Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of 60°, by what fraction will the illuminance change?

The illuminance will not change.

Light from a point source falls on a small area placed perpendicu…

If several lenses of focal lengths $f_1, f_2, f_3 \ldots$ are kept in contact with each other, then what will be the effective focal length of this combination?

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Write names of those radiations of electromagnetic radiations which are :(i) used in satellite communication (ii) used to study crystal structure (iii) produced as a result of decay of radioactive nucleus (iv) wavelength in between 350 nm and 770 nm (v) absorbed by the ozone layer (vi) produce extensive heating effect.

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Name the electromagnetic radiations used for studying the crystal structure of solids.

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What is the nature of effective forces in the nucleus?

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Write any two drawbacks of Rutherford's experiment related to atomic structure.

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A charge 'q' is placed at the centre of a cube of side l.What is the electric flux passing through each face of the cube?

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When a current is established in a wire, the free electrons drift in the direction opposite to the current. Does the number of free electrons in the wire continuously decrease?

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What is doping?

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A copper cube of mass 200g slides down on a rough inclined plane of inclination 37° at a constant speed. Assume that any loss in mechanical energy goes into the copper block as thermal energy. Find the increase in the temperature of the block as it slides down through 60cm. Specific heat capacity of copper $= 420Jkg^{-1}K^{-1}$.

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On what does the color dispersion power of a prism depend?

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