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## Physics library

### Course: Physics library>Unit 15

Lesson 1: Reflection and refraction

# Specular and diffuse reflection

Specular reflection, which occurs with smooth surfaces like mirrors, causes light rays to reflect at the same angle as they hit the surface. Diffuse reflection, which occurs with rougher surfaces, scatters light rays in different directions. The video also introduces double reflection, which occurs when light reflects off of one surface and then reflects off of another surface before reaching your eye. Created by Sal Khan.

## Want to join the conversation?

• If build a box with mirrors, all on the inside, so the entire inside of the box is all mirrors, and will all give off specular reflections, and then cut a circular hole in the top and insert an ON light bulb,
will the light level keep on building up? Or are the photons adsorbed or "die" somehow?

I understand that the actual light bulb may absorb some of the light, with a diffuse reflection.

But hypothetically, if the entire light bulb had a specular reflection and it was completely sealed, would the light keep bouncing around the box, even after the light is turned off?
• That's a cool question.
I think that in real life, light will always find a way out in form of heat or something, but hypothetically it would be cool if we could lock light in a box :p
• Just to clarify, in a diffused reflection, the angle of incidence and angle of reflection are not always equal? Since the reflections are going in all sorts of crazy directions.
But in a specular reflection, the angle of incidence and angle of reflection are always equal.
• Technically, you're wrong. The angle of incidence is technically the angle between the incident ray and the NORMAL and the angle of reflection is between the reflected ray and the NORMAL (again). Diffused reflection is observed when the surface isn't smooth. This means that at different points on the surface, the normal is different. The angle of reflection is STILL equal to the angle of incidence at THAT POINT and the NORMAL is unique at THAT point in the surface. Hope I'm not too confusing.

EDIT - This is almost a broad explanation of what Colin said :P
• When i look at a light source and close my eyes immediately, why does it appear green?
• In our eyes there are microscopic tiny sensors which are sensitive to light.They have 3 colors red,blue and green.If you look at a very bright thing such as the sun and close your eyes immediately you will see red.This means that red is the most sensitive to light.When you see any light source glowing brightly and when you see it and close your eyes immediately you will see green.In your case you might have seen a brightly glowing bulb.So green is less sensitive to light than red.Finally if you see a light source glowing dimly you will see blue color.So blue is least sensitive to light.One interesting fact for you that when you look at the sun immediately and close your eyes firstly you will see red color.But as the light intensity seen by our eyes decreases you will see green color then and finally you will see blue color and then you will see only black.The order of sensitivity of the receptors are:
Red > Green > Blue
Red -----------> Most Sensitive
Green------------> Less Sensitive
Blue------------> Least Sensitive
Hope you like it
• Why do we see images of us in glass at times ?
• That's because glass reflects light (specular reflection). But it only reflects a fraction of it. Most of the light goes through glass.
• Is there any proof for why angle of incidence is equal to the angle of reflection as far as specular reflection is concerned?
• An intuitive proof is very easy if you assume that all the basic laws of physics stay the same if you switch the direction of time (at least of classical physics, and let's disregard thermodynamics for a second).
Take a film of two billard balls colliding. Now play it backwards. There is no way to tell, which direction actually happened.
Therefore the incoming and the outcoming angle of a beam of light have to be the same.
• if in diffused reflection light scatters away then we must not be able to see it . then how on the world are we seeing all this objects ?
• Keep in mind that the diagram he made is a microscopic view of an object on the atomic level. There are billions of light rays and billions of surfaces. So while you see things, a lot of the light is diffused. That's why when you look at something, it isn't like looking into the sun, but you still get enough light to see it.
• On a micro level, is all reflection specular, with the angles the same as they reflect off the various surfaces, and the diffuse reflection is caused by inconsistent surfaces, or does some light not reflect at an angle equal to the angle it hit the surface at?
• YES! All light reflects at equal angles of incidence and reflection. It is the changing orientation of the normal on certain surfaces that are not smooth (like the paint on your wall) that causes this shift, and diffuse reflection occurs. Diffuse reflection refers more to the collection of the reflected rays. If the incident rays are parallel, but the reflected rays are not, diffuse reflection occurred.
• Am I right in saying that the light would be continuously reflected forever? Would it ever stop reflecting or would it just carry on for ever?
• each time light reflects, a little is absorbed or refracted so it will eventually all be gone

ok??
• I am confused on the difference between diffused and specular, if you consider...

consider a round / unsmooth object. If you zoom in really far -further and further, eventually you will find a point where it looks like a geometric shape with multiple sides (think of a circle on the computer -if you look hard enough the pixels are still square at the edge), right? So wouldn't diffused reflection just be specular but bouncing on many surfaces?
See the image I made below because just words is too confusing:
http://imgur.com/je1xdIQ