Home » 2013 » November » 05

Daily Archives: November 5, 2013


Does light experience friction?

Does light experience friction?

Asked rashmi (VIII B, KV AFS Bidar)


Night Light

FRICTION  is defined as the force which opposes relative motion between two surfaces in contact.

If you mean friction in the normal sense as above (as discussed in class), then the answer is “NO“.

Light is a form of energy and not a particle. But it is true that light shows some particle behaviour also.

But, there is a slowing down when light interacts with matter. But, in a homogeneous medium, light has a constant speed. The speed of light is different in different medium.  If we go to the atomic or molecular picture of matter, then we can imagine that light is interacting with a molecule, during the time of interaction, the light can be assumed to have been absorbed by the particle and then it is re-radiated. the re-radiated light travels to the next atom and interacts/absorbed. this slows down the average speed of light, but is constant for a particular homogeneous medium.

The speed of light is maximum in vacuum where there is no matter for light to interact.

So, in between two atoms also, light travels with its branded 3L kmps speed.

Also refer:

  1. http://phasing.org/2009/09/25/does-light-have-friction/
  2. http://answers.yahoo.com/question/index?qid=20100425222814AA2LUvq

Relative velocity of image with respect to object

A point object is moving with a velocity u=2i+j-k m/s in front of a stationary plane mirror.The magnitude of relative velocity of image with respect to object is maximum if normal will be along?


When we walk, is there a short time when both our feet don’t touch the ground?

When we walk, is there a short time when both our feet don’t  touch the ground?

Iliana Perez

Yes. The time interval depends how fast you walk.
Watch the video below

Motion in upward direction – numerical

Sir, I am not able to find the answer to this question.It is :
When a particle is projected vertically upwards with an initial velocity of 25m/s. During third second of its motion, which of the following statement is correct ?
a: displacement of the particle is 30m.
b: distance covered by the particle is 30m.
c: distance covered by the particle is 2.5m
d: none of these

Posted by Nidhish Saga





Light and matter possess dual nature. Means they have properties of matter as well as waves at the same time. So, if we say that light is a form of matter or vice versa, it cannot be falsified.

Light (which is a form of radiant energy) and matter are inter-convertible too. matter can be converted to energy and vice versa.

The energy released by the complete conversion of a mass “m” of any matter is given by Einstein’s equation, E = mc2

Problem from Mechanics, :A small boy is throwing a ball towards a wall ….”

A small boy is throwing a ball towards a wall 6 m in front of him. He releases the ball at a height of 1.4 m from the ground. The ball bounces from the wall at a height of 3m, rebounds from the ground and reaches the boy’s hand exactly at the point of release. Assuming the two bounces (one from the wall and the other from the ground) to be perfectly elastic, how far ahead of the boy did the ball bounce from ground ?

Asked Amal Byju

(Answer to be posted soon)

Difference between reflection and scattering

Please explain the exact difference between reflection and scattering of light.


Asked A N Srinivasa Murthy




Reflection is the general term and scattering is also called diffused reflection.


Reflection happens when light striking the interface between two media returns to the same media.


Scattering can refer to irregular reflection or a deviation from the predicted angles of reflection.


Scattering is the phenomenon which helps us to see the objects.


A highly scattering surface is said to have a matte finish and a less scattering surface appears glossy. Here the microscopic irregularities on the surface is increasing the scattering.


We see our image in a plane mirror due to regular reflection. A matte finish paper (or a newspaper) can never show us even a faintest image of our face when we are looking into it, because there the scattering is more and almost no regular reflection is taking place.


Regular reflection is identified from the following idea;


Diffuse Reflection


If a parallel beam of light is incident on a surface and the reflected beam is also parallel, then the reflection is regular.

If the reflected beam is not parallel, then the type of reflection is irregular, diffused or scattered.


%d bloggers like this: