Why do we have illusion?

1. Why do we have illusion? • The reflected light that we see comes from the reflection of a surface. However, we do not usually interpret it as a reflection (we don’t see the surface). We always think the light travels on a straight line! Thus • There is a moon in the lake, some try to get it. • Somebody is cut into two halves, but he/she is still alive and talking!

2. How can you tell the image from the real thing? • Most times, we cann’t! Consider the reflection of a tree on the other side of a lake from the water. If the tree is very close to the water, you have a difficult time to distinguish the image from the object. • Movies: Camera moves slowly from a mirror image to a real person…. • On the other hand, if the tree in high on the bank, you don’t see the image near the root. So the reflection is not complete.

3. Relation between the Reflection Intensity and Angle • The amount of reflected light depends on the materials involved and on the angle at which the light hits the interface. • If you look straight done into a lake, you may see the bottom of it and there is little reflection in this angle. • If you look at a point farther away, you see mainly the reflected light from sun and sky. (Grazing incidence)

4. Subsun • The subsun is the most common sub-horizon halo.  It is a direct reflection of the sun from millions of crystals in clouds acting together as a giant mirror.   Plate crystals are the usual source, and rays can reflect externally off the uppermost horizontal face or internally from the lowermost.

5. When water freezes, it sometimes form a flat-plate, hexagonal crystal which falls flat through atmosphere

6. A bright subsun looking down from 11,000m over Italy, late afternoon 18th July 1995.  The halo was intermittently visible over the southern and central parts of the country covered by a thin cirrus sheet. The halo elongated and contracted as the tilts of the plate crystal mirrors changed. Reflections from lakes and rivers shine through the cloud.  Video by Les Cowley.

7. Sun-pillar • A spectacular orange sunrise and sunset provides the backdrop for a sun-pillar, an upward reaching shaft of light that is formed by reflection of sunlight from flat ice-crystal plates in light clouds. • not exactly horizontal, there are reflections at many different angles • Sun pillars are commonly seen at sunset and sunrise in blowing snow. Moonlight can produce them as well (over a body of water)

10. Specular Reflection • Reflections from smooth surfaces, such as polished metal, glass and water is called specular reflections.

11. Score card for Specular reflection • Pluses • Mirror • Radar.. • Minus • Don’t know where the mirror is (you might bump on to a mirror, but it makes a small room look bigger). • Problem with driving on a rainy day. You cannot see the road.

12. Diffused Reflection • Most reflections we see are diffused reflection from a rough surface (paper, cloth, skin, tree, buildings), which give an overall brightness. The reflected light goes in all possible directions.

13. In diffused reflection, every light-ray obeys the law of reflection. • You eyes assume the object is located where the diffuse reflection happens

14. Score card for diffused reflection • Plus • Tell us where the object is • Minus • Need strong light to see the reflection

15. Multiple Reflection • Have you been in a barber shop? Light can be reflected again and again and you see an endless series of images. You can see infinite number of reflections? • Not quite • Mirrors are not perfectly parallel. • Each refection gets dimmer. • Your head will cover up the images. • Infinite path can not be covered in a finite time.

16. Play with a java applet • http://www.phy.ntnu.edu.tw/java/optics/image.html

17. Two Mirrors in right angle If the light incidents in the same plane formed by the normals of the two mirrors, no matter what direction is the incidence, the light always is reflected back into that same direction

18. Corner reflector • The three mirrors arranged to have three mutually perpendicular faces have the property of being retro-directive; i.e. it will reflect all incoming rays back along their original directions. When viewed from any angle, the image of the observer always appears at the vertex of the three mirrors. Alternatively a laser beam directed to the reflector comes straight back.

19. picture of a corner reflector

20. Single Mirror with Several Images • The primary: Mirror reflection. • Secondary: reflection from the surface of the glass. • Secondary: the primary reflection reflects again from the surface of the glass and then reflects from the mirror.