Explanation

The frequency of light remains the same when it travels from one medium to another.

The relation between wavelength and refractive index is

$$ \lambda \propto \frac{1}{n} $$

or

$$ n \lambda = \text{constant} $$

For light travelling in water,

$$ n_1 \lambda_1 = \frac{4}{3} \times 540 $$

For light travelling in the second medium,

$$ n_2 \lambda_2 = \frac{3}{2} \times \lambda_2 $$

Since frequency is constant,

$$ n_1 \lambda_1 = n_2 \lambda_2 $$

$$ \frac{4}{3} \times 540 = \frac{3}{2} \times \lambda_2 $$

$$ \lambda_2 = \frac{\frac{4}{3} \times 540 \times 2}{3} $$

$$ \lambda_2 = 480\ \text{nm} $$

Hence, the wavelength of light in the second medium is

480 nm

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