point f-focal length A=P=pole Sun demo.

A curved mirror is a small part of a sphere’s surface. Light rays parallel to principal axis are reflected to one point – the principal focus F Also in reverse – headlights. f=r/2

2 vital rules for converging mirrors Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection. Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection

Ray Diagrams Ray diagrams are drawn to find details of an image. Size Nature (inverted or upright, virtual or real, enlarged or diminished) Position

How to draw ray diagrams Draw principal axis Mirror line Object

Next a ray parallel to the principal axis is reflected through F

Next a ray from the top is drawn thro’ F and reflected parallel to the principal axis A ray aimed at the pole reflects at the same angle

Exercises Draw a ray diagram similar to the one before but with the object located behind C. And between C and F.

Draw a ray diagram with the object between F and the mirror.

Note that image is upright, magnified and virtual.

And finally, with the object at F

In Summary

Convex (Diverging) Mirrors Convex mirrors follow similar rules.

Important rules for convex mirrors Ray parallel to principal axis reflects as if it comes from the focus Ray aimed at focus reflects parallel to principal axis Ray aimed at P reflected straight back

Exercise Draw an accurate ray diagram showing the image formed from a 3.0 cm object 7.0 cm in front of a convex mirror of focal length 5.0 cm. How high does the image appear? Describe the image from these words enlarged, same size, diminished, upright, inverted, real, virtual.

Formulae for spherical mirrors Descartes’ formula di distance to image do distance to object

Magnification m=hi/ho = di/do

Exercise Michael is shaving with his face 12 cm from a concave mirror of focal length 12.5 cm. What is the magnification of the image of his face?

di =-300cm ( negative sign shows virtual image) m = di/do = 300/12=25 Answer 1/f = 1/di + 1/do 1/12.5 = 1/di + 1/12 di =-300cm ( negative sign shows virtual image) m = di/do = 300/12=25

Exercise A convex mirror has f 10 cm. Calculate the position and size of a 10cm object 6cm in front of mirror.

Answer do = 6 cm, f = -10 cm, ho = 10 cm 1/di=1/10 – 1/6 di = -3.75 cm m = di/do = 0.625 hi = mho = 0.625 x 10 = 6.25 cm

Activity 5b on page 67