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## Class 12 Physics (India)

### Course: Class 12 Physics (India)>Unit 9

Lesson 1: Reflection and plane mirrors

# Specular and diffuse reflection 2

Specular and Diffuse Reflection 2. Created by Sal Khan.

## Want to join the conversation?

• I'm still not quite sure why the specular reflection is white. Why was energy not absorbed from that light ray, returning white light, yet the diffused reflection returns red? Its seems both are reflections of the same light source and both types of rays would have certain wave lengths absorbed equally.
• Specular reflection occurs at the very surface of the apple. Often apples are waxed so that they are extra shiny (it makes the colors seem more intense). Just like a mirror or a shiny white light everything is reflected. White light is a mix of all colors, so all colors get reflected.

Specular reflection is not always white - think of mirrors made of silver, gold and copper. Silver reflects all visible light more or less equally. Gold and copper do not. Pure gold absorbs blues and greens and has a yellow appearance. Copper absorbs yellow too, so that it has a red-orange appearance. This has to do with the way light interacts with the electrons of these metals. This wasn't properly understood until the 20th century.

Diffuse reflection is a bit more complicated than Sal's easy to understand explanation. The apple appears red because of pigments that absorb the other colors. This happens below the surface where the pigments are. Depending on the surface (I assume this is true of apples - I haven't looked at them) the light is bounced back and force in between the layers, in this case in the apples skin. These multiple reflections make the light diffuse, because the light is reflected differently between each of the layers (e.g., closer to of further from the white flesh).
• So, is what we see in our mirror a specular reflection of us that is a diffuse reflection of the light source?
• correct, its the portion that gets into our eyes.
• Sal said that the apple was reflecting because it was waxed nicely, but whywould waxing change themake up of the surface of the apple? Wouldnt it still be an uneven surface regardless of the wax? Why does the wax increase the reflecton?
• The wax is like water and water is reflective.like how polished wood is shiny
• maybe a little out of the box question.
if light gets absorbed by the object then shouldn't the object gain mass as it is absorbing the light particles or is it that the light particles dont have mass as it is travelling at the speed of light ? quite confused here
• Lights is electromagnetic waves and we can say it made of photons and photons do not have mass
• Sal said that in diffused reflection some part of the light is absorbed by the apple. And since light is in effect just energy particles, does this mean that the apple keeps more getting energetic the more you keep it in the light?(given that it isn't decaying)
• Yes the apple is gaining energy from the light that it absorbs and it warms up because of it. The apple is in turn radiating this heat increase out as infrared light that we can not see.
• this question has been asked earlier in the first video of specular and diffuse reflection. the question was that during diffuse reflection will the angle of incidence be equal to angle of refection?
i am still not clear because there were 3 different answers to the question which still makes it unclear.
hence according to me the angles should be equal because its the normal's function to divide the angle and even the law. but my book says that law is only obeyed in smooth surface. thus this makes it unclear . so can someone please answer.
• the angle of reflection is in fact equal to the angle of incidence at all times. It does not matter whether the surface is smooth or uneven. When you come to think of it , the particles of light in the incident ray fall only on a minute point so when it gets reflected you take it as a plane surface and therefore the normal is always perpendicular to the surface ( which is a minute point).

hope this cleared our doubt :)
• is the specular reflection caused due to total internal reflection??
• Specular reflection is the mirror-like reflection of light (or of other kinds of wave) from a surface, in which light from a single incoming direction (a ray) is reflected into a single outgoing direction. Such behavior is described by the law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal, thus the angle of incidence equals the angle of reflection ( in the figure), and that the incident, normal, and reflected directions are coplanar. This behavior was first discovered through careful observation and measurement by Hero of Alexandria (AD c. 10–70
• how do things absorb (or reflect) certain wavelengths and not others?
• It has to do with the energy levels of the atoms, molecules and inter molecular bonds. If the difference in these levels is the same as the energy of a photon then it can be absorbed.