Light with an energy flux of falls on a perfectly reflecting surface at normal incidence. If the surface area is , the average force exerted on the surface is [2014]

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Q.
Light with an energy flux of $25 \times 10^{4}$ $W$ $m^{- 2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15$ ${cm}^{2}$ , the average force exerted on the surface is [2014]

1 $1.25 \times 10^{- 6} N$

2 $2.50 \times 10^{- 6} N$

3 $1.20 \times 10^{- 6} N$

4 $3.0 \times 10^{- 6} N$

Ans.
(2)

Here, Energy flux, $I = 25 \times 10^{4}$ $W$ $m^{- 2}$

Area, $A = 15$ ${cm}^{2} = 15 \times 10^{- 4} m^{2}$

Speed of light, $c = 3 \times 10^{8} {ms}^{- 1}$

For a perfectly reflecting surface, the average force exerted on the surface is

$F = \frac{2 I A}{c} = \frac{2 \times 25 \times 10^{4} {Wm}^{- 2} \times 15 \times 10^{- 4} m^{2}}{3 \times 10^{8} {m s}^{- 1}}$

$= 250 \times 10^{- 8} N = 2.50 \times 10^{- 6} N$

Solution

Q.
Light with an energy flux of $25 \times 10^{4}$ $W$ $m^{- 2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15$ ${cm}^{2}$ , the average force exerted on the surface is [2014]

1 $1.25 \times 10^{- 6} N$

2 $2.50 \times 10^{- 6} N$

3 $1.20 \times 10^{- 6} N$

4 $3.0 \times 10^{- 6} N$

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Solution

Q. Light with an energy flux of 25×104Wm-2 falls on a perfectly reflecting surface at normal incidence. If the surface area is 15cm2, the average force exerted on the surface is [2014]

1 1.25×10-6N

2 2.50×10-6N

3 1.20×10-6N

4 3.0×10-6N

Ans. (2) Here, Energy flux, I=25×104Wm-2 Area, A=15cm2=15×10-4m2 Speed of light, c=3×108ms-1 For a perfectly reflecting surface, the average force exerted on the surface is F=2IAc=2×25×104 Wm-2×15×10-4m23×108m s-1 =250×10-8N=2.50×10-6N

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Q.
Light with an energy flux of $25 \times 10^{4}$ $W$ $m^{- 2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15$ ${cm}^{2}$ , the average force exerted on the surface is [2014]

1 $1.25 \times 10^{- 6} N$

2 $2.50 \times 10^{- 6} N$

3 $1.20 \times 10^{- 6} N$

4 $3.0 \times 10^{- 6} N$