# 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.

## Presentation on theme: "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."— Presentation transcript:

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 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. 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 ! Somebody is cut into two halves, but he/she is still alive and talking !

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. 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…. 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. 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.

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. 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 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. If you look at a point farther away, you see mainly the reflected light from sun and sky. (Grazing incidence) (Grazing incidence)

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. 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.

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

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.

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. 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 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) Sun pillars are commonly seen at sunset and sunrise in blowing snow. Moonlight can produce them as well ( over a body of water)

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

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

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. 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.

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

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

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

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

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

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. 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.

picture of a corner reflector

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

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