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## Class 10 Physics (India)

### Course: Class 10 Physics (India)>Unit 1

Lesson 1: Reflection of light

# Laws of reflection

In this video, we will explore what diffused (scattered) & specular (regular) reflections are, and also learn the rules governing the reflection of light. Created by Sal Khan.

## Want to join the conversation?

• Light can travel through space, but why can't sound?
• as light can travel in any medium, so it can travel through space. but sound needs a medium to propagate and it needs a medium like air to travel further as the air particles move in a wave when sound travels. if there are no molecules in space, the sound won't travel.
• Our book tells us that even for irregular surfaces,
``i = r``
while the video says the opposite (at around ). Which is correct here?
• `i = r.`

This law always holds true. But the angles are dependent on the situation of the normal - the line perpendicular to the surface. In irregular surfaces, as the surface is knobbly, the normal points a different way at every point. So even though `i = r`, it doesn't always seem like it.
• if the mountain diffused light, Why would all the light necessarily fall on water ? some light may hit the ground also and we would not see a complete image in the water . But how do we get a complete image then??
• The fact that mountains diffuse light is the reason why we can see it. The reflected rays from the mountain are directed in all directions. Some of these fall on the land, some towards the sky, some on the water surface. The light rays that reflect towards our eyes enable us to see the mountain. However, the reflected rays that are directed towards the surface of the lake do not reach our eyes directly. They reflect specularly off the surface of the lake and then reach our eyes, hence we see the image of the mountain in the lake. But the surface of land is not smooth, so there, once again, diffused reflection takes place and don't get any image.
• Why do we draw a normal from the point of incidence? Dont it essentially means that angle of incidence is imaginary?
• A ray of light is incident on a totally reflecting surface at a certain point. This point is called point of incidence.

If we draw a line which is perpendicular to the reflecting surface at the point of incidence, this line is called the normal i.e., normal is an imaginary line perpendicular to the surface reflecting the light.

Angle between the incident ray and the normal is called angle of incidence. Angle between the reflected ray and the normal is called angle of reflection.

The law of reflection states that when a ray of light is reflected off a surface, the angle of incidence is equal to the angle of reflection.

I think that the angles of incidence and reflection are imaginary ones. They help us understand things better. Perhaps it’s because it’s easier to visualize a reflection about that line rather than a symmetry with regard to the tangent to the surface. It’s mostly just a convention.

Pls do point out if I'm wrong.

Hope this helps!
• Is the first law of reflection imaginary?
• There are two laws of reflection:
1) The incident ray, reflected ray and normal lie on the same plane.
2) Angle of incidence is equal to angle of reflection.
In case you are referring to the first law,to some extent yes it is imaginary because a plane is a human made concept ( does not have any physical existence) but it is nevertheless important. Take two pencils and hold them together by connecting their sharpened ends. The law tells us that if these two pencils are light rays, they can only exist in a 'V' format.The normal would be lying 90 degrees to the surface. If you try moving one pencil forward or backward, notice that all three( incident ray, normal, and
reflected ray) can never be in the same plane.
If you are talking about law 2, then it is more practical. It is also important because it gives us a basic foundation of light's nature that angle of incidence is always equal to angle of reflection.
• Why do we prefer convex mirror in the cars and other vehicles?
• Convex mirrors are commonly used as rear-view (wing) mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. Thus, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
Hope this will help.
• what is the meaning of sign convention
• Sign conventions are some 'convetional rules' which are used to know and decide the positions of objects and images with the help of + and - signs.
If you don't know about the notion of conventions then I can explain it to you as follows:
You might surely know about the number system- 0,1,2,3,4....-which is the English numereal system.
But there are numeral systems of other languages, like the hindi numeral system (though it is not used much substantially ,nowadays ). We have just accepted them without any scientific reason because we needed a common language to communicate with other people . Therefore, these numeral systmes are also conventional, and even languages thmeselves are conventional.