Ultraviolet light of wavelength `800 A` and `700 A` when allowed to fall on hydrogen atoms in their ground states is found to liberate electrons with kinetic energies `1.8eV` and `4.0eV`, respectively. Find the value of Planck's constant.

Explanation

To find the value of Planck's constant using the given information about the photoelectric effect, we can follow these steps:

### Step 1: Identify the given data
- Wavelengths of ultraviolet light:
- \( \lambda_1 = 800 \, \text{Å} = 800 \times 10^{-10} \, \text{m} \)
- \( \lambda_2 = 700 \, \text{Å} = 700 \times 10^{-10} \, \text{m} \)
- Kinetic energies of liberated electrons:
- \( K_{\text{max1}} = 1.8 \, \text{eV} \)
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