Question

A pulse of light of duration 100ns is absorbed completely my a small object initially at rest. Power of the pulse is 30mW and the speed of light is $\bf 3×10^{-8}ms^{-1}.$The final momentum of the object is?​

Answer 1

Explanation:

We know that

$\boxed{\sf p=\dfrac{E}{c}}$

$\\ \qquad\quad\sf{:}\longrightarrow p=\dfrac{P\times t}{c}$

$\\ \qquad\quad\sf{:}\longrightarrow \dfrac{30\times 10^{-3}\times 100\times 10^{-9}}{3\times 10^8}$

$\\ \qquad\quad\bf{:}\longrightarrow 10^{-17}kgms^{-1}$

Answer 2

Sol.(B)

t=100×10

−9

sec, P=30×10

−3

Watt,

C=3×10

8

m/s

Incident energy =Pt

By debroglie wavelength, λ=h/p

and Energy of photon is E=hc/λ

From above two equations, E=pc

Momentum p=

C

Pt

=

3×10

8

30×10

−3

×100×10

−9

=1.0×10

−17

kg ms

−1

So, the answer is option (B).

Check out the attachment please