Dual Nature of Radiation and Matter

Two sources of light emit with a power of 200 W. The ratio of number of photons of visible light emitted by each source having wavelengths 300 nm and 500 nm respectively, will be :

1. 5:3
2. 3:5
3. 1:5
4. 1:3

A convex lens of focal length 40 cm forms an image of an extended source of light on a photoelectric cell. A current I is produced. The lens is replaced by another convex lens having the same diameter but focal length 20 cm. The photoelectric current now is :

1. I2
2. 4 I
3. 2 I
4. I

The threshold frequency of a metal with work function 6.63 eV is :

1. 16 1015 Hz
2. 16 1012 Hz
3. 1.6 1015 Hz
4. 1.6 1012 Hz