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A 1.5 kW (kilo-watt) laser beam of wavel...

A 1.5 kW (kilo-watt) laser beam of wavelength 6400 Å is used to levitate a thin aluminium disc of same area as the cross section of the beam. The laser light is reflected by the aluminium disc without any absorption. The mass of the foil is close to

A

`10^(-9) kg`

B

`10^(-3) kg`

C

`10^(-4) kg`

D

`10^(-6) kg`

Text Solution

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The correct Answer is:
D
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A cylindrical block of length 0.4 m and area of cross-section 0.04 m^2 is placed coaxially on a thin metal disc of mass 0.4 kg and of the same cross - section. The upper face of the cylinder is maintained at a constant temperature of 400 K and the initial temperature of the disc is 300K. if the thermal conductivity of the material of the cylinder is 10 "watt"// m-K and the specific heat of the material of the disc is 600J//kg-K , how long will it take for the temperature of the disc to increase to 350 K? Assume for purpose of calculation the thermal conductivity of the disc to be very high and the system to be thermally insulated except for the upper face of the cylinder.

Knowledge Check

  • A 1.5 kW (kilo-watt) laser beam of wavelength 6400 Å is used to levitate a thin aluminium disk of same area as the cross section of the beam. The laser light is reflected by the aluminium disk without any absorption. The mass of the foil is close to -

    A
    `10^(-9) kg`
    B
    `10^(-3) kg`
    C
    `10^(-4) kg`
    D
    `10^(-6) kg`

  • A beam of light of wavelength 6400 Å is incident normally on a parallel glass plate of 5cm thick and R.I. 1.6. The beam takes the same time to travel from source to incident surface and through the glass plate. The distance of source from incident surface is

    A
    7.5 cm
    B
    6 cm
    C
    10 cm
    D
    8 cm

  • A radiation of wavelength 200nm is propagating in the form of a parallel surface. The power of the beam is 5mW and its cross-sectional area is 1.0 mm^(2) . Find the pressure exerted by radiation on the metallic surface if the radiation is completely reflected.

    A
    `3.33xx10^-5Nm^(-2)`
    B
    `4.33xx10^-5Nm^(-2)`
    C
    `5xx10^-2Nm^(-2)`
    D
    None of the above

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